Advances in Insect Physiology, Volume 23

Advances in Insect Physiology

Volume 23

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Advances in Insect Physiology edited by

P. D. EVANS Department of Zoology, The University Cambridge, England

Volume 23

u ACADEMIC PRESS Harcourt Brace Jovanovich, Publishers London San Diego New York Sydney Toronto Tokyo

Boston

ACADEMIC PRESS LIMITED 24-28 Oval Road London NWl 7DX United States Edition published by ACADEMIC PRESS INC. San Diego, CA 92101

This book is printed on acid-free paper Copyright 0 1991 by ACADEMIC PRESS LIMITED

All Rights Reserved

No part of this book may be reproduced in any form by photostat, microfilm, or any other means, without written permission from the publishers A catalogue record for this book is available from the British Library ISBN 0-12424223-0 Typeset by Latimer Trend & Company Ltd, Plymouth and printed in Great Britain by St Edmundsbury Press Ltd, Bury St Edmunds, Suffolk

Contributors S. A. Corbet

Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK C. P. Ellington

Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK R. E. Page, Jr

Department of Entomology, University of California, Davis, C A 95616, USA M. P. Pener

Department of Zoology, Hebrew University of Jerusalem, 91904 Jerusalem, Israel G. E. Robinson

Department

05Entomology,

University of Illinois, Urbanu, I L 61801, USA

Retiring Editor V. B. Wigglesworth Advances in Insect Physiology, 1963-1990

Contents Contributors

V

Locust Phase Polymorphism and its Endocrine Relations M. P. PENER

1

A Fresh Look at the Arousal Syndrome of Insects S. A. CORBET

81

The Genetics of Division of Labour in Honey Bee Colonies R. E. PAGE, J R and G. E. ROBINSON

117

Aerodynamics and the Origin of Insect Flight C. P. ELLtNGTON

171

Subject Index

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Locust Phase Polymorphism and its Endocrine Relations M. P. Pener Department of Zoology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

1 Insect polymorphism and its endocrine aspects 1 1.1 Polymorphism 1 1.2 Polymorphism and endocrine factors 3 2 Phase polymorphism 4 2.1 Locusts 4 2.2 Some other insects 7 3 Phase characteristics and related endocrine eEects 8 3.1 Morphology, morphometrics and anatomy 8 3.2 Colouration 12 3.3 Reproduction 21 3.4 Hopper development 26 3.5 Physiology, biochemistry and molecular biology 28 3.6 Cytology 30 3.7 Behaviour and activity 31 4 Endocrine organs, hormones and their role in phase transformation 37 4. I The corpora allata and juvenile hormone 37 4.2 The prothoracic (=ventral) glands and ecdysteroids 45 4.3 Neurosecretory cells, corpora cardiaca and neurohormones 48 5 Pheromones 49 6 Concluding remarks 52 References 55 Addenda 75

1 1.1

insect polymorphism and its endocrine aspects POLYMORPHISM

The term polymorphism roughly means that distinct morphological differences, reflecting and often including physiological, behavioural and/or oecological differences, occur simultaneously or recurrently among conspecific individuals. Although the phenomenon is easily recognized and well known in many insect orders, its exact definition runs into difficulties. For example, most authors would not apply the term polymorphism to sexual ADVANCES IN INSECT PHYSIOLOGY VOL 23 ISBN 6 1 2 4 2 4 2 2 3 4

Copyrrghr 0 1991 Academic Press Limited AN rights of reproduction in any form reserved

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dimorphism, that is to morphological difference(s) other than in the genitalia between males and females, neither to a sequence of different stages in the course of metamorphosis (larva, pupa, adult). Some others, however, may extend the meaning of polymorphism to include these cases. For a comprehensive overview, definitions and related terminology the paper of Richards (1961) with its open discussion and the recent articles of Hardie and Lees (1985) as well as of Pener (1985) may be consulted. In many instances, polymorphism is under the strict control of the genotype. This “genetic polymorphism” (Ford, 1961) is independent of environmental factors, except for long-term effects through selection pressure. In contrast, immediate environmental factors, such as photoperiod, temperature, humidity, diet, and/or population density, often play a major role in the determination of the actual morph, although obviously the potential to exhibit such polymorphism in response to extrinsic factors is genetic. Moreover, the genotype may modify the phenotypic responses to environmental cues and the genetic ability to express environmentally controlled polymorphism is itself subject to selection pressure. In some cases, morphologically similar conspecific insects exhibit environmentally induced major physiological and/or behavioural differences. Michener (196 1) suggested calling such insects polyphenic rather than polymorphic. Liischer (1976b) extended the meaning of “polyphenism” to include morphological differences. Hardie and Lees (1985, p. 443) defined polyphenism “. . . as the occurrence of two or more distinct phenotypes which can be induced in individuals of the same genotype by extrinsic factors.” This definition makes the term polyphenism practically synonymous with the older usage of environmentally regulated polymorphism. The latter includes physiological and behavioural “polymorphism”, as well as “facultative polymorphism” as used by Nijhout and Wheeler (1982), all contrasting to “genetic polymorphism” (see above). Although from the etymological standpoint “polyphenism” is more correct, I prefer the older term, “polymorphism”, because it is deeply rooted in the literature. Locust phase polymorphism is environmentally regulated, but it is regarded as a “continuous polymorphism”. This term (see Kennedy, 1956, 1961) means that a continuous range of intermediates exists between the two extreme phases. This subject will be discussed later (see Section 2.1), but its definition again presents some difficulties because continuous “trivial” variations, exhibited by all insects, for example in body length, are never considered to be polymorphism. Although in the case of continuous polymorphism the differences between the extremes are more marked and the range covered by the intermediates is much wider than in trivial variations, attempts to make definitions which clearly separate the two phenomena run into quantitative difficulties and arbitrariness. How much difference and how

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wide a range of intermediates are needed for unequivocal distinction? It does not help that in the case of continuous polymorphism a given population, made up from individuals which have experienced similar environmental conditions, usually covers only a limited part of the full range. A population of a species exhibiting only trivial variations may also cover only an environment-dependent part of the full range. For example, body length (trivial variation) of a population which has experienced food shortage (environmental factor), may well be smaller than that of a population with abundant food. These two populations may respectively cover the lower and upper parts of the full range of this trivial variation.

1.2

POLYMORPHISM AND ENDOCRINE FACTORS

In environmentally controlled insect polymorphism (including physiological and behavioural “polymorphism”), extrinsic cues induce sensory and/or nutritional inputs. These are somehow coupled to the mechanisms which make a “decision” to prefer a certain morph over the other(s), then substantiate this preference in the course of development. Components of the endocrine system are usually involved in these mechanisms and are often major factors in the control of polymorphism. The first studies on hormonal effects on insect polymorphism were carried out on locusts by P. Joly (1949, 1951). Although he drew some preliminary misconclusions, these were soon corrected (P. Joly and L. Joly, 1954; L. Joly, 1954; P. Joly, 1956; P. Joly et al., 1956). From the mid 1950s onward, publications on endocrine effects on insect polymorphism became more frequent. Up to the early 1960s most of them were devoted to locust phase polymorphism, culminating in the comprehensive experimental works of L. Joly (1 960) and Staal(l961). Even in this early period, however, some studies already dealt with endocrine aspects of other kinds of polymorphism. For example, colour polymorphism in the grasshopper, Acrida turrita, was found to be affected by the corpora allata (P. Joly, 1952) and brain implantation was reported to influence wing polymorphism in the cricket, Gryilus campesIris (Sellier, 1955). Toward the turn of the decade, the first experimental studies of endocrine effects on caste polymorphism in lower termites (Luscher, 1961 and some further references therein) and on wing polymorphism in aphids (Lees, 1961) appeared. Although differences in the volume of the corpora allata (CA) between queens and workers of the honey bee, Apis mellifera, had been reported quite early (Lukoschus, 1955, and other contemporary publications by the same author), the subject of endocrine effects on caste determination in Hymenoptera gained momentum only about 15 years later. Advances in the endocrine aspects of polymorphism in

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4

aphids and social insects (termites, bees, ants) were summarized in a book edited by Liischer (1 976a). More recently, endocrine effects on insect polymorphism were reviewed by Nijhout and Wheeler (1982) and Hardie and Lees (1985). Other recent reviews, dealing with more restricted aspects of the subject, are those of Lees (1983) on aphids, Pener (1983) on locust phases, relevant sections in the articles of Brian (1979) and of De Wilde and Beetsma (1982) on social insects, and a section on wing and flight related polymorphism in Pener’s (1985) chapter.

2 Phase polymorphism 2.1

LOCUSTS

The theory of locust phases was formed by Uvarov (1921) in a taxonomic revision of the genus Locusta. He concluded that L. migratoria and L. danica, previously regarded as two distinct species, are respectively the swarming and the solitary forms or “phases” of the same species; these forms are capable of transforming into one another and are connected by intermediate forms. He also claimed that the South African brown locust, Locustana pardalina, has similar swarming and solitary phases. The phase theory was soon extended to other locust species and phase transformation was verified both experimentally and by field evidence (for a comprehensive study see Faure, 1932). Within the first decade which followed Uvarov’s (1921) paper, the terminology was formalized. The swarming crowded phase and the more sedentary isolated one were given the latinized names “gregaria” and “solitaria”, respectively. The intermediates were named as phase “transiens”, and some authors even made the distinction between “congregans” and “dissocians”. At this time, the term phase was used in three senses: ( 1 ) as a loosely formalized taxonomical unit at the intraspecific level; (2) as an oecological concept in relation to population density, separating migrating swarms or hopper bands of locusts from those found in isolation or at intermediate densities, and as an explanation, or even as “The Cause”, of the periodicity of locust outbreaks; and (3) as a biological phenomenon or status, to separate the different forms of the continuous density-dependent locust polymorphism, with the overt or implicit understanding that various phases differ not only morphologically, but also in other (physiology, behaviour, etc.) aspects. The relationship and the extent of correlation between taxonomical, oecological and biological (polymorphism) concepts of locust phases led to confusion resulting in much argument. The taxonomical concept, still

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retained for example in Key’s (1950) review, was gradually abandoned also by Uvarov (1 966) himself, though the latinized names are often used even today because of historical reasons. It also became increasingly evident (see contemporary reviews by Key, 1950; Kennedy, 1956, 1961, 1962; Gunn, 1960) that locust outbreaks are not caused by phase transformation; phase change does not precede but follows changes in population density. When locusts exhibiting “solitaria” morphology and colour but “gregaria” behaviour had been found in migrating bands or swarms (see references in the reviews above), the oecological and biological (polymorphism) concepts of the term phase were largely uncoupled. Today most authors agree that locust phases do not designate migrating or non-migrating locusts; they are used to characterize locust polymorphism, and the differences in locomotor activity (see Section 3.7) are considered to be one phase characteristic among others. This does not mean that locust phase polymorphism, locust migration and the periodical appearance and disappearance of locust outbreaks are not correlated; moreover, these phenomena presumably co-evolved. But their correlation is less rigid than it was regarded some decades ago. The better fit of the different phases to live in relative isolation or in groups in the field may be considered as oecological aspects of the phase polymorphism or even as oecological phase characteristics. According to present views, typical locust species show density-dependent phase changes in morphology, colouration, reproduction, development, physiology, biochemistry, molecular biology, cytology, behaviour and oecology. The extreme phases are usually named by the non-latinized terms “gregarious” and “solitary” (or “solitarious”). Full-scale phase differences seem to be limited to the field. Locusts reared in the laboratory under conditions of crowding and in isolation respectively only approach the characteristics of the gregarious and solitary phases. Such laboratory populations are often named as “crowded” and “isolated”, instead of “gregarious” and “solitary”, though this distinction is not consistent in the literature. As already stressed, locust phase polymorphism is continuous. Not only are all kinds of intermediates found between the extreme phases, but phase transformation itself is continuous and its induction is not instar-specific. In other words, phase characteristics can be shifted in either direction, and the direction of the shift is reversible in any stadium (except for eggs in the ground), all in response to appropriate changes in density. Some phase characteristics, such as behavioural patterns and some components of adult colour, respond within the same instar to changes in density. Other characteristics, like hopper colouration, exhibit changes in the next and/or subsequent instars. Some phase characteristics, for example the colour of the hatchlings, are affected by parental density. Phase transformation is cumulative, a phase shift starting in an instar progresses in the following ones and

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M. P. PENER

also in the next generation; a full-scale phase transformation may take several generations. Many density-dependent phase characteristics, such as colour and morphometrics, are also affected by further environmental factors, especially by humidity and temperature. Different locust species show somewhat different phase characteristics and the amplitude of the change of a given phase characteristic is often species-dependent and sometimes sex-dependent. Phase polymorphism of locusts, phase characteristics and the amplitudes of the changes in phase characteristics were comprehensively surveyed in Uvarov’s (1966) and Albrecht’s (1967) books. Other reviews on the subject within the last two decades include those by May (1971), Nolte (1974), Cassier (1974), and Pener (1983). 1 have defined locusts as short-horned grasshoppers (Orthoptera: Acrididae) which “. . . meet two criteria: 1) They form at some (rather irregular) periods dense groups comprising huge numbers, bands of hoppers and/or swarms of winged adults which migrate; and 2) they are polymorphic in the sense that individuals living separately differ in many characteristics from those living in groups” (Pener, 1983, p. 379). The first criterion is based rather on the same considerations which were somewhat dissociated from phase above; but now it is used for the definition of locusts and not of locust phases. In fact, this criterion is much older; it fits even the description of the locusts in the Old Testament, for example as the plague inflicted on the Pharaoh and the People of Egypt (Exodus, Chapter 10, Verses 4-19), presently attributed to the desert locust (Schistocevca gregaria). The second criterion is phase polymorphism as discussed above. Typically locust species satisfy both these interrelated, but not completely correlated, criteria. Many species of acridids tend to aggregate, and/or to migrate, and/or to exhibit more or less rudimentary phase polymorphism (see Uvarov, 1966, pp. 369-375, 1977, pp. 142-1 50; Jago, 1985). Thus, the borderline between gregarious grasshoppers and locusts is not very strict. For example, species of the genus Melanoplus (Catantopinae, here and below, Uvarov’s 1966 classification is used for the subfamilies) exhibit some phase polymorphism (Dingle and Haskell, 1967 and references therein), but they are still regarded as gregarious or migratory grasshoppers. The Australian plague locust, Chortoicetes terminifera (Oedipodinae), and the Moroccan locust, Dociostaurus maroccanus (Gomphocerinae), are considered to be locusts, even though the amplitude of their phase polymorphism is less extreme than that of some other locusts. A few New World species of the genus Schistocerca (Cyrtacanthacridinae), such as the Central American S. piceiforms and the South American S. cancellata, are also regarded as locusts; owing to taxonomical confusion, these have been often named as S. paranensis and S. americana, respectively, in the literature (for taxonomical considerations see Jag0 et al., 1979; Harvey, 1981). The most typical locust species are: (1) the migratory

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7

locust, Locusta migratoria (Oedipodinae), with its various subspecies, most of which have recently been considered to be just geographical races, although the subspecies status, L. m. migratorioides, is retained for all nondiapausing populations from the tropics and from the southern hemisphere (Farrow and Colles, 1980); (2) the brown locust, Locustana pardalina (Oedipodinae), from South Africa; ( 3 ) the red locust, Nomadacris septemfasciata (Cyrtacanthacridinae), from Central and South Africa; and (4) the desert locust, Schistocerca gregaria (Cyrtacanthacridinae), for a few years incorrectly named as S. americana gregaria (rectified by Jago et al., 1979), from tropical and subtropical areas of the Old World extending from the west coast of Africa to about the eastern border of India. These four species will be referred to below only by their generic names. Different locust species belong to several different subfamilies. This situation probably indicates that locust phase polymorphism evolved several times, by convergent evolution, within the family of acridids. The speciesdependent differences, both in certain phase characteristics and in the amplitude of their changes, may well be explained by this assumption. As already mentioned, many acridids exhibit tendencies of aggregation, migration and more or less rudimentary phase polymorphism (Uvarov, 1966, 1977; Jago, 1985). Typical locust species probably constitute only evolutionary culminations of these tendencies. Also, extreme phase polymorphism is found only in a relatively small number of acridid species; other kinds of polymorphism, such as non-density-dependent colour polymorphism (reviews by Rowell, 1971; Fuzeau-Braesch, 1985), are exhibited by a much larger number of species. It seems that evolution often led to the incorporation of colour polymorphism into the more complex phase polymorphism, but again such incorporation probably occurred independently in different locust species, or at least in different subfamilies. Altogether, locust phase polymorphism, a highly complex phenomenon when any single species is considered, becomes even more complicated when species-dependent differences are taken into account.

2.2

SOME OTHER INSECTS

Density-dependent continuous polymorphism also occurs in some other insects. The term phase polymorphism has often been employed to cover such cases. The phenomenon is especially well known in several species of Lepidoptera (see Faure, 1943a,b; Matthee, 1945, 1946, 1947; Long, 1953; Iwao, 1962; as examples of earlier publications). There is no comprehensive review on lepidopteran phase polymorphism, but the reader may consult some recent research articles for up-to-date information (e.g. Tojo et al.,

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1985a,b; Simmonds and Blaney, 1986; Fescemyer and Hammond, 1986; Fescemyer et al., 1986). These reveal that the term phase is used in a double sense, in relation to polymorphism exhibited mostly by larvae and to migratory tendencies (flight) of the adults. As in locusts (see Section 2.1), these two phenomena are surely interrelated, but it remains to be established how strictly they are correlated. The endocrine regulation of lepidopteran phase polymorphism is outside of the scope of the present review, but it may be mentioned that juvenile hormone (JH) is involved in this regulation (Yagi, 1976; Yagi and Kuramochi, 1976; Tojo et al., 1985a; Fescemyer et al., 1986; Fescemyer and Hammond, 1988). Nevertheless, as in locusts (see Section 4.l), JH may not necessarily constitute the sole endocrine factor in the control of lepidopteran phase polymorphism (cf. Tojo et al., 1985a). Density-dependent continuous polymorphism has also been found in some other insects, for example in Tettigoniidae (Chopard, 1949; Verdier, 1958; Robinson and Hartley, 1978), in Phasmida (Key, 1957) and in Gryllidae (Fuzeau-Braesch, 1960).

3

Phase characteristics and related endocrine effects

Any characteristic which shows density-dependent changes in locusts is considered to be a “phase characteristic”. Such phase characteristics, reflecting differences between gregarious and solitary locusts, are found (and obviously often intermingled) in morphology, anatomy, colour, reproduction, development, physiology, biochemistry, molecular biology, cytology, behaviour and oecology. There are so many phase characteristics that they cannot be fully surveyed in the present review. Only major, or recently discovered ones, especially those which were subject to studies on hormonephase interrelations, will be discussed below. For a comprehensive treatment of phase characteristics and an extensive list of references the reader is referred to Uvarov’s (1966, 1977) and Albrecht’s (1967) books.

3.1

MORPHOLOGY, MORPHOMETRICS AND ANATOMY

In locusts, as in other acridid species, adult males are smaller than adult females. However, the relative difference in body size between the two sexes may be phase dependent, or conversely, the relative differences in body size between the phases may be sex dependent. In Locusta, Schistocerca and Numaducris solitary or isolated females are somewhat larger than conspecific gregarious or crowded ones, but in the males of these species the situation is

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reversed. Thus, in these cases, the differences in size between the females and the males is smaller in the gregarious than in the solitary phase. However, in Locustana, Dociostaurus maroccanus and Chortoicetes terminifera gregarious adults of each sex are larger than conspecific solitary adults of the same sex. The differences in body size between gregarious and solitary locusts have not been claimed to be affected by endocrine factors. The pronotum is high, its median carina is convexly arched and so forms a crest in late hoppers and adults of solitary Locusta, but is rather straight or even slightly concave in gregarious hoppers and adults of this species (Fig. 1). Similar, but much less marked, differences may exist also in other locusts (see Faure, 1932, measurements of Locustana; and Dirsh, 1953, drawings of Schistocerca). No endocrine effects on phase-related differences in the shape of the pronotum have been claimed, and P. Joly (1956) and P. Joly et al. (1956) explicitly stated that the median carina of the pronotum does not change its gregarious shape after implantation of extra CA into crowded Locusta. These authors concluded, therefore, that this phase characteristic does not seem to be dependent on the CA.

FIG. 1 Lateral view of the pronotum in adults of Locusta: (a) solitary or isolated locusts; (b) and (c) gregarious or crowded locusts. Linear magnification, c. x 4.5.

The morphological differences between gregarious and solitary locusts are mainly quantitative and are usually studied by measurements of various parts of the body and their relative comparisons. Up to the late 1950s only

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simple morphometrical ratios were employed, especially the E/F ratio (length of the fore wing : length of the hind femur) and the F/C ratio (length of the hind femur : maximum width of the head). The latter, introduced by Dirsh (1951, 1953), is considered to be a better parameter for expression of phase differences. The F/C ratio is higher, whereas the E/F ratio is lower, in solitary than in gregarious locusts (cf. Dirsh, 1951, 1953; Uvarov, 1966). P. Joly and L. Joly (1954) and P. Joly (1956, 1962) reported that implantation of extra CA into crowded hoppers of Locusta leads to a decrease of the E/F ratio after the moult to the sixth instar (which is the adult in normal development of this species). These authors even obtained “hypersolitary” E / F ratios and they related the results to a “solitarizing” effect of the CA. Staal(l961) obtained similar results, but he related them to metathetelic disturbances in metamorphosis rather than to a phase shift. P. Joly (1962) found no effect of implanted CA on the F/C ratio in Locusta. However, in more comprehensive experiments, Staal (1961) observed a rather complex situation, finding the following effects on the F/C ratio in crowded Locusta: (A) an increase (of this ratio) in the metamorphic moult from normal fifth-instar hoppers to normal (sixth-instar) adults; (B) an increase in the adults obtained from hoppers isolated after hatching; (C) an increase in the adults obtained after implantation of extra CA into secondinstar hoppers; but (D) a decrease in the often metathetelic sixth-instar locusts obtained after implantation of extra CA into young fifth-instar hoppers. From these results Staal (1 961) concluded that although early implantation of extra CA and isolation both cause a somewhat parallel increase of the F/C ratio, the isolated (solitary) locusts do not seem to be simply slightly metathetelic or neotenous forms of the crowded ones. Owing to the fact that the F/C ratio increases during normal metamorphosis to the adult, prevention of this increase, that is F/C values lower than in the normal adult, would be characteristic of metathetely or neoteny. Indeed, such lower values are obtained after implantation of CA into the young fifth-instar hoppers. Isolation, however, causes an increase of the F/C ratio, just the opposite to the shift expected in metathelic creatures. Implantation of ventral glands (VG), equivalent in acridids to the prothoracic glands of other insects, into first- or second-instar hoppers of Locusta led to some disturbances in metamorphosis (Staal, 1961). A portion of the locusts had already reached adult morphology by the fifth instar. These adults were somewhat smaller than normal (sixth-instar) adults, but they exhibited typical adult E/F and almost adult F/C ratios. A smaller portion of the locusts did not show such precocious metamorphosis; they became fifthinstar hoppers with hypertrophied wing buds. They then moulted to giant adults which exhibited very high, “hypergregarious”, E/F ratios (Staal, 1961; Staal and De Wilde, 1962). Carlisle and Ellis (1962) reported that a positive

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correlation exists between the size of the ventral glands and the F/C ratio in Schistocerca. Although these simple morphometrical ratios are still used, they were rightly criticized by several authors who introduced into phase morphometrics more complicated, but also more correct and informative, statistical methods based on multivariate analysis (Albrecht and Blackith, 1957; Blackith and Albrecht, 1959; Stower et al., 1960; Symmons, 1969; Lauga, 1976a,b; reviews by Blackith, 1972; Lauga, 1977a). Stower et a/. (1960), employing discriminant functions, showed that non-density-dependent environmental factors also affect phase-related morphometrics; in very general terms, high temperatures shifted the morphometrical results toward the solitary phase in Schistocerca. High temperatures and increasing daylength (Albrecht and Lauga, 1978), as well as high humidity (Albrecht and Lauga, 1979) also induced a “solitarizing” effect on the morphometrics of Locusta. Unfortunately, these advanced statistical techniques have not been employed for investigating effects of endocrine factors on phase morphometrics. A notable exception is Lauga’s (1977b) study, which is related, however, to morphometrics of hatchlings (see Section 3.3.3). Thus, relevant knowledge is practically limited to endocrine influences on the simple morphometrical ratios (see above). These indicate that endocrine factors may affect phase morphometrics, but separation of the effects into disturbances of metamorphosis and into phase shifts run into difficulties and the interpretation of the results are not sufficiently clear. Solitary adults of Schistocerca and Nomadacris have usually one more stripe in the compound eyes than conspecific gregarious adults. The number of eye stripes in the adult of these species is equal to the number of instars including the adult instar itself (see Albrecht, 1955, and further references therein). Thus, the difference reflects the fact that solitary locusts of these species usually have an additional hopper instar (see Section 3.4). The sternal hairs of adult Locusta are longer in insects reared under crowding and under dry conditions; implantation of extra CA leads to a decrease of the length of these hairs (Staal, 1961). However, as the sternal hairs are much shorter in hoppers than in adults, it is again difficult to distinguish between metathetelic disturbances and a phase shift. In Locusta, isolated adults and fifth-instar hoppers have more basiconic and coeliconic sensilla on the antennae than the respective crowded locusts (Greenwood and Chapman, 1984). This finding may indicate that isolated locusts do not constitute a slightly metathetelic form of the crowded ones, because the number of antenna1 sensilla markedly increases from the fifthinstar hopper to the adult; thus, metathetelic adults would be expected to have fewer sensilla. However, no actual experiments were carried out on the possible effects of endocrine factors on this phase characteristic.

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Cassier and Delorme-Joulie (1976) and Cassier (1977) reported that crowded adult males of Schistocerca have a thicker epidermis (= hypodermis) and more numerous gland cells (which presumably produce the so-called “maturation-accelerating pheromone”; see Section 3.3.1) than isolated adults or crowded adult females. They claimed that solitary or gregarious characteristics of the epidermis of adult males are determined by the endocrine balance at about the moult from the fourth to the fifth instar ( = from the penultimate to the last hopper instar in crowded Schistocerca). The presence of ecdysone alone, or of both ecdysone and JH, at this moult was supposed to induce at the next moult (to the adult), gregarious-type or solitary-type male epidermis, respectively. These conclusions were drawn from experiments which included allatectomy, CA implantation, injection of JH or of ecdysone, but all endocrine manipulations were carried out on crowded locusts. Unfortunately, these authors did not allatectomize isolated locusts at the postulated critical period (close to the moult from the penultimate to the last hopper instar) to investigate whether the epidermis differentiates to the gregarious-type or to the solitary-type in the adult male. Since nymphs have a thin epidermis with few gland cells, it is possible that the results obtained with the crowded locusts reflect metathetelic disturbances in metamorphosis rather than a real phase shift. Investigating gland cells in the abdominal epidermis of adults and fifth-instar hoppers of Locusta, Ali (1987) also found differences between crowded and isolated locusts; this study, however, did not test the possible influences of endocrine factors. The number of ovarioles i s higher in female hatchlings obtained from eggs laid by isolated mothers in Locusta (Albrecht et al., 1958, 1959), Nomadacris (Albrecht, 1959) and Schistocerca (Papillon, 1960). Endocrine effects on this characteristic will be discussed in relation to reproduction (see Section 3.3.3). Some further differences in the morphology of gregarious and of solitary locusts, such as the armature of the hind femur and hind tibia in adults of Schistocerca and Locusta, and the number of stridulatory pegs on the fore wing of adult Locusta, were listed by Uvarov (1966, and references therein; see also Fuzeau-Braesch et al., 1973, for a later publication). The effect of endocrine factors on these differences are not known.

3.2

COLOURATION

3.2.1 General aspects, hoppers and associated hopper-adult features Acridids frequently exhibit environmentally regulated or phenotypic colour polymorphism (reviews by Uvarov, 1966; Albrecht, 1967; Rowell, 1971; Fuzeau-Braesch, 1972, 1985), also termed polychromatism (Fuzeau-Braesch,

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13

1985). Three major types of such colour polymorphism occur in this family: ( I ) “homochromy”, that is adaptation of the colour to that of the neighbouring background; ( 2 ) “green-brown’’ polymorphism; and (3) phase or densitydependent colour polymorphism. An acridid species may show none of them, or only one type, or a combination of two, or even all three types, and often there are differences between hoppers and adults of the same species. Hoppers of Locusta constitute a well-investigated example of a highly complex case. In this species, the three types of colour polymorphism are superimposed one upon another in a kind of hierarchy (Fig. 2). First-instar hoppers, originating from eggs laid by gregarious or crowded females, are dark, and crowded hoppers in the later instars are dirty orange with black patterns. In contrast, hatchlings from eggs laid by solitary or isolated females are light grey, and solitary hoppers have a uniform colour without black patterns (Faure, 1932; Gunn and Hunter-Jones, 1952; Hunter-Jones, 1958). This is the density-dependent or phase colour polymorphism and it stands at the highest level of the hierarchy; the gregarious colour excludes further kinds of colour polymorphism. Solitary hoppers exhibit the so-called “greenbrown” polymorphism; under high humidity they are uniformly green, whereas under low humidity the colour is uniform but not green, although not necessarily “brown” (Faure, 1932; Staal, 1961; Albrecht, 1964, 1965). The green colour excludes the third type of colour polymorphism, the “homochromy”, which stands at the lowest level of the hierarchy. Under low humidity, solitary hoppers adjust their colour- whitish-cream, straw yellow, beige, buff, brown, grey, black or intermediate shades-to match the colour of the underlying background in the field, or the inside colour of the cages/ containers in the laboratory (Faure, 1932; Hertz and Imms, 1937; Albrecht, 1965). Locustana also shows all three kinds of colour polymorphism; their hierarchical order and the environmental factors which affect colour polymorphism in this species seem to be similar to those in Locusta (cf. Faure, 1932). In contrast, Schistocerca (Hunter-Jones, 1958; Stower, 1959) and Nomadacris (Faure, 1932; Michelmore and Allan, 1934; Lea and Webb, 1939; Burnett, 1951) exhibit mostly phase colour polymorphism. In Schistocerca, first-instar hoppers originating from eggs laid by solitary or isolated females are rather uniformly light pale green; this colour becomes more intense, often emerald green, without black patterns, in later instars of solitary hoppers. First-instar hoppers from eggs laid by gregarious or crowded females are dark with little light areas (practically black); gregarious hoppers are creamish with black patterns in the second and third instars and bright yellow with black patterns in the later ones. Hunter-Jones (1962) observed that isolated hoppers of Schistocerca kept under high humidity are uniformly green without black patterns, whereas those kept under dry

14 isolation (PHASE COLOUR

black patterns)

-

Solttary colour (uniform colour)

-

I

factor

I

Block-

M. P. PENER

black

I

( 'GREENIBROWN'

POLYMORPHISM )

llght -Whitish

cream

BACKGROUND ( 'HOMOCHROMY')

More or less respective shades of these colours

FIG. 2 Diagrammatic presentation of the three different kinds of superimposed colour polymorphism in hoppers of Locusta rnigrutoriu. The environmental factors which exert the major effect on each type of colour polymorphism (in parentheses) are boxed. The appropriate changes in the environmental factors are underlined, whereas the actual colour(s) exhibited by the hoppers are not. Endocrine effects on colour are encircled. For further details, see text. CA, corpora allata; CC corpora cardiaca; JH, juvenile hormone; NSC, neurosecretory cells.

conditions show a wide colour variation-green, yellow-green, olive green, shades of brown, as well as other colours-and also exhibit some of the black patterns of the gregarious hoppers. Stower (1959) found brown solitary hoppers of Schistocerca with vestigial black patterns in dry field habitats. The non-green solitary hoppers in dry environments may be regarded as a "brown" form of a somewhat limited humidity-dependent green-brown polymorphism, but this consideration does not account for the vestigial black patterns. On the other hand, the reduced black patterns may reflect an incomplete phase colour change under dry conditions, but this explanation disregards the fact that the bright yellow component of the gregarious colour is entirely missing. Obviously, these two interpretations are not mutually exclusive. Hoppers of Nomadacris also exhibit a clear phase colour polymorphism and possibly a limited green-brown polymorphism; the details are rather parallel to those observed in hoppers of Schistocerca, including a tendency of retention of reduced black patterns in non-green solitary hoppers (Faure, 1932; Michelmore and Allan, 1934; Lea and Webb, 1939; Burnett, 1951). Even the relatively rigid gregarious colouration of locust hoppers is further influenced by factors unrelated to density. Low temperatures increase the black patterns, whereas high temperatures reduce them both in the field (Stower, 1959) and in the laboratory (Dudley, 1964). At intermediate

LOCUST PHASE POLYMORPHISM

15

densities great variations of intermediate colourations are observed (see for example Stower, 1959, colour plates). Both green-brown polymorphism and homochromy are subject to considerable individual variations; results of field observations or of laboratory experiments often reveal a distinct phenotypic dominance of a certain colouration, but usually some minority of the population shows deviations from this dominant colouration. Also, changes in density, or in other environmental factors, experienced by a certain hopper instar, do not necessarily result in an immediate full-scale colour change; the next instar may exhibit various intermediate colourations, and stabilization of the colour may occur only in later instar(s). Considering all these factors and effects, almost endless variations exist in the actual colouration of locust hoppers (see colour plates by Faure, 1932; Stower, 1959). All three types of phenotypic colour polymorphism in acridids are due to “morphological colour changes” (cf. Raabe, 1982, 1983; Fuzeau-Braesch, 1985), that is the visible colouration changes due to alterations in synthesis, oxidation-reduction, degradation, etc., of one or more pigments. Insect pigments or biochromes (recent reviews by Needham, 1978; Fuzeau-Braesch, 1985) are well investigated, both in relation to acridids and their colour polymorphism, including locust phase colour polymorphism (reviews by Goodwin, 1952; Nolte, 1965; Uvarov, 1966; Albrecht, 1967; Rowell, 1971; Fuzeau-Braesch, 1985). Nevertheless, as outlined by Rowell (1971) and Fuzeau-Braesch (1989, clarification of the exact relation between pigments and actual (=visible) colouration runs into considerable difficulties. According to present concepts (see reviews above), four groups of pigments make major contributions to the visible colouration of locusts. These are the melanins, ommochromes (formerly also termed insectorubin), bile pigments and carotenoids. Some other pigments, such as pterins, may also play some role (cf. Nolte, 1965; Bouthier, 1966). The green colour of solitary hoppers under high humidity is attributed to a mixture of a blue bile pigment and yellow caroteinoid(s) (Goodwin and Srisukh, 1951; Goodwin, 1952), or to a green bile pigment alone (PassamaVuillaume, 1965), all linked to proteins. The former view was accepted by most authors, including Dadd (1961) who obtained blue or greenish-blue locusts on a carotenoid-free diet, and Rothschild et al. (1977). Rowell (1971) has pointed out that the two views may be somewhat reconciled by assuming that the bile pigment (mesobiliverdin-protein complex) is sufficiently labile to change between blue and green. The black patterns of the gregarious hoppers are attributed to cuticular melanins overlying dark areas of epidermal ommochromes; these two groups of pigments show, therefore, a close spatial association in this case (Nickerson, 1956; Fuzeau-Braesch, 1965, 1966). The same pigments seem to be

16

M. P. PENER

responsible in Locusta also for the dark-blackish colour of the “homochrome” solitary insects obtained on black background, though in this instance the ommochromes may predominate and the black colour is less restricted to well-defined patterns. Interestingly, daily repeated short (1 min) treatments of crowded Locusta with CO, induces solitary characteristics, including solitary colouration and disappearance of the black patterns, but these “solitarized” locusts are as able to exhibit black homochromy as normal isolated ones (Fuzeau-Braesch and Nicolas, 198I ; and references therein). The situation is unclear for the pigments responsible for the yellow or orange component of the gregarious hcppers on the one hand, and for the “brown” colour shades of the solitary “homochrome” hoppers on the other. Various authors have advocated a definite role of one or more pigments in different or even in the same species, and if all the relevant publications are considered, practically all the major groups of pigments have been claimed to play a role. For details and discussion of the often controversial findings and conclusions, the reader is referred to Rowell’s (197 1) comprehensive review. The effects of endocrine factors on locust colouration have usually been investigated only at the level of visible colours, leaving the effects on the pigments open to speculations. Such speculations are frequent in the literature, but their value is very limited due to the lack of firm knowledge on the exact relationships between visible colours and pigments (see above). Experimental work on the hormonal effects on the pigments is much needed; meanwhile, however, a meaningful discussion is necessarily restricted to visible colouration. Implantation of extra CA, or administration of JH or JH analogues, to crowded hoppers induce the green solitary colour (Fig. 2). This effect was first demonstrated in Locusta (P. Joly and L. Joly, 1954; L. Joly, 1954; P. Joly et al., 1956) and was amply reconfirmed in both Locusta (L. Joly, 1960: Staal, 1961; P. Joly, 1962: P. Joly and Meyer, 1970; NEmec et at., 1970; FuzeauBraesch, 1971; Roussel, 1975d, 1976a,b; Couillaud et al., 1987) and Schistocerca (Novak and Ellis, 1967; Roussel and Perron, 1974; Mordue (Luntz), 1977). Even green isolated hoppers became greener after implantation of extra CA (Ellis and Novak, 1971), and injection of JH to isolated non-green (homochrome) hoppers of Locusta also induced a green colour (Nicolas, 1977). Surgical allatectomy of green isolated hoppers led to high mortality, but available results indicated loss of the green colour after such an operation (L. Joly, 1960; Staal, 1961). Employing precocene-induced chemical allatectomy (Pener et al., 1978), these results were reconfirmed by M. P. Pener and J. De Wilde (unpublished); chemical allatectomy of green isolated Locusta hoppers led to the disappearance of the green colour. In these experiments, however, the resulting colouration was similar to that of the “brown” homochrome hoppers, and so markedly different from the gregarious

LOCUST PHASE POLYMORPHISM

17

colouration! Thus, although the relationship between JH and green colour seems to be absolute, endocrine manipulations involving CA/JH in isolated Locusta hoppers shifts one solitary characteristic to another, from green to homochrome after chemical allatectomy as in the experiments of M. P. Pener and J. De Wilde (unpublished), or from homochrome to green as in Nicolas’ (1977) study. The green-colour promoting effect of experimentally induced JH surplus in crowded Locusta is correlated with a marked regression, up to complete disappearance, of the black patterns after the next moult(s). However, the mechanism underlying the black-pattern reducing effect of JH is unknown; it may be direct, or mediated through a negative interaction with the blackcolour inducing neurosecretory factor (see below), and/or the green colour may exert a negative feedback at the level of the epidermis on the production of the pigments responsible for the black colour. It must be emphasized that the green-colour inducing effect of CA/JH is not confined to locusts and phase polymorphism. Many acridids from different subfamilies exhibit green-brown polymorphism without any density-dependent colour polymorphism (cf. Uvarov, 1966; Rowell, 1971) and JH promotes the green colour also in these cases. Implantation of extra CA into hoppers of Acrida turrita (Acridinae) induced green colour (P. Joly, 1952) and similar treatment led to similar results (Rowell, 1967) in Humbe tenuicornis, Gustrimargus africanus (both Oedipodinae) and Acanthacris rujicornis (Cyrtacanthacridinae). Green adults were even obtained in A . ruficornis, despite the fact that such adult colouration has not been observed in natural populations. Moreover, JH also promotes green colour in nonacridid insects, especially in larvae of Lepidoptera (review by Raabe, 1983; for a later work see Fescemyer and Hammond, 1988). Bioassays for J H were even based on the green-colour inducing effect of the hormone in larvae of Manduca sexta (Truman et a[., 1973; Fain and Riddiford, 1975), and a black larval mutant of this species was shown to be caused by a reduction in J H titres, due to a reduced CA activity (Safranek and Riddiford, 1975; Kramer and Kalish, 1984). The existence of an endocrine factor which promotes the black colour in locusts was first claimed by Nickerson (1954, 1956), who injected haemolymph of crowded Schistocerca hoppers into isolated ones and obtained an increase of the gregarious black patterns. Injection of haemolymph from isolated to crowded locusts had no effect on the colour. Staal(l961) observed an increase of the black patterns after implantation of extra corpora cardiaca (CC) into hoppers of Locusta, noting that the relevant factor from the CC may be identical with the haemolymph factor found by Nickerson. Staal’s findings were confirmed and the factor from the CC was traced back to some neurosecretory cells (NSC) in the protocerebrum (Girardie and Cazal, 1965;

18

M. P. PENER

Girardie, 1974; Bouthier, 1976). However, this NSC-CC factor, or a factor of similar origin (Fig. 2), also promotes black colour in isolated Locusta (Nicolas and Ismai‘l, 1978), and thus it may be involved in the control of the black homochrome response. Like the effect of JH on green colour, the black-colour promoting effect of this NSC-CC factor does not seem to be restricted to locusts or phase polymorphism. A factor from the same organs is responsible for the black homochrome response of the grasshopper Oedipoda coerulescens (MoreteauLevita, 1972a,b; Moreteau, 1975), which exhibits marked homochromy but neither green-brown nor phase colour polymorphism. A neurohormonal factor also promotes dark or black colour in some non-acridid insects, but the site(s) of synthesis and/or release of this factor depends on the species (Raabe, 1982, 1983, and references therein), and there is no clear evidence that this factor is identical in all species. Bursicon, itself a neurohormone, also affects darkening. Padgham (1976b) reported that bursicon induces “melanization” (darkening) and sclerotization in crowded Schistocerca hatchlings; the hormone is also present in pale hatchlings, but in these it only induces sclerotization. These findings indicate that the integument of the dark and pale hatchlings reacts differently to the same hormone. According to Vincent (1972) bursicon is present in the brain, CC and ventral nerve cord of Locusta, but it is released from the last abdominal ganglion. In contrast, Padgham (1976a) claims that in Schistocerca hatchlings a melanizing factor, presumably bursicon, is released from neu.rosecretory axon terminals in fine nerves posterior to the metathoracic ganglion, possibly due to some signal originating from the prothoracic ganglion. Clearly, more work is needed on bursicon production and release, on its role in darkening, as well as on its relation to the NSC-CC darkening factor in locusts. Partial extirpation of the ventral glands (VG), equivalent in acridids to the prothoracic glands (PG) of other insects, was claimed to induce gregarious colouration in isolated Schistocerca (Ellis and Carlisle, 1961; Carlisle and Ellis, 1962). In contrast, following partial extirpation of the VG, or after implantation of extra VG, Staal (1 961) observed no appreciable colour changes in Locusta. Since the VG (or PG) secrete ecdysone (or immediate precursors which are rapidly converted to ecdysone; cf. Warren et al., 1988), the fact that ecdysteroid titres do not differ markedly between isolated and crowded locusts (see Section 4.2) seems to agree with Staal’s findings. 3.2.2 Adults Green solitary adults, usually obtained from green hoppers, are well known in Locusta and Locustana (Faure, 1932). After the first few days following fledging Locusta adults are no longer able to become green (Albrecht, 1965),

LOCUST PHASE POLYMORPHISM

19

but green adults may show fading of this colour under dry conditions (Albrecht, 1965), or after being transferred to a crowd of adults (Pener, 1976b). Homochromy is more restricted in solitary adults of Locusra than in hoppers, but the black homochrome response of isolated adults is still marked (see colour plates by Fuzeau-Braesch, 1965, 1985). Solitary adults of Schistocerca and Nomadacris are not green, even if they are obtained from uniformly green hoppers, and adults of these species d o not show homochromy. Gregarious or crowded locusts exhibit a quite consistent course of colour changes during adult life strongly associated with sexual maturation. Solitary or isolated adults do not show such changes, or, as in Locusta, the colour change is limited to the hind wings. Crowded fledglings of Schistocerca are pink, but after a few days the colour turns to pinkish-beige, then to beige or brown. Eventually females become yellowish and males bright yellow after the onset of full sexual maturation (Chauvin, 1941; Norris, 1954; Pener, 1967a). Somewhat similar courses of colour change also occur in gregarious or crowded adults of Nomadacris (Michelmore and Allan, 1934), Locustana (Faure, 1932) and Locusta. In Locusta, the development of the bright yellow coiour over the body is restricted to crowded males (for a detailed system scoring the yellow colour see Pener et al., 1972); crowded females show yellowing only on the hind wings. Isolated Locusta do not become yellow over the body, but both sexes show marked yellowing of the hind wings (Pener, 1976b). In all cases the yellow colour coincides with sexual maturation. Beside density, temperature also affects yellowing being more intense at higher temperatures. The yellowing of crowded adult locusts depends completely on the CA and the J H they produce. Allatectomy of last-instar hoppers or of young adults prevents yellowing, whereas reimplantation of CA or administration of J H reinduces it in Schistocerca (Loher, 1961; Pener, 1965b, 1967a,b; Odhiambo, 1966a; Amerasinghe, 1978b, Pener and Lazarovic;.i,1979), Locusta (Girardie, 1966; Girardie and Vogel, 1966; Pener et al., 1972; Pener, 1976b) and Nomadacris (Pener, 1968). Allatectomy of sexually mature yellow adults results in the fading of the yellow colour (Pener, 1965b). The yellowing restricted to the hind wings of isolated Locusta adults also depends on JH; after allatectomy it does not take place (Pener, 1976b). However, despite this absolute relationship between CA, J H and yellowing, JH is not the primary factor responsible for the differences in the yellow colour between gregarious and solitary adults. Isolated adults may have higher JH titres than crowded ones (see Section 4.1), nevertheless they d o not become yellow. Moreover, implantation of extra CA into isolated adults does not induce yellowing, whereas transfer of isolated adults into newly formed crowds does, even without implantation of extra CA (Pener, 1976b). Thus, J H is necessary but

20

M. P. PENER

not sufficient for yellowing to occur, and the epidermis and/or the relevant pigment system(s) react differentially to J H in crowded and isolated adults. Goodwin (1952, and references therein) related the colour of crowded fledglings, pink in Schistocerca and grey-brown in Locusta, to ommochromes (under the name insectorubin) and claimed that the yellow colour of the crowded adults is due to accumulation of carotenoids, especially p-carotene, in the integument. This claim has been widely cited as an established fact (Nolte, 1965; Uvarov, 1966; Fuzeau-Braesch, 1985), but the situation may be more complicated. Allatectomy of crowded adult Schistocerca males affected ommochrome and purine content of the integument (Ballan-Dufrangais, 1978). Moreover, unpublished results (J. Gross, M. P. Pener and M. Rothschild) showed that p-carotene content in the integument of allatectomized non-yellow males increases similarly to that in normal yellow males of crowded Schistocerca. Again, more experimental work is needed to clarify the role of pigments underlying the visible yellow colour of gregarious adult locusts and to reveal the role of J H at the level of the pigments. 3.2.3 Some conclusions

The available experimental evidence clearly demonstrates that endocrine factors do affect several major components of locust colouration. Nevertheless, none of these factors seem to play a physiologically primary causal role in locust phase colour polymorphism. By inducing yellow colour in crowded adults, JH promotes a gregarious characteristic, but the competence of the adult’s integument to become yellow is governed by some density-dependent unknown intrinsic factor(s). The lack of response of isolated adults to the yellow-colour inducing effect of the JH cannot be explained by the assumption that the relevant competence of the adult integument was already fixed by endocrine events during hopper development, because simple transfer of isolated adults to a crowd changes the competence and leads to yellowing. By inducing green colour in hoppers and fledglings, J H promotes solitary characteristics, but the effect is not restricted to locusts since it exists also in other acridids which show “green-brown” polymorphism without any density-dependent phase colour polymorphism. Moreover, the very existence of the “brown” homochrome forms of solitary locusts demonstrates that the green colour is not a necessary characteristic of the solitary phase. Relevantly, promotion of this “brown” solitary colour by JH has never been observed (cf. Pener, 1976b). The black-colour promoting NSC-CC factor again may not be confined to locusts, because it seems to play a role also in the black homochrome response of grasshoppers which show no phase colour polymorphism.

LOCUST PHASE POLYMORPHISM

21

Phase colour polymorphism is much less frequent in acridids than “greenbrown” polymorphism or homochromy, and each major locust species is taxonomically closely related to many acridids which exhibit the latter types. It seems, therefore, that some not necessarily simple mechanisms, which control “green-brown” polymorphism by CA activity and JH titres, and black colour by the NSC-CC factor in grasshoppers, were “co-apted” during evolution to play partial roles in a more complex phase colour polymorphism. The increasing JH titres in the adults, primarily related to reproduction, might have also been so “co-apted” for inducing yellow colouration with sexual maturation in gregarious locusts. Although the phase-dependent adaptive advantage of this yellow colour is not clear, again many non-locust acridids show adult colour changes in relation to sexual maturation (see Uvarov, 1966). It must be emphasized, however, that no coherent picture accounting for all aspects of locust phase colour polymorphism emerges even by addition of all these partial mechanisms.

3.3

REPRODUCTION

3.3.1 Maturation and maturation-accelerating pheromone The period elapsing between fledging and sexual maturation is shorter in isolated than in crowded Locusta (Norris, 1950; Pener, 1976a), but the situation is reversed in Schistocerca (Norris, 1952; Papillon, 1968). Norris (1964) related this difference to a differential balance of two counteracting, possibly pheromonal, effects. In both species, presence of mature males accelerates the maturation of young adults, but that of young males retards maturation. In Locusta, the inhibitory effect is more prominent and/or longer lasting, whereas in Schistocerca the activatory effect dominates. Under crowding, therefore, change-over from inhibition to acceleration occurs relatively later in Locusta than in Schistocerca. In both species, however, these counteracting effects result in a more or less synchronous sexual maturation of the crowded locusts. This synchrony may be considered as an advantage for gregarious adults because, to be adapted for living in swarms, their activities, for example migratory flight or reproduction, should be synchronized. The maturation-accelerating effect exerted by mature males on young adults in Schistocerca (Norris, 1954) was shown to be due to a CA-dependent production of a maturation-accelerating pheromone (Loher, 1961; Amerasinghe, 1978’0); in fact, the presence of allatectomized males, or of allatectomized females, inhibits the maturation of young adult males (Norris and Pener, 1965). From histological and ultrastructural studies on the number of

22

M. P. PENER

gland cells in the epidermis (see also Section 3.1) and the state of activity of these cells, Cassier and Delorme-Joulie (1 976) and Cassier (1977) concluded that isolated adult Schistocerca males do not produce maturation-accelerating pheromone. In contrast, Amerasinghe (1978a) observed that sexually mature non-yellow isolated males and yellow crowded males (see Section 3.2.2) equally accelerate the maturation of young males and concluded that the isolated males also produce maturation-accelerating pheromone. However, the experimental findings on which these controversial conclusions are based do not seem to exclude the possibility that crowded adult males may produce more pheromone than isolated ones, and so the difference in pheromone production is possibly quantitative. In any case, under complete isolation the pheromone cannot exert an actual effect on other locusts. 3.3.2

Male sexual hehaviour

A further reproduction-related phase characteristic is that crowded Locusta males exhibit markedly more intense mating behaviour than isolated ones (Pener, 1976a). Male mating behaviour is known to be affected by the CA in some acridids (review by Pener, 1986), but in Locusta the effect is phase dependent. Allatectomy drastically reduces the intensity of this behaviour in crowded males, but has only minor effects in isolated ones (Pener, 1976a). In this instance, therefore, by elevating the intensity of the sexual behaviour in crowded males, the CA (or JH) promotes a gregarious phase characteristic. However, as in the case of induction of yellow colour in adult males (see Section 3.2.2), J H does not constitute a primary physiological causal factor for the phase-related differences in the intensity of male sexual behaviour. Implantation of extra CA into isolated males does not affect the eventual intensity of this behaviour and so does not elevate it to the level shown by crowded males (Pener, 1976a). Thus, the target organs (presumably the nervous system in this case) again seem to respond differentially to JH in crowded and isolated Locusta.

3.3.3 Female reproductive parameters and maternal effects on hatchlings The reproductive potential or fecundity of isolated females is higher than that of crowded ones in Locusta (Norris, 1950; Albrecht et al., 1958), Nomadacris (Albrecht, 1959; Norris, 1959) and Schistocerca (Papillon, 1960, 1970). This difference is primarily because parental density markedly affects the number of ovarioles developing in the embryo; in female hatchlings originating from eggs laid by isolated females of Locusta (Albrecht et al., 1958, 1959), Nomadacris (Albrecht, 1959) and Schistocerca (Papillon, 1960), this number is higher than in those originating from eggs of conspecific

LOCUST PHASE POLYMORPHISM

23

crowded females. The effect of the density is cumulative; the number of ovarioles is highest in locusts kept under isolation for several consecutive generations (Albrecht et al., 1958; Albrecht, 1959; Injeyan and Tobe, 1981a). For example, Injeyan and Tobe (1981a) reported that the average number of ovarioles was 110 in crowded females of Schistocerca, and it increased to 130 and 154 in the second and fourth generation of isolated locusts, respectively. However, despite the differences in the number of ovarioles, the average weight of an egg pod (Albrecht et al., 1958) and the average vitellin content per ovary (Injeyan and Tobe, 1981a) are about equal in crowded and isolated females, because the eggs of the latter are smaller and lighter. Thus, the number of eggs per pod laid by isolated females is higher at the expense of egg size and vitellin content per egg. Consequently, hatchlings from eggs laid by isolated females are smaller and morphometrically different (see also Section 3.1) from those originating from crowded females (Lauga, 1974, 1976b). The colour differences between isolated and crowded hatchlings (see Section 3.2.1) are also related to maternal density and/or to the differences in egg (and hatchling) size (Hunter-Jones, 1958; Papillon, 1960, 1968). The number of ovarioles constitutes a theoretical upper limit for the number of eggs in an egg pod, but in practice the latter is markedly lower because usually a considerable portion of the oocytes are resorbed before completing development. The proportion of such resorbed oocytes is higher in crowded than in isolated females; thus, the actual number of eggs per egg pod in crowded females in even less than that expected due to their fewer number of ovarioles (Injeyan and Tobe, 1981a). The rate of egg laying (number of egg pods produced per female per week), the total number of pods laid during the life span of a female, and proportion of viable eggs (per cent of eggs which hatch) are also distinctly lower in crowded than in isolated Locusta (Norris, 1950; Albrecht et al., 1958). Similar differences between crowded and isolated females also exist in other locusts, although for some of these reproductive parameters they may be less extreme (Schistocerca: Norris, 1952; Papillon, 1970; Nomadacris: Albrecht, 1959). Although the differences in the reproductive potential between crowded and isolated locusts are well demonstrated, certain factors, related more to the experimental methods than to real phase characteristics, may contribute to these differences. Isolated locusts are handled individually in laboratory cultures and they experience no competition for food or for egg laying space. It is known, for example, that as the number of egg pods laid into the same oviposition vessel in the laboratory increases, the viability of the eggs decreases (Chamberlain, 1980). Also, as isolated locusts are usually kept in small containers, they may spend less time and energy in searching for oviposition vessels. These presumably better conditions may reduce the number of oocytes resorbed, and increase the rate of egg laying, as well as the

24

M. P. PENER

proportion of viable eggs in the isolated locusts. Owing to these better conditions and/or to real phase differences, the life span of isolated adults is longer than that of crowded ones (Norris, 1952, 1959; Pener, 1976a; for a contradicting result see Norris, 1950; and for the experimental difficulties in comparing the life span of isolated and crowded locusts see Albrecht, 1967, p. 60). The longer life span of isolated adults may account for the higher total number of pods produced. On the other hand, it seems unlikely that such handling-dependent factors play a causal role in the difference of the ovariole-number in the progeny. Oocyte development strictly depends on the CA and JH in locusts (L. Joly, 1960; Highnam et al., 1963; Pener, 1965b, 1967a; Strong, 1965; Girardie, 1966; Roussel, 1975d, 1976a,b; Lazarovici and Pener, 1977), as well as in other acridids (see table IV-2- 1 by Engelmann, 1983). Allatectomy of sexually mature females results in rapid resorption of the developing oocytes (Pener, 1965b, 1967a). JH induces vitellogenin production in the fat body (Chen et al., 1976, 1979; Abu-Hakima, 1981; Wyatt et al., 1987) and mediates vitellogenin uptake by the developing oocytes (Ferenz et al., 1981). Thus, J H is necessary to induce and maintain the female’s reproduction implying that more active CA and/or higher JH titres are responsible for the higher fecundity of isolated females. In regard to fecundity, therefore, JH may promote solitary characteristics. The relevant experimental findings should be examined separately for the so-called ”immediate” and “transmitted” effects. The former term covers effects on the adult females, whereas the latter means effects becoming overt in the hatchlings, that is effects “transmitted” to the progeny. Considering the “immediate” effects, implantations of two pairs of active extra CA into crowded adult Locusta females led to increased fecundity; the number of eggs per pod, rate of egg laying and total number of pods laid were higher than in the controls (Cassier, 1965a). However, unilateral allatectomy, or unilateral allatectomy coupled with severance of the allatal nerve of the remaining gland, in isolated Locusta females reduced these parameters (Cassier, 1966a,b). Thus, the trends observed after implantation of extra CA were similar to those induced by isolation, whereas surgical treatments, presumably reducing JH titres, led to effects similar to those caused by crowding. However, this parallel may be coincidental and the results of the endocrine manipulations may be related to the vitellogeneticgonadotropic effects of the CA without any phase shift. The life span is longer in isolated than in crowded adults (see above and also Cassier, 1965a), but implantation of extra CA, claimed to induce solitary characteristics, also shortened the life span of the adult females (Cassier, 1965a). Implantations of three pairs of CA into isolated Locusta females was reported to decrease the number of eggs per pod, though implantation of 15 pairs led to an increase

LOCUST PHASE POLYMORPHISM

25

(Albrecht and Cassier, 1964). Only the latter finding was included in Cassier’s (1 970) summary paper. Implantation of extra CA into crowded adult females of Schistocerca reduced the life span and some of the reproductive parameters, but it increased the number of eggs per pod (Cassier and Papillon, 1968). According to Injeyan et al. (1981), high J H titres in both crowded and isolated Schistocerca females may reduce the viability of the eggs and so lower the proportion of those which hatch. Furthermore, Albrecht et al. (1958) found that the relationship between density and reproductive potential is not simple in Locusta. Females crowded as hoppers then isolated as adults produced more eggs (c. 1500 eggs per female) than those kept continuously under isolation (c. 1000 eggs), whereas females isolated as hoppers then crowded as adults produced even fewer eggs (c. 150) than continuously crowded ones (c. 300 eggs). In the light of these findings, phasedependent “immediate” differences in female fecundity may have a more complex causal relation than a simple effect exerted solely by adult density on adult CA activity. Although CA activity and J H are undoubtedly necessary for oocyte development in locusts and other acridids (see above), the exact relationships between these endocrine factors and the quantitative aspects of various reproductive parameters are not fully understood. Injecting exogenous J H into allatectomized females of Locusta, LazaroviGi and Pener (1977) observed that with increasing doses the rate of oviposition increased, but the number of eggs per pod was unaffected. Couillaud et al. (1984) found that pre-severance of the allatal nerves (NCAI, or NCAI and 11) drastically reduced the JH biosynthetic activity of the CA in crowded Locusta, but had no effect on the rate of growth of the first generation of oocytes. These authors concluded that the reduced amount of JH produced by the denervated CA is sufficient for normal vitellogenic growth of the oocytes, but surprisingly in subsequent work (Couillaud et al., 1985) the lower activity of the denervated CA was found to be correlated with a higher JH titre in the operated females. The latter finding throws doubts on the interpretation of Cassier’s (1 966a,b) results; Cassier assumed that unilateral allatectomy and unilateral allatectomy coupled with severance of the allatal nerve reduce J H titres. Couillaud et al. (1987) reconfirmed the lower activity of the CA after pre-severance of the NCAI; despite the very low activity, such glands induced considerable vitellogenic oocyte growth after being implanted into allatectomized females, though the rate of this growth was somewhat lower than that obtained after implantation of “control” (without pre-severance of the NCAI) C k . Assessing JH biosynthetic activity of the CA of isolated and crowded Schistocerca females, Injeyan and Tobe (1981b) found a substantially higher glacd activity during the first half of the first gonotrophic cycle and a corresponding earlier appearance of vitellogenin in isolated females;

26

M. P. PENER

nevertheless, the completion of the cycle was delayed in these females in comparison with crowded ones. Among several feasible explanations of these results is the possibility that density may alter the responsiveness of the target tissues (fat body and/or oocytes in this case) to the J H (Injeyan and Tobe, 1981b). In conclusion, regarding the “immediate” effects of the CA/JH on reproductive parameters, the evidence is not clear enough conclusively that a higher activity of the CA and/or an increased J H titre promote the relevant solitary phase characteristics. Regarding the “transmitted” effects, Cassier (1965a) found that implantation of extra CA into crowded adult females of Locusta led to lower weights, a higher proportion of light coloured individuals and a higher number of ovarioles in hatchling progeny, all solitary characteristics (see above). Unilateral allatectomy of isolated mothers resulted in an increased weight of the hatchlings (Cassier, 1966a), and unilateral allatectomy combined with severance of the allatal nerve yielded similar results and also induced darkening in some hatchlings (Cassier, 1966b). Implantation of extra CA into crowded Locusta mothers shifted the morphometrics of the hatchlings to approach those of hatchlings obtained from eggs laid by isolated females (Lauga, 1977b). Similar implantations also increased the number of ovarioles in female hatchlings, though this increase was much smaller than that induced by isolation of the mothers (Lauga, 1976b). Implantation of extra CA into crowded mothers of Schistocerca resulted in a decrease of weight and in a shift toward a green hatchling colour (Cassier and Papillon, 1968). On the other hand, Injeyan et al. (1979) reported that exogenous J H treatment of the eggs obtained from crowded Schistocerca females caused disturbances in embryogenesis and hatching, but did not induce solitary characteristics in the hatchlings. Although the evidence that the CA and J H promote solitary characteristics seems firmer for the “transmitted” than for the “immediate” effects, the complex relationship between CA activity and haemolymph J H titres after surgical manipulations (cf. Couillaud et al., 1985) prevents clear conclusions being drawn. Also, as already outlined by Cassier (1965a) and Lauga (1976b), differences in CA activity and/or J H titres may exert effects on other endocrine organs and these may also affect phase characteristics.

3.4

HOPPER DEVELOPMENT

In Schistocerca and Nomadacris, as well as in some non-locust acridids, a stripe is present in the compound eye of the first-instar hoppers and at each consecutive moult an additional stripe appears at the anterior margin of the

LOCUST PHASE POLYMORPHISM

27

eye, while the earlier stripe(s) move(s) backwards (see Uvarov, 1966; and references therein). Thus, the number of eye stripes in an adult (usually six or seven in Schistocerca and seven or eight in Nomadacris) is equal to the number of instars, including the adult instar itself, in the life history of that adult. Actual observations on the number of moults (mostly in the laboratory), or just counts of the number of eye stripes (mostly in field populations), revealed that in isolated laboratory breedings, or in solitary field populations, Schistocerca and Nomadacris tend to add an “extra” nymphal instar, thus they usually moult once more than conspecific crowded or gregarious locusts (Burnett, 1951; Albrecht, 1955; and references therein). Albrecht (1955) concluded that in these two species, the density of the parents by affecting the weight and size of the hatchlings determined the number of nymphal instars in the progeny. Hatchlings originating from eggs laid by isolated mothers are smaller (see Section 3.3.3), and subsequently they undergo an extra nymphal instar. Although the correlation between hatchling size and the extra instar is clear, some findings of Injeyan and Tobe (198la) indicate that the hopper density experienced after hatching may also affect the number of instars. Isolating hatchlings obtained from eggs laid by crowded Schistocerca mothers, these authors observed that about 25% of the nymphs underwent an extra moult, that is an extra instar. In consecutive generations of isolated Schistocerca, the proportion of locusts exhibiting the extra nymphal instar increased to c. 90%, but failed to reach 100% within six generations. Isolated females showed the extra nymphal stage more frequently than did isolated males (Injeyan and Tobe, 1981a). Moulting and metamorphosis are controlled by hormonal factors and the difference in the number of instars between solitary and gregarious locusts may reflect a somewhat different programming of related endocrine events. Owing to the extra instar, the metamorphosis of the solitary locusts may be regarded as retarded and the state of the solitary adult as somewhat neotenous; these considerations led to the assumption that higher JH titres may be responsible for the extra nymphal instar in solitary locusts (Kennedy, 1956). However, there is no solid experimental evidence to support this theory. Although JH biosynthetic activity in the CA was found to be somewhat higher in isolated than in crowded penultimate and last-instar hoppers of Schistocerca (Injeyan and Tobe, 1981b), this fact itself does not demonstrate a causal role of the JH in the induction of the extra nymphal instar. In many acridids that do not show phase polymorphism, the number of instars is subject to individual variations. In some instances the larger females have one instar more than the smaller males, and in phylogenetically more advanced groups and/or in groups having a smaller size the number of nymphal instars is fewer (cf. Uvarov, 1966, pp. 286289). However, there is no evidence which relates these differences to the overall differences in CA

28

M. P. P E N E R

activity or in JH titres. Furthermore, the case of Locusta indicates that there may be no relationship between a higher J H titre and an extra nymphal instar, or if such relationship exists, it cannot be easily reconciled with the green-colour inducing effect of the J H (see Section 3.2.1). Both crowded and isolated Locusta undergo five nymphal instars. However, an extra nymphal instar can be obtained by isolating both mothers and offspring and keeping the offspring under low humidity; neither isolated hoppers originating from crowded mothers (even kept under low humidity) nor isolated hoppers kept under high humidity (even if they originate from isolated mothers) exhibit this extra instar frequently (Albrecht, 1965). But because of the low humidity experienced by the offspring, the hoppers which undergo the extra instar are all “brown”; high humidity, which induces green colour in isolated hoppers (see Section 3.2. l), actually prevents (!) the extra instar. The assumption that a higher J H titre is responsible for the extra instar in these “brown” isolated hoppers seems to contradict the green-colour promoting effect of JH. Since the latter is amply demonstrated (see Section 3.2. l), one has to assume either that there is no causal relation between the higher JH titre and the extra instar, or that environmental humidity alters the response of the integument to J H (cf. discussion by Pener, 1976b) so drastically that the higher titre which induces the extra moult is insufficient to induce green colour under low humidity, whereas the lower titre which prevents the extra moult under high humidity is sufficient to d o so.

3.5

PHYSIOLOGY, BIOCHEMISTRY AND MOLECULAR BIOLOGY

The rate of oxygen consumption is higher in crowded than in isolated Locusta (Butler and Innes, 1936; Blackith and Howden, 1961; Roussel, 1963a) and Schistocerca (Gardiner, 1958; Pener, 1965a), though in firstinstar hoppers this difference is not always clear. Chauvin (1941) found that the amplitude of the respiratory movements was higher in crowded than in isolated Schistocerca hoppers. Allatectomy reduced 0, consumption in crowded adult females of Locusta independently of ovarian development (Roussel, 1963b), thus the effect of allatectomy was somewhat parallel to that of isolation. Implantation of extra CA into crowded hoppers of Locusta did not alter the rate of 0, consumption, in spite of inducing a green solitary colour (Roussel, 1963a). Thus, for 0, consumption, either the CA promote a gregarious characteristic, or at least phase-dependent differences in 0, consumption are not affected by the CA. The life span of isolated adults is longer than that of crowded ones (see Section 3.3.3). Allatectomy drastically increases the longevity of adult locusts (Wajc and Pener, 1969; Pener, 1972). This effect of allatectomy is clear in

LOCUST PHASE POLYMORPHISM

29

both isolated and crowded Locusta adults, whereas implantation of extra CA results in a slight decrease of the adult life span under both conditions of density (Pener, 1976a). In this case, therefore, J H deficiency (allatectomy) seems to promote a solitary characteristic (longer life span). However, although both allatectomy and isolation shift the life span in the same direction, the operation induces a much larger shift than the change in density (cf. Pener, 1976a). In the last two hopper instars and in the adult, heartbeat frequency is higher in isolated than in crowded Locustu (Roussel, 1972a,b, 1973, 1975a; Queinnec, 1973). Allatectomy decreased heartbeat frequency under both conditions of density, but allatectomized isolated locusts exhibited a higher rate than allatectomized crowded ones; in fact, the relative difference was even more marked in allatectomized than in normal locusts, because the decrease induced by allatectomy was smaller in isolated insects (Roussel, 1975b,c). Thus, although CA and J H affect heartbeat frequency, they do not seem to be responsible for the relevant phase difference, and Roussel(1975c) concluded that the target organ, presumably the heart itself, of isolated and crowded locusts may respond differentially to JH. Investigating some biochemical parameters in crowded and isolated fifthinstar hoppers of Locusta and Locustana, Matthee (1945) found no significant density-dependent differences in lactic acid content per wet body weight, but observed a higher uric acid content of the haemolymph in isolated hoppers of both species. The p H of the haemolymph, measured only in Locusta, did not differ significantly between isolated and crowded hoppers. However, the fat content of the body was significantly higher in crowded hoppers of both species. Blackith and Howden (1961) also found a higher fat content in hatchlings originating from eggs laid by crowded females of Locusta, Schistocerca and Nomaducris than in those originating from conspecific isolated females. These results on the fat content may be related by some circumstantial evidence to the CA; allatectomy leads to an increase of fat content in adult male locusts (Odhiambo, 1966b; Strong, 1968), thus the effects of crowding and allatectomy-induced JH-deficiency on fat content are somewhat parallel. However, this parallel may be coincidental because gregarious locusts are more active than solitary ones, but allatectomy seems to reduce locomotor activity (see Section 3.7.2) and the effect of allatectomy on fat content may be explained by an accumulation of non-utilized fat. No data are available on the effect(s) of CA on fat content in isolated locusts. Marty et al. (1972) found some differences in haemolymph proteins between crowded and isolated albino mutants of Locustu. Nolte (1977) recorded that the cyclic AMP content of the testis is lower in isolated than in crowded fifth-instar hoppers and young adults of Locusta.

30

M. P. P E N E R

Genin et al. (1986) have shown that there are differences in cuticular hydrocarbons between crowded and isolated Locusta. In a subsequent paper (Genin et al., 1987), phase-dependent differences were also found in the aliphatic ethers of the cuticular waxes in the same species. Possible endocrine effects on these differences, however, have not been investigated. Recently, Colgan (1987) has studied developmental changes of isoenzymes, mostly associated with glycolysis, in Locusta. Offspring of crowded mothers were either isolated, or placed in a small crowd, less than an hour after hatching, and the effect of hopper density was investigated on six enzymes. In spite of limited developmental stages (first two nymphal instars) and a limited period (only 14 days) during which the hoppers experienced the different densities in these experiments, there were two major findings: (1) the aldolase phenotype found in hatchlings and in crowded hoppers was replaced by a novel isoenzyme in the isolated hoppers; and (2) the levels of two glycerol-3-phosphate dehydrogenase isoenzymes were higher in the isolated than in the crowded hoppers. Feeding of young crowded hoppers on JH1 solution, or injection of JH1, failed to induce the appearance of the novel aldolase isoenzyme which was found to be induced by isolation. These experiments (Colgan, 1987) are the first efforts to investigate phase polymorphism at a molecular level, presumabIy reflecting changes in the underlying gene expression.

3.6

CYTOLOGY

One or two supernumerary chromosomes, termed B chromosomes, are often present in Locusta (Lespinasse, 1973, 1977). The proportion of locusts with such B chromosomes is different in different geographical strains of the species and is subject to selection within the same strain (Lespinasse and Nicolas, 1975, 1981). However, daily repeated 1-min exposure to CO, promotes solitary characteristics in crowded Locusta (review by FuzeauBraesch and Nicolas, 1981). Correlating these phenomena, Lespinasse and Nicolas (1975, 1981) found that the higher the B chromosome frequency in different geographical strains or in selected laboratory populations of the same strain, the more intense was the “solitarizing” effect of the CO,. These authors suggested that races or populations with higher B chromosome frequency have a greater flexibility for phase change. Nolte (1964a,b, 1967, 1968, 1974, and some of his other publications) reported that in Locusta, Locustana and Schistocerca chiasma frequency is higher in gregarious than in solitary field populations; in the laboratory crowding increases, whereas isolation decreases, chiasma frequency. However, Dearn (1974a,b) found no evidence that there is any relationship

LOCUST PHASE POLYMORPHISM

31

between phase status, or density, and chiasma frequency in Schistocerca and Locusta and strongly criticized Nolte’s methods and findings. This debate is still open, but it may be added that Nolte (1967, 1968) regarded albino locusts as an extreme solitary phase, because they d o not have gregarious black patterns (see Section 3.2.1) and show low chiasma frequency largely unaffected by density. Although chiasma frequency is indeed low in albino strains of Schistocerca and Locusta (Dearn, 1977) and obviously (by the definition of the term “albino”) they do not have black patterns even when crowded, these albinos do exhibit density-dependent changes in other phase characteristics (Pener, 1965a; Dearn, 1977) and so they cannot be considered as extreme solitary locusts unresponsive to density. Except for a claim by Nolte (1968) that injection of haemolymph from crowded to isolated hoppers increased chiasma frequency, nothing is known about possible endocrine effects on cytological phase characteristics, nor about relations between such phase characteristics and endocrine events.

3.7 3.7.1

BEHAVIOUR AND ACTIVITY

Hoppers

When locust populations reach high densities in the field, the hoppers form large groups, termed “bands”. The individual hoppers within the band exhibit more or less synchronous activities and they show positive reactions to one another, that is they actively aggregate (Ellis and Ashall, 1957; review by Uvarov, 1977). In the laboratory, experimental parameters reflecting the tendency to aggregate, such as proportion of locusts forming groups or the per cent of time spent in groups, were much higher for hoppers which had been kept under crowded conditions than for those which had been maintained under isolation (Ellis, 1959, 1962a,b; and references therein); in the latter case, the values of these parameters did not exceed those expected by random distribution. It was also discovered that group formation strongly depends on an habituation to “being touched”, which is acquired by crowded hoppers a day or two after hatching and “learned” by isolated ones after being placed into a crowd (Ellis, 1962a,b, 1963a,b; Ellis and Pearce, 1962). Experimentally induced “training” in aggregation enhanced aggregation behaviour in previously isolated Locusta hoppers and 4 h of such training increased the aggregation behaviour of isolated Schistocerca hoppers to the level observed in continuously crowded ones (Ellis, 1963b). Thus, a change in behaviour leading to active aggregation is apparently the first phase characteristic to be affected by crowding.

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M. P. PENER

The most characteristic and impressive activity of a hopper band in the field is marching (Uvarov, 1977). The general impression created by a marching band is that all hoppers are moving all the time in the same direction; but detailed data (Ellis and Ashall, 1957; Stower, 1963) reveal that only a portion of the hoppers show actual displacement at any given time, an individual hopper often halts, and the direction of the displacement of different individuals or of small groups within the band is not necessarily parallel. The aggregation behaviour of the marching hoppers presumably keeps the band coherent; hoppers advancing beyond the edge of the band were frequently observed to turn back and rejoin the main bulk (Ellis and Ashall, 1957; Stower, 1963; for further references see Uvarov, 1977). Marching can be induced and studied in the laboratory where crowded hoppers march around the floor of the cage in circles (Ellis, 1950, 1951). Isolated hoppers placed among crowded ones, or into a newly formed crowd, also march, but they spend much less time marching and march more slowly than crowded hoppers (Ellis, 1950, 1951). Crowding of previously isolated hoppers leads to an increase of marching behaviour within 24 h, which may be related to the habituation to “being touched” and to the induction of active aggregation behaviour (see above); however, a longer period of crowding, 15-16 days, is needed to bring the intensity of marching behaviour of these hoppers to the level shown by continuously crowded ones (Ellis, 1964). This long-term effect may be related to slower physiological adaptations to the high activity needed for fully intense marching. In any case, locomotor activity is much higher in gregarious than in solitary hoppers. Carlisle and Ellis (1963) reported that injections of ventral ( = prothoracic) gland extracts temporarily reduced the marching activity of Locusta hoppers. Haskell and Moorhouse (1963) bathed ventral nerve cord preparations of crowded Schistocerca adults in the haemolymph of fifth-instar hoppers and observed by electrophysiological techniques that haemolymph taken within 12 h of the moult, or at actual ecdysis, reversibly depressed the activity of metathoracic motor neurons, but enhanced interneuron activity in the ventral nerve cord. Haemolymph taken from hoppers in the mid-intermoult period had no such effects. These findings were correlated with the fact that the locomotor activities of hoppers are low close to the ecdysis, but high in the intermoult period. An extract from Bombyx pupae containing ecdysone and 20-hydroxyecdysone induced effects similar to those observed with hopper haemolymph taken about the time of the moult. Haskell and Moorhouse (1963) suggested that the effects are due to ecdysteroids, whose level was then assumed to be high at ecdysis. This explanation, however, is not consistent with more recent findings which clearly and uniformly show that body or haemolymph ecdysteroid levels of locust hoppers are high at about two-thirds to four-fifths of the intermoult period, but practically nil

LOCUST PHASE POLYMORPHISM

33

close to the actual moult (Wilson and Morgan, 1978; Hirn et al., 1979; Baehr et ul., 1979; Gande et al., 1979). Thus, the effects reported by Haskell and

Moorhouse (1963) may not be caused by ecdysteroids, but by some other factor(s) in the haemolymph of moulting or close-to-moult hoppers. It is more difficult to explain the results obtained with the Bombyx extract, but the authors themselves pointed out that the minimum effective dose was very high and the extract was impure. It is possible, therefore, that some impurities caused the effects, or that the presumably very high concentration of ecdysteroids induced some coinciding pharmacological effects. 3.7.2 Adults The final moult within a hopper band in the field is usually more or less synchronized and the resulting adults remain together, now forming a “swarm” (see Uvarov, 1977). Locusts in a swarm show active grouping or aggregation behaviour, best demonstrated in field studies on migratory flights (see below). Crowded adult locusts also show aggregation behaviour in the laboratory (Gillett, 1972) and the parameters reflecting the intensity of this behaviour are higher for crowded than for isolated adults (Gillett, 1973). Norris (1963, 1970) reported that crowded Schistocerca females exhibit a clear group behaviour in egg laying which may be related to a pheromonal factor. Nothing is known about possible endocrine influences on the aggregation behaviour of adult locusts. A major behavioural difference between solitary and gregarious adult locusts in the field is the migratory group flights of the latter. Extensive studies on Schistocerca (Rainey, 1963, 1976; and references therein; Waloff, 1972; review by Uvarov, 1977) revealed that the actual displacement of a flying locust swarm depends mainly on atmospheric air currents, but active aggregation behaviour of the locusts is a major factor keeping the swarm coherent. Smaller groups or “streams” of locusts within the swarm fly together in the same direction, but different streams are oriented to different directions, so the orientation of the individual locusts in different parts of the swarm may be effectively random and not parallel to the direction of the displacement of the whole swarm. However, when a stream of locusts flies, or is carried away by local air turbulances, beyond the edge of the swarm, the insects change orientation and fly back into the swarm. It may be recalled that hoppers in bands show a somewhat similar behaviour (see Section 3.7.1). Because of the intense migratory flights, gregarious adults are considered to be more active than solitary ones, though the latter may also make quite long individual flights by night (see Uvarov, 1977, p. 74). Laboratory studies on flight performance of Schistocerca confirmed that crowded adults fly much

34

M .P. PENER

more intensely than isolated ones (Michel, 1970a,b, 1980a,b); thus, in relation to flight the former are indeed more active than the latter. The results and conclusions of numerous studies on the effects of the CA or JH on locust flight activity (Wajc and Pener, 1971; Goldsworthy et al., 1972; Michel, 1973a; Wajc, 1973; Lee and Goldsworthy, 1975, 1976; Kutsch, 1983) were recently summarized (table 2 by Pener, 1985), and discussed together with some unpublished data. This discussion (Pener, 1985) reveals that the experimental results are controversial and cannot be amalgamated into a decisive picture. Different authors employed different conditions and experimental parameters for measuring flight activity, and these differences may well contribute to the apparent conflicts. However, the controversy itself indicates that the CA or JH probably do not play a cardinal regulatory role in locust flight, though minor effects, some of them possibly mediated by differential physiological ageing of allatectomized and control locusts (cf. Lee and Goldsworthy, 1975), may not be excluded. Allatectomy of adult Schistocerca males was reported to reduce spontaneous locomotor activity (Odhiambo, 1965, 1966~).However, allatectomy leads to a complete absence of male sexual behaviour in this species (Pener, 1986; and many references therein) and the reduced “spontaneous locomotor activity” may be explained, at least partially, by the lack of some movements which are related to sexual behaviour but not recognized as such, for example searching movements of the male for finding a female. Nevertheless, some data indicate that allatectomized males are rather sluggish (Wajc and Pener, 1969) and that the CA exert a stimulatory effect on the excitability of locusts (Cassier, 1963, 1964a,b, 1965b). If the questionable conclusion that CA and J H stimulate flight and/or locomotor activity and/or excitability of adult locusts is accepted, it would be in accord with a higher activity of the glands and/or higher JH titres in the more active gregarious or crowded locusts. Michel (1972a) implanted ventral glands (VG), taken from 5-day-old isolated adults of Schistocerca into 20-day-old crowded ones and observed a considerable but temporary decrease in flight activity of the recipients. Similar implantations of presumably degenerating VG taken from 5-day-old crowded adult donors have less effect, or no effect at all. From these results Michel (1972a) concluded that the VG exert an inhibitory effect on locust flight activity and suggested that the persistence of the VG in isolated adults (cf. Carlisle and Ellis, 1959; Cassier and Fain-Maurel, 1969a) may be responsible for their reduced flight activity. In the light of the controversial results in different studies on the effect of the CA on locust flight and related methodological problems (see above), this single report on the effect of the V G on flight activity requires independent confirmation. Michel and Bernard (1973) reported that electrocoagulation of the pars intercerebralis (PI), including the median neurosecretory cells (MNSC),

LOCUST PHASE POLYMORPHISM

35

drastically reduced flight duration in Schistocerca males, but implantation of the PI, or of whole brains, did not improve flight. In contrast, Goldsworthy et al. (1973) found no effect on flight performance after electrocoagulating the PI-MNSC in mature (18-25-day-old) Locusta males. In a later study, Goldsworthy et al. (1977) observed that electrocoagulation of the PI-MNSC of newly fledged (I-day-old) Locusta males moderately reduced flight performance tested on day 19 and administration of a synthetic JH to the PIcoagulated locusts partially restored flight performance. These authors suggested, therefore, that the PI-MNSC most likely affect flight activity by an activatory effect on the CA. Such an overall activating effect of the PI on the CA is well demonstrated in locusts (see Section 4.3). Goldsworthy et al. (1977) also observed that electrocoagulation of parts of the brain in the immediate lateral vicinity of the MNSC reduced flight performance more drastically than did electrocoagulation of the median area presumably comprising only the-MNSC. In conclusion, there is no clear evidence that a neurohormonal factor originating from the MNSC is directly involved in the regulation of locust flight activity. Recently, Diederen et al. (1988) found that during a single l-h period of flight, stainable neurosecretory material and label originating from injection of radioactive amino acids, both increased in the PI and storage lobes of the corpora cardiaca (CC) of crowded Locusta. In contrast, Highnam and Haskell (1964) reported that daily repeated flight induced/enhanced release of stainable neurosecretory material from the MNSC and CC, and led to an increase of CA volume and oocyte growth in crowded and isolated Schistocerca and in crowded Locusta, but not in isolated adults of the latter species. Michel (1972b, 1973b) claimed that the storage lobes ( = neurohaemal part) of the CC promote sustained flight, whereas the glandular lobes do not affect flight in Schistocerca. The former part of this claim has not received decisive confirmation, and circumstantial evidence (cf. Diederen et al., 1988) may indicate that it is not valid, at least in Locusta, whereas the latter part was decisively disproved and amply criticized (see Mordue and Goldsworthy, 1974; and references therein). The glandular lobes of the CC were found to exert a crucial effect on locust flight (Goldsworthy et al., 1972, 1973; Jutsum and Goldsworthy, 1977); these lobes produce adipokinetic hormones (AKH) in locusts (reviews by Stone and Mordue, 1980; Goldsworthy, 1983; Beenakkers et al., 1985a,b; Orchard, 1987), which control the major metabolic events necessary to provide fuel and energy for sustained flight. Presently three AKHs and their chemical structure are known from locusts, AKHI from both Locusta and Schistocerca, AKHII of Schistocerca, and AKHII of Locusta, the latter two differing only by one amino acid (Siegert et al., 1985; see also Orchard, 1987). All of them belong to a “family” of insect and crustacean small peptide neurohormones with similarities in their

36

M. P. PENER

chemical characteristics. The vast literature on AKH cannot be reviewed here; detailed studies, especially with AKHI, revealed that the hormone promotes formation and release of diacylglycerol from the fat body, activates glycogen phosphorylase in the fat body, induces marked hyperlipaemia in the haemolymph, changes the lipophorin profile of the haemolymph resulting in a much improved lipid transport and stimulates diacylglycerol oxidation in the flight muscles (see reviews above). In conclusion, all major endocrine organs have been claimed to affect locust flight activity. However, the only thoroughly investigated and unequivocal effects are those of the glandular lobes of the CC and of AKHs secreted by them. In the light of this fact, and of 20 years of intensive related research, it is astonishing that nothing is known about AKH in isolated locusts! Possible differences in AKH production, AKH content of the CC, rate of AKH release and haemolymph titre, as well as possible differences in adipokinetic responses (that is the response of the target systems to AKH), may well be responsible, or may contribute, to the differences in flight activity between crowded and isolated locusts, but this subject has not been investigated. Besides its other functions as a neurotransmitter and a neuromodulator in insects, including locusts (see Evans, 1982), octopainine may exert a general excitatory effect (Orchard, 1982). Rough handling and other irritations (Orchard et al., 1981), or flight (Goosey and Candy, 1980) quickly elevate haemolyniph octopamine levels, and it seems that the octopamine induces some rapid adipokinetic-like responses which occur mostly or partially before the AKH-induced slower and more prolonged adipokinetic responses take place (Orchard et al., 1981, 1982; Orchard and Lange, 1983, 1984; review by Orchard, 1987). Octopamine may also stimulate the process of oxidation in the flight muscles (see Goldsworthy, 1983). Fuzeau-Braesch and David (1978), Fuzeau-Braesch et al. (1979) and Benichou-Redouane and Fuzeau-Braesch (1982) reported that the octopamine content of whole heads and of various components of the nervous system is higher in isolated than in crowded fifth-instar hoppers and adults of Locusta. Treatment of crowded locusts with CO,, which induces “solitarizing” effects, increased octopamine content to the level found in isolated ones (see also Fuzeau-Braesch and Nicolas, 1981). In contrast, Morton and Evans (1983) obtained no differences in octopamine levels of nervous tissue, muscles, or whole heads between crowded and isolated Schistocerca adults. These authors strongly criticized the work of Fuzeau-Braesch and David (19’78) and of FuzeauBraesch et al. (1979), and in relation to the controversy between their results and those of Benichou-Redouane and Fuzeau-Braesch (1982), they implied that the differences may be related to high individual variations; they also

LOCUST PHASE POLYMORPHISM

37

cited the opinion of Fuzeau-Braesch and David, as a personal communication, that the differences in the results are likely to be due to differences in methodology and/or in the species of locusts used. Octopamine receptors in the brain were reported to be less sensitive in isolated than in crowded Locusta (David and Fuzeau-Braesch, 1979). Dopamine content was found to be about five times higher in crowded than in isolated Locusta, but no significant differences were found in noradrenaline content (Fuzeau-Braesch, 1977a,b).

4

4.1

Endocrine organs, hormones and their role in phase transformation THE CORPORA ALLATA A N D JUVENILE HORMONE

Staal(l961) reported that the volume of the CA is larger in isolated than in crowded adults of Locusta, except in adult males kept under low humidity. However, the results of Staal (1961, appendix 1) also showed a marked interaction between density and humidity; in crowded locusts the humidity had little effect on the volume of the CA, whereas in isolated ones high humidity led to a considerable increase in gland volume. In another experiment of the same study, CA volumes measured in the fifth instar were found to be larger in hoppers which had been kept isolated from the later part of the third instar than in those which had been maintained continuously under conditions of crowding. Thus, differences in density experienced during one (the fourth) instar were sufficient to affect gland volume. Highnam and Haskell (1964) studied CA volume and its increase during the sexual maturation of adult female locusts under various experimental conditions. These authors found that the maximum volume of the CA, as related to oocyte length, was quite similar in isolated flown and unflown and in crowded flown females of Locusta kept without males. However, the major increase in gland volume occurred at a smaller oocyte length in the crowded flown females than in the isolated (flown or unflown) ones. The steepest increase in this species was observed in unflown crowded females kept without males, and maximum gland volumes in this group greatly exceeded those in the other three groups. Thus, again an interaction was found, in this case between the effects of density and flight (=intense locomotor activity) on CA volume. The results obtained by Highnam and Haskell (1964) in Schistocerca were somewhat different. In adult females kept without males, the maximum volumes of the CA were quite similar in unflown isolated, flown isolated and unflown crowded locusts and a little smaller in flown crowded ones, but the increase in gland volume was steeper in the crowded

38

M. P. PENER

than the isolated females. The highest gland volumes and steepest increase were found in crowded females kept with mature males producing maturation-accelerating pheromone (see Section 3.3.1); such females also showed the shortest period of sexual maturation. Regardless of density and flight, maximum volume of the CA in adult Schistocerca females coincided with 46 mm length of the proximal oocytes. Measuring CA volume in the penultimate and last-instar female hoppers and in adult females of Schistocerca, Injeyan and Tobe (1981b) recorded consistently larger volumes in isolated than in crowded locusts. These findings somewhat differ from those of Highnam and Haskell (see above), but direct comparison may not be justified because the isolated locusts of Injeyan and Tobe were reared for two or more generations under strict isolation and all exhibited an extra hopper instar (see Section 3.4), whereas Highnam and Haskell separated their locusts from a crowded stock only at the moult to adult. Dale and Tobe (1986) found larger CA volumes in isolated than in crowded adult females of Locusta during the first 8 days after fledging. Considering that sexual maturation is quicker in isolated than in crowded Locusta adults (see Section 3.3. l), these results correlate well with densitydependent differences in maturation time. Unfortunately: these authors did not report on CA volumes in older females and it is probable that the experiments were stopped before the CA of the belatedly maturing crowded females completed growth. Thus, it cannot be decided whether maximum gland volumes, or only the rate of increase in gland volumes were different between the crowded and isolated locusts. Altogether, it seems that CA volume is larger in isolated than in crowded locusts. However, this conclusion is obscured by non-density-dependent effects (humidity, flight) on the volume of the glands. Also, except for the results of Injeyan and Tobe (1981b) in Schistocerca females, the various findings in adult locusts may reflect density-dependent changes in the rate of sexual maturation, rather than an effect of density on CA volumes per se. Finally, it must be kept in mind that differences in CA volume do not necessarily parallel differences in gland activity (see Feyereisen, 1985; Tobe and Stay, 1985). Injeyan and Tobe (1981b) reported that JH biosynthetic activity of the CA, assessed by radiochemical assay in vitro (for details and references in relation to this technique see Tobe and Stay, 1985), was higher in isolated than in crowded penultimate and last-instar female hoppers of Schistocerca. In the same study, the activity of the CA was found to be slightly lower in crowded than in isolated adult Schistocerca females, but major differences were temporal; the isolated locusts exhibited relatively higher rates of J H synthesis earlier in the first gonotrophic cycle. This earlier activity of the CA

LOCUST PHASE POLYMORPHISM

39

correlated well with a shorter period from fledgling to first appearance of the vitellogenic oocytes in the isolated females. However, in spite of the initially higher gland activity, vitellogenic oocyte development was slower in the isolated females, and eventually the crowded females completed the first gonotrophic cycle earlier than the isolated ones. JH biosynthetic activity of the CA was similar in crowded and isolated adult Locusta females within the first 5-6 days after fledging, but on day 8 gland activity was much higher in the isolated locusts (Dale and Tobe, 1986). As no data were presented for older females, the difference found in the 8-day-old females may be related to the shorter maturation time of isolated Locusta adults. Recently, Dale and Tobe (1988) found that addition of the calcium ionophore A23187 to the medium significantly increased in vitro J H biosynthetic activity and/or release in CA taken from 3-, 5- and 8-day-old adult crowded Locusta females. A23187 had a similar effect on glands of isolated females, but the increase was not statistically significant at any of these ages. These results constitute an additional CA and J H related difference between crowded and isolated adults. As one possible and/or partial explanation of these findings, the authors suggested that incubation in the ionophore resulted in a significant elevation in JH I11 production rates by CA of crowded locusts only because “initial” rates, that is without the ionophore, were lower in these locusts. Again, no females older than 8 days were investigated. Employing the Galleria bioassay, L. Joly and P. Joly (1974) and L. Joly et al. (1977) found higher haemolymph JH titres in isolated than in crowded fourth- and fifth-instar hoppers of Locusta. These authors have also observed that in isolated young Locusta adults J H titres increased much more rapidly with age than in crowded ones, but detailed inspection of their text and tables reveals that maximum values were only slightly higher in the isolated locusts. Using the more reliable method of gas chromatography-mass spectrometry, Dale and Tobe (1986) found low J H 111 titres in I-day-old adult Locusta females and no differences between isolated and crowded locusts at this age. The titres were much higher on day 4, and the increase was approximately twice as great in isolated than in crowded females. Lack of data for older females again prevents a determination of whether the latter result reflects a genuinely higher titre in the isolated locusts, or just an earlier increase of the titre in correlation with their earlier sexual maturation. Fuzeau-Braesch et al. (1982) assessed JH titres in last-instar hoppers and adults of Locusta, comparing crowded, isolated green, isolated homochrome (light coloured), and artificially “solitarized” (by CO,, cf. Fuzeau-Braesch and Nicolas, 1981) locusts. Except for higher J H 111titres in the artificially “solitarized” (= CO, treated) locusts, no clear differences were found; thus, these authors concluded that their results do not confirm the assumption that isolated locusts have higher J H titres, and consequently differences in JH titres may not be a

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M. P. PENER

primary cause of locust phase transformation. However, Fuzeau-Braesch et al. (1982) employed a radioimmunoassay for assessing J H titres, the accuracy of which is subject to criticism (cf. Granger and Goodman, 1983; Tobe and Stay, 1985). Also, Fuzeau-Braesch et al. (1982) claimed to find J H I and J H I1 in their locusts, but when more reliable physicochemical methods are employed, no J H I or I1 can be detected in Locusta (Huibregtse-Minderhoud et al., 1980; Bergot et al., 1981; Pener et al., 1986), or in other orthopteroid insects (Loher et al., 1983). In fact, occurrence of JH I, I1 or 0 has been demonstrated convincingly only in Lepidoptera (Schooley et al., 1984). Thus, the results of Fuzeau-Braesch et al. (1982) may not be accepted without doubts. Judging all the available data, it seems that CA activity and J H titres are higher in isolated than in crowded locusts, but this conclusion is still open to some doubts, especially in relation to adults. As already outlined (see above and Section 3.3.1), sexual maturation is quicker in isolated than in crowded Locusta adults and JH is necessary for this maturation. The higher CA activity and higher JH titres in isolated as compared to those in crowded Locusta adults of the same chronological age ( = equal number of days after fledging) found by L. Joly and P. Joly (1974), L. Joly et al. (1977) and Dale and Tobe (1986, 1988), may just reflect an earlier increase of gland activity and of hormone titres in the isolated locusts. Thus, for a more meaningful comparison of isolated and crowded adults, CA activity and/or J H titres may be related not to chronological age but rather to physiological events, such as per cent of increase in oocyte length, during at least the whole first gonotrophic cycle, and preferably during several consecutive cycles. Unfortunately, no such data are available for Locusta adults. The more comprehensive data of Injeyan and Tobe (1981b) on Schistocerca indicate that the differences in CA activity between isolated and crowded adult females are mostly temporal. Even if we accept without restrictions that CA activity and J H titres are higher in isolated than in crowded locusts, such differences do not necessarily prove a primary causal role of CA/JH in phase change. Table 1, summarizing the effects of CA/JH on phase characteristics, clearly shows that CA/JH promote solitary features in many instances, but promote gregarious features, or do not exert a relevant effect, in many others. Also, in some instances, an effect assumed to be exerted on phase may have alternative interpretations (see for example “E/F morphometric ratio” in Table 1). Moreover, as already outlined (Pener, 1976a,b, 1983, 1985; Roussel, 197%; Injeyan and Tobe, 1981b; Hardie and Lees, 1985), the response of various target organs to J H may be different in isolated and crowded locusts (see for example “yellow colour in adults” in Table 1).

TABLE I Effects of the CA and/or JH on locust phase characteristics. Phase characteristics, for which no experimental findings nor circumstantial evidence are available in relation to an effect of CA/JH, are not included in the table Phase characteristics investigated (in parentheses: difference between phases")

In relation to the given phase characteristic, the CA/JH* Promote solitary feature(s)

1 . E/F morphometric ratio (higher in G) 2. F/C morphometric ratio (higher in S) 3. Shape of pronotum (convex in S, straight or concave in G) 4. Sternal hairs (longer in G) 5. Thickness and number of gland cells in epidermis of adult male (more numerous in G) 6. Green colour (absent in G, may be present in S)

Promote gregarious feature(s)

+?

Effect may be related to disturbed metamorphosis Different authors draw somewhat different conclusions

+ + -c

+

Details and references are given in section

Do not exert an effect, or the effect is not clear

+ +?

Remarks, notes, doubts; see text for details

3.1

3.1

3.1

Effect may be related to disturbed metamorphosis Effect may be related to disturbed metamorphosis

3.1

Green colour is not a necessary characteristic of the solitary phase, effect is not restricted to locust phase polymorphism

3.2.1-2-3

TABLE 1 Continued Phase characteristics investigated (in parentheses: difference between phasesa)

In relation to the given phase characteristic, the CA/JHb Promote solitary feature(s)

Promote gregarious feature(s)

colouration (absent in G, may be present in S)

8. Black patterns in hoppers (present in G, absent in S except in black homochromy)

+?

9. Yellow colour in adults (present in G, absent in S except on hind wings of

Locustu) 10. Intensity of male sexual behaviour (more intense in G) 11. Fecundity of adult females (higher in S) 12. Adult life span (longer in S)

+?

+?

+

Details and references are given in section

CA/JH induce green colour in homochrome hoppers, thus shift one solitary characteristic to another Effect may be due to green colour induction, or due to interactions with a black colour promoting neurosecretory factor Target organ (epidermis) of isolated and crowded adults reacts differently to CA/JH Target organ (nervous system?) of isolated and crowded adults may react differently to CA/JH Results are open to different interpretations

3.2.1

Do not exert an effect, or the effect is not clear

+

7. “Homochrome”

Remarks, notes, doubts; see text for details

3.2.1

3.2.2-3

3.3.2

3.3.3 3.3.3 and 3.5

13. Adult reproduction; effects “transmitted” to the progenyc

+?

14. Number of hopper instars = number of eye stripes in some species (higher in S of some species)

+?

15. Rate of 0, consumption (higher in G) 16 Heartbeat frequency (higher in S)

17 Fat content (higher in G)

Conclusion is open to slight doubts; not all experimental data support the conclusion +?

+?

19. Adult flight and/or spontaneous locomotor activity (higher in G)

+?

+ +?

18. Isoenzymes in hoppers (some qualitative and quantitative differences between G and S)

+?

+?

The claim that CA/JH may induce an extra hopper instar is entirely theoretical; some experimental data do not support this claim

+?

3.3.3

3.4 (see also 3.1)

3.5 CA/JH do affect heartbeat frequency, but this effect is not responsible for phase differences Evidence is entirely circumstantial J H treatment did not induce isoenzymes which were induced by isolation; as a “negative” result, it is not entirely conclusive Experimental data and evidence are not clear

3.5

3.5 3.5

3.7.2

G = gregarious or crowded locusts; S = solitary or isolated locusts. Although every effort was made to present an objective picture, this task is practically impossible when results are conflicting or open to different interpretations. Some other authors may prefer removal or insertion of a question mark (which reflects some uncertainty) in relation to some effects. These include: weight of hatchlings (higher in G), colour of hatchlings (differences between G and S), number of ovarioles in female hatchlings (higher in S) and morphometrics of hatchlings (hatchlings of S are smaller and morphometrical analyses show differences between G and S ) .

44

M .P. PENER

The view that higher CA activity or higher J H titres induce the solitary phase in locusts is rather deeply rooted in the literature (Kennedy, 1956, 1961, 1962; May, 1971; Cassier, 1974, and many other publications; for a more recent review firmly holding this opinion see Nijhout and Wheeler, 1982). The reason for this situation may be because most of the cases in which CA/JH promote (or are assumed to promote) solitary characteristics were discovered and recognized earlier and received more attention, as well as wider publication, than those cases in which CA/JH promote (or are assumed to promote) gregarious characteristics. However, almost three decades ago P. Joly (1962, p. 77) had already concluded that the “. . . problem of physiological determination of locust phases cannot be explained on the basis only of differential activity of the corpora allata”. The information compiled in Table 1 of the present review indicates that this conclusion is valid even today. It may be assumed that the “phase status” in a particular instar is fixed by CA activity and/or JH titres in the previous instar(s). For example, adult phase characteristics may be fixed by J H titres in the hopper stage(s), or phase characteristics of first-instar hoppers may be fixed by J H titres in the adult mothers, etc. According to this assumption, differences in CA activity and/or JH titres in a certain instar lead to “pre-conditioning” of some target organ(s) to react differentially to J H in a subsequent instar. Although this assumption may be valid in some cases in relation to some phase characteristics, it cannot be held for all cases and for all phase characteristics. For example, we have already detailed that JH induces yellow colour in crowded adults, but does not do so in isolated ones (see Section 3.2.2). However, the lack of yellowing in isolated adult males of Locustu cannot be explained by the assumption that higher JH titres experienced during isolated hopper life fixed the competence of the adult’s epidermis not to respond by yellowing to the JH, because transfer of isolated adults to a crowd changes this competence and results in yellowing (Pener, 1976b; see also Section 3.2.3). The three simple experimental facts, amply demonstrated in Locustu, that: (1) the body of the adult males does not become yellow as long as they are kept isolated, (2) the CA/JH are necessary for the induction of yellow body colour in crowded adult males, and (3) crowding of previously isolated adult males does induce yellow body colour, cannot be reconciled with the assumption that lack of yellowing in isolated males is solely a result of a higher J H titre in any instar. Nijhout and Wheeler (1982) advocated a major role of J H in the control of locust phase polymorphism. However, their review is focused on a model of a JH-induced gene-switching mechanism as the basis of insect polymorphism and the conclusions drawn may be biased by preference for the model. As far as locust phase polymorphism is concerned, these authors disregarded or

LOCUST PHASE POLYMORPHISM

45

ignored almost all the evidence which may contradict the model. Some experimental findings indicate that the period of sensitivity to green-colour inducing and metamorphosis controlling effects of JH is different within a hopper instar (see, for example, Joly, 1968, pp. 290-294; NEmec et al., 1970; NEmec, 1971; De Wilde, 1975) and some authors (NEmec et al., 1970; NEmec, 1971; De Wilde, 1975) simply extended the findings related to green-colour induction to a general idea of an overall phase determining effect. Following this attitude, Nijhout and Wheeler (1982, p. 116) have stated that “it is clear now that the JH-sensitive period for the determination of larval in contrast to adult characters occurs at a different time than the JH-sensitive period for solitary versus gregarious phase determination” and again (p. 117) “. . . phase differentiation depends largely on the presence or absence of JH at a critical period . . .”. One may wonder when the “critical period” is for JH to affect, for example, pronotal shape (which is not influenced by JH), or nongreen (homochrome) solitary hopper colour, or competence of the adult epidermis to become or not to become yellow. Eventually, by referring to the black-colour-promoting neurosecretory factor (see Section 3.2. I), even Nijhout and Wheeler (1982) admit that the development of phase characteristics does not depend exclusively on JH. In conclusion, the differences in CA activity and J H titres, presumably existing between isolated and crowded locusts, may constitute an additional physiological phase characteristic responding to density. At the same time these differences may also cause appropriate changes in some, but not in all phase characteristics. Very probably, CA activity and J H titres stand not at the beginning, but somewhere in the middle, of a chain of events and physiological causal factors which are responsible for phase transformation. Thus, the hypothesis that the solitary phase is just a neotenous form induced by permanent or even specifically timed higher J H titres seems to be at best an oversimplification.

4.2

THE PROTHORACIC ( = VENTRAL) GLANDS AND ECDYSTEROIDS

The prothoracic glands, often termed “ventral glands” (VG), of acridids are located ventrolaterally in the posterior part of the head (P. Joly et al., 1956; Strich-Halbwachs, 1959; Staal, 1961). These glands are somewhat diffuse, not compact and there is no easy way to reach them. It is thus difficult to ensure their complete removal by surgery. Nevertheless, the results of a comprehensive study on the effects of ventral-glandectomy and implantations of VG carried out by Strich-Halbwachs (1959) in Locusta (see also, Strich-Halbwachs, 1954, 1958; Strich, 1955; Halbwachs et al., 1957) agreed well with the concept that these glands control moulting. However, Staal

46

M. P. PENER

(1961) was unable to confirm this conclusion, probably because of imperfect removal of the glands. Hoffmann and Koolman (1974) observed no moult after careful extirpation of the VG and, using the Calliphora bioassay, they did not detect ecdysone activity in homogenized tissues of ventral-glandectomized fifth-instar Locusta hoppers. Hirn et al. (1979) obtained ecdysteroid production by VG of Locusta in vitro, and more recently Reichhart and Charlet (1986) found that addition of brain-corpora cardiaca extracts to the medium enhanced ecdysone production by VG in vitro, presumably due to the presence of an ecdysiotropic factor (prothoracicotropic hormone?) in these tissues. Thus, there is no doubt that the VG of acridids and the prothoracic glands of other insects are the same organs. Ellis and Carlisle (1961) reported that the VG are larger in isolated than in crowded young adults of Schistocerca, but they did not describe the method of measuring the size of these diffuse and irregularly shaped glands. In an earlier report, Carlisle and Ellis (1959) observed that the VG degenerate in crowded adults of Locusta and Schistocerca, but persist in isolated adults of these species. Later work on Locusta somewhat modified this claim. Cassier and Fain-Maurel (1968) and Fain-Maurel and Cassier (1969) found that under long days (LD= 16 : 8) the VG degenerate, but under short days (LD= 12 : 12) they persist in crowded Locusta adults. However, in isolated green adults the VG persist under high humidity but degenerate in non-green isolated adults kept under dry conditions (Cassier and Fain-Maurel, 1969a). Moreover, the persisting VG of the green isolated adults show cyclic ultrastructural changes in correlation with cycles of oocyte development (Cassier and Fain-Maurel, 1969b), though it has not been demonstrated that such persisting glands actually produce ecdysone. Cassier (1969) related the persistence of the VG in crowded adults kept under short days and in isolated adults kept under high humidity to a “hyperactivity” of the CA in such locusts. However, the physiological importance (if any) of the persisting VG in adult locusts is not clear and later Cassier (1974) considered that this persistence is rather a result than a cause of some phase change. Kiiqiikekgi (1969) found that the VG in developing embryos of Schistocerca were somewhat smaller in those originating from isolated mothers than in those originating from crowded ones, but the difference did not exceed that expecied due to the smaller size of eggs and embryos from isolated mothers (see Section 3.3.3). Although suitable techniques for testing ecdysone biosynthetic activity of the VG in vitro are available (cf. Hirn et al., 1979; Reichhart and Charlet, 1986), such techniques have not been utilized to investigate possible differences in VG activity between isolated and crowded locusts. Several independent studies, however, demonstrate that ecdysteroid titres are similar, or at least not markedly different, between crowded and isolated Locusta

LOCUST PHASE POLYMORPHISM

47

(L. Joly et al., 1977; Ismail et al., 1979; Fuzeau-Braesch et al., 1982) and Schistocerca (Wilson and Morgan, 1978). Only few publications claim an effect of the VG on phase characteristics. Ellis and Carlisle (1961) and Carlisle and Ellis (1962) reported a positive correlation between the F/C morphometric ratio (see Section 3.1) and the size of the VG in Schistocerca, and observed that partial extirpation of the VG promoted gregarious colouration in hoppers of Schistocerca, though not in Locusta. The same authors (Carlisle and Ellis, 1963) also found reduced marching behaviour in crowded Locusta hoppers after injection of VG extracts (see Section 3.7. I), and Michel (1972a) observed that implantation of presumably active VG decreased flight activity in Schistocerca adults. All these reports would indicate a “solitarizing” effect of the VG. Finally, Haskell and Moorhouse (1963) claimed that ecdysteroids reduce the activity of certain motor neurons, an effect which seems to agree with the lower locomotor activity of solitary locusts. However, the results of Haskell and Moorhouse (1963) were probably not caused by ecdysteroids, or they were caused by a pharmacological effect of a presumably very high concentration of ecdysteroids (for details see Section 3.7.1), and Michel’s (1972a) findings may need further confirmation (see Section 3.7.2). In Locusta, Staal(l961) did not find an appreciable effect of implanted VG on colour, nor on the F/C and E/F morphometric ratios, except in some “giant adults” which exhibited very high “hypergregarious” E/F ratios (see Section 3. l), probably reflecting disturbances in metamorphosis rather than a shift toward the gregarious phase. KiiGiikekSi (1969) observed no clear relation between VG in embryos and colour of hatchlings in Schistocerca. The investigations of Strich-Halbwachs (1954, 1959) and Halbwachs et al. (1957) were focused on the effect of VG on moulting rather than on possible phase changes, but as far as information relevant to phase may be gained from these studies, the VG did not affect E/F and F/C ratios, nor colour, in Locusta (see also the discussion of Strich-Halbwachs’ results by Girardie and Joly, 1968). In the light of the similarity of ecdysteroid titres in crowded and isolated locusts (see above), it seems unlikely that the VG and ecdysteroids play a major ’causal role in locust phase transformation. This conclusion leaves the reports of Ellis and Carlisle (1961), Carlisle and Ellis (1962, 1963) and Michel (1972a) unexplained, except if one assumes that the VG produce some other hormone(s) beside ecdysteroids (cf. L. Joly et al., 1969; Hoffmann and Weins, 1974; L. Joly and Schneider, 1976; see also discussion by Wilson and Morgan, 1978; and a recent article by Charlet et al., 1988), and that these “other hormones” affect phase.

48

4.3

M. P. PENER NEUROSECRETORY CELLS, CORPORA CARDIACA A N D NEUROHORMONES

The activity of the CA is regulated by complex neurosecretory and nervous signals from the brain (reviews by Raabe, 1982; Feyereisen, 1985; Tobe and Stay, 1985). In locusts, the overall effect of the brain on the CA is activatory; an allatotropin, presumably a neurohormone, seems to be a major factor in this activation (see reviews above and later studies by Gadot and Applebaum, 1986; Gadot et al., 1987a,b), though further neurosecretory and/or nervous activating and inhibitory effects may also be involved in the regulation of locust CA activity (cf. Tobe and Stay, 1985; and a later study by Baehr et al., 1986). The activity of the prothoracic glands in insects is also controlled by the brain, primarily by the prothoracicotropic hormone(s) (reviews by Raabe, 1982; Bollenbacher and Granger, 1985). In agreement with this concept, Reichhart and Charlet (1986) have shown that brain-CC extracts exert an ecdysiotropic effect on the VG of Locusta in vitro. It seems, therefore, that any effect on phase polymorphism which may be related to differential activity of the CA and/or the VG in solitary and gregarious locusts, may be retraced to corresponding superimposed differences in the regulatory activity of the brain or brain and CC. Since neurohormones seem to play a major role in the control of CA and VG activity, they may be involved in the regulation of phase polymorphism as much as the CA or VG themselves, though the effect(s) of such neurohormones on phase may be considered as indirect. However, possible phase-related differences in neurohormones which activate or inhibit CA or VG activity, have never been investigated. Highnam and Haskell (1964) studied the amount of neurosecretory material in the pars intercerebralis neurosecretory cells (PI-NSC) and CC of isolated and crowded adult females in relation to oocyte growth and the effect of flight upon maturation in both Schistocerca and Locusta. Generally, they found that experimental conditions which led to slow maturation of the oocytes also led to the accumulation of neurosecretory material in the PINSC and CC system. Conditions which enhanced oocyte maturation also promoted the release of neurosecretory material from this system. These correlations, however, do not reveal underlying causal relationships and do not clarify even possible indirect effects (CA activity?) of neurosecretion on phase. Other, possibly more direct, neurohormonal effects on phase polymorphism have also been poorly investigated. There is evidence that an NSC-CC neuroendocrine factor promotes the black patterns which are characteristic of gregarious hoppers, but the identity of this factor is unknown; it may also be involved in the control of the black homochrome response of solitary hoppers and may not be limited to locusts (for details see Sections 3.2.1 and

LOCUST PHASE POLYMORPHISM

49

3.2.3). The relationship between biogenic amines which may function as neurohormones and locust phase polymorphism is unclear; certain experimental findings are controversial (see Section 3.7.2) and even if phasedependent differences exist in some biogenic amines, their causal role and mode of involvement in phase change are as yet obscure. The fact that repeated short daily treatment with CO, induces solitary phase characteristics in crowded Locusta may also be considered as an indication that neural factors are involved in phase change (review and interpretation by Fuzeau-Braesch and Nicolas, 1981). The nervous system seems to be a probable target organ for the CO,, but the actual effect(s) may be neurosecretory, nervous, or both. Bernays (1980) suggested that a factor from the CC reduces locomotor activity in crowded hoppers of Locusta. The role of this putative factor in behaviour-related phase differences has not been investigated, though it may be assumed, as a working hypothesis, that this factor promotes the more sedentary behaviour (no marching, inferior flight performance, see Sections 3.7.1 and 3.7.2, respectively) of solitary locusts. Alternatively, absence of this factor may promote the more active gregarious behaviour. As already outlined (see Pener, 1985; and Section 3.7.2), despite extensive studies and hundreds of research articles on AKH in crowded locusts, there is not a single publication devoted to AKH in isolated ones and nothing is known on possible AKH-related differences between gregarious and solitary locusts. All the available information constitutes only circumstantial evidence, intermingled with speculations, about the role of neurohormones in the regulation of locust phase polymorphism. The lack of more solid and exact evidence is mainly because the subject has not received sufficient attention and experimental efforts.

5 Pheromones Pheromones are often classified as “primers” and “releasers”; the latter have short-term effects and trigger preprogrammed behaviour in the receiving animal, whereas primer pheromones induce long-term effects, changing the physiology and/or behaviour of the receiving animals so that they become physiologically (or even morphologically) different and/or react differently to environmental stimuli from those animals which have not been exposed to the primer (Wilson and Bossert, 1963; for a more recent review see Weaver, 1983). Endocrine factors may regulate production of, and/or responsiveness to, both kinds of pheromones, but only primers may induce endocrine changes. Although a preprogrammed behaviour induced by a releaser may

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also affect subsequent behaviour and physiology, including endocrinology, of both the releaser producing and receiving animals, in such cases (for example, successful copulation mediated by some sex pheromone) not the releaser itself, but the pheromone-induced execution of the behaviour causes long-term physiological effects. This classification of the pheromones into releasers and primers is useful, but in some cases no easy distinction can be made, or a pheromone may act as both a releaser and a primer (see maturation-accelerating pheromone, below). The relationship between locust phases and pheromones may be two-fold; phase may affect pheromone production and/or reception, or (primer) pheromones may affect locust phase changes. The presumably chemotactile ( = contact) pheromonal factor which promotes group behaviour in oviposition of Schistocerca (and possibly of Locusta) females (see Section 3.7.2) seems to be a releaser. Production of this pheromone may be affected by phase, because there is some evidence that isolated locusts probably produce less of this pheromone than crowded ones (Norris, 1970). Nothing is known about the possible involvement of endocrine factors in the production or the reception of this pheromone. Gillett et al. (1976) were unable to demonstrate that there are other releaser pheromones which have marked or consistent effects in promoting immediate aggregation in Schistocerca, and concluded that visual and/or mechano-tactile factors play a more important role in locust grouping than chemical ones. However, more recent findings (FuzeauBraesch et al., 1988) may somewhat modify this conclusion (see below). The CA/JH-dependent maturation-accelerating pheromone produced by sexually mature adult Schistocerca males (see Section 3.3.1) seems to be both a releaser and a primer. It was reported to induce the “vibration reaction” in mature and immature adults of both sexes (Loher, 1961). Amerasinghe (1978a) confirmed this effect of the pheromone on mature males, but not on immature ones. By inducing the “vibration reaction” the pheromone acts as a releaser. However, regarding its maturation-accelerating effect (see Section 3.3. l), the pheromone acts as a primer; it presumably accelerates reproduction-related adult development of the endocrine system. The double involvement of the CA/JH in this system is interesting; they affect pheromone production in a male and they are affected by the pheromone in the receiving locust. If other effects of crowding (such as initial inhibition of maturation in both Locusta and Schistocerca and maturation acceleration by mature Locusta) are also pheromonal (see Section 3.3.1), these may also be considered as primers. There is some dispute about whether differences in production of maturation-accelerating pheromone between isolated and crowded mature Schistocerca males (see Section 3.3.1) d o exist, but no claim has been made that this pheromone, or other putative maturation-affecting pheromones, play a causal role in phase transformation.

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Based on results indicating the promotion of gregarious black hopper colouration (Nolte, 1963) and increase in long-term gregarious behaviour (Gillett, 1968; Ellis and Gillett, 1968), a so-called “gregarization pheromone” was found to be produced by locusts. This pheromone seems to be a primer (Gillett, 1968), and it was proposed to induce or intensify gregarious phase characteristics. Subsequent reports on this “gregarization pheromone”, however, are somewhat controversial. Nolte et al. (1970, 1973) and Nolte (1974, 1976) have concluded that the gregarization pheromone is produced in the crop of the alimentary tract, is present in the faeces of hoppers and it is actually 5-ethyl-guaiacol ( = 2-methoxy-5-ethylphenol); they named it “locustol”. Nolte (1977) even postulated that locustol somehow promotes the production of cyclic AMP, and the latter promotes transformation from solitary to gregarious phase. Nolte and co-workers regarded chiasma frequency as a major and decisive phase characteristic (see Section 3.6), and unfortunately tested the effect of the gregarization pheromone or locustol mostly or only on this parameter. Chiasma frequency, however, may not be a good phase indicator (see Section 3.6) and phase-related conclusions based on this parameter need further independent support. Also, Nolte (1977) drew his conclusions on the relationships between locustol, cyclic AMP and phase from circumstantial evidence, again using chiasma frequency as the main parameter. Although in this study some other parameters were also employed, the results obtained for these are not very convincing; for example, the differences in the mean F/C ratios varied between 3.25 and 3.47 in different experimental groups of “solitary controls”, and this range was larger than the differences induced by injection of locustol (from 3.25 to 3.11, or from 3.37 to 3.25, or from 3.46 to 3.34, in different experimental groups) or of cyclic AMP (from 3.46 to 3.35). On the other hand, Gillett (1975a,b) reported that a gregarization pheromone in Schistocerca affects some phase characteristics, such as colour or certain components of behaviour, but it has no or doubtful influence on some other phase characteristics including morphometrics and number of eye stripes (=number of instars, see Section 3.4). Like Nolte’s group (see above), Gillett and Phillips (1977) also found that the faeces of the hoppers constitute the source of a “gregarizing” factor, but they showed in the same study that the faeces of the adults have a “solitarizing” influence. Later Gillett (1983) confirmed these results, but she reported that while extracts of hopper faeces exerted the appropriate “gregarizing” effects, locustol (presumably a synthetic preparation) did not. Also, Gillett (1983) found that the gregarization pheromone from the faeces of the hoppers is perceived by the antennae, whereas Nolte el al. (1970) and Nolte (1974) claimed that locustol is received and/or perceived through the spiracles. Fuzeau-Braesch et al. (1988) found phenol, veratrole and guaiacol, but no 5-ethyl-guaiacol ( = Nolte’s “locus-

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tol”), in vapour condensed from the atmosphere of cages of crowded Locusta and Schistocerca. The mixture of these three compounds and also pure guaiacol or phenol, tended to increase aggregation behaviour without acting, however, as attractants. Perhaps these substances only amplify aggregation behaviour when coupled with other, visual and/or tactile (see above and Gillett et al., 1976), stimuli. If so, these phenolic compounds may be regarded as releaser pheromones with a complementary role. Fuzeau-Braesch et al. (1988) neither investigated the exact source (faeces?) of these substances, nor their possible primer effect(s) on phase transformation. In conclusion, a gregarization pheromone does seem to exist in the faeces of hoppers and to act as a primer, but its identity, importance and mode of action in phase transformation are not yet fully understood. The “solitarizing” influence of adult faeces (Gillett and Phillips, 1977; Gillett, 1983) may indicate a further primer pheromonal effect on phase, though the practical role of this effect in natural phase changes seems to be rather obscure. Possible relations between phase-affecting primer pheromones and endocrine factors have not been studied; for example, there is no report in the literature on possible effects of CA/JH on the production of the relatively well documented gregarization pheromone (regardless whether it is locustol or some other substance) which is found in the faeces of hoppers.

6

Concluding remarks

During the previous large-scale locust plague, in the 1950s and early 1960s, basic research on locusts flourished, especially in the UK and France, with overt or unexpressed hopes that Old World locust problems would be solved through a better understanding of fundamental locust biology. It was largely accepted that the factors affecting, and basic processes underlying, locust phase transformation may lead to practical control of these insects. This period coincided with the maturation of insect endocrinology as an established branch of biology (cf. Wigglesworth, 1954). Interest in locust research on the one hand, and in the relatively new promising field of insect endocrinology on the other, led to the investigation of endocrine effects on locust phases. The momentum so gained maintained the research in the 1960s and early 1970s, long after the locust plague had declined in the mid 1960s. This research produced an extensive literature, especially on the effects of CA/JH. This is well reflected in the bibliography of the present review but which is by no way comprehensive for this period. However, the vast amount of work invested into the subject did not yield uniformly accepted concepts about the role of the CA and JH in the control

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of locust phase changes, except the effect on green colouration. Mostly this latter effect led to the superficial confirmation of the claim that CA/JH induce the solitary phase (see Sections 3.2.3 and 4.1). Authors familiar with the complexity of locust phases usually did not accept this oversimplified conclusion without serious restrictions (Joly, 1962; Rowell, 1971 ; Pener, 1976b, 1983; Hardie and Lees, 1985), but its repetition in the literature created a feeling among insect endocrinologists not much specialized in locusts that the problem of endocrine control of locust phases has been “solved” and that there is not much room for further research. This attitude decelerated endocrine research on locust phases. Applied aspects have played even a more crucial role in this declaration. It has been long known that density is the primary extrinsic factor which controls locust phases, and phase change does not precede but follows changes in density. Consequently for forecasting locust outbreaks, increases in population density rather than phase characteristics have been surveyed in the field. Phase was considered as unimportant from the practical standpoint. Moreover, there were no serious locust outbreaks for over 20 years, from the mid 1960s until the mid 1980s, leading to the then increasingly accepted opinion that locusts are pests of the past and research on their phase change has no applied justification. The recent locust outbreak which started in 1985-86 and reached a devastating culmination in 1988, convincingly refuted the concept that locusts are pests of the past and rekindled interest in locusts and their phase polymorphism. Locust research is again considered to be important from the applied standpoint (Anonymous, 1989). The primary role of density in locust phase changes is not doubted, but the rather mechanistic view that phase is unimportant and phase change is not a target for applied research is losing its foothold. Thus, for example, it is realized today that if change from the solitary to the gregarious phase, especially the marching behaviour of hoppers and/or the swarming behaviour of adults, can be prevented despite an increase in densities over critical levels, the locusts may not be able to emigrate from localized areas. Present and future insect growth regulators (IGRs), such as hormone analogues and anti-hormonal agents, may serve as possible candidates for such prevention of major displacements of gregarious locusts. Locusts which would be unable to make long-distance collective emigrations from localized areas, would be easy targets to limited conventional or integrated control measures. Most of the locusts may meet death even without control measures because of starvation imposed by food limitations in such localized areas. In the present review I wished to emphasize that the problem of endocrine effects on locust phase changes is far from being “solved”. Probably, we are nearer to the beginning than to the end of the road. In the light of the great

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progress made in insect endocrinology and other relevant branches of science during the last 15 years or so when locust phase-related endocrine research was rather neglected, we are now able to reframe old questions, pose new ones and make novel working hypotheses which can be attacked by modern methods of molecular biology and computer analysis. Thus, for example, are there AKH-related differences between gregarious and solitary locusts (see Section 3.7.2)? What is the fuel for the intense marching of gregarious hoppers (see Section 3.7.1)? Is there an endocrine control of the marching behaviour and/or of fuel mobilization for marching, and if so, are there differences in the related endocrine mechanism(s) between marching gregarious and non-marching solitary hoppers? Do the CA/JH (claimed by some authors to play a primary role in locust phase changes, cf. Nijhout and Wheeler, 1982), or other hormones, affect aggregation behaviour? It was demonstrated almost 40 years ago that marching can be induced and investigated in the laboratory, and almost 30 years ago that aggregation behaviour can be well studied even in laboratory cages (see Section 3.7. I and the relevant publications of Ellis and co-workers cited in the same Section). Today these behavioural patterns can be studied by using time-lapse video cameras, and the data can be analysed by computerized methods. JH analogues or anti-juvenile agents (precocenes) can be administered instead of time-consuming implantations of CA or surgical allatectomy. Phase characteristics at the molecular level can be (cf. Colgan, 1987) and should be discovered and neuroendocrine effects on these should be investigated. In spite of the fact that colour-affecting neurohormones have been isolated and characterized from other insects (see, for example, Matsumoto et al., 1988), not much is known on the black-patterns/black-colour-promoting neurohormonal factor in locusts (see Section 3.2.1). Possible effects of this neurohormone on phase characteristics other than the black patterns, and of other putative neurohormones on phase changes, are poorly investigated and practically unknown (see Section 4.3). Possible endocrine or neuroendocrine effects on production, release and perception of phase-affecting primer pheromones have not been studied (see Section 5). During the last few years peptidergic insect neurohormones have become promising candidates for non-conventional insect control. In contrast to earlier concepts, it is now agreed that insect neurohormones may be developed to “IGRs” by genetically engineered microorganisms, especially by baculoviruses (Keeley and Hayes, 1987; Keeley, 1988; Menn and Bofkovec, 1989). Baculoviruses also infect locusts, though as a pathogen they kill them very slowly (cf. Bensimon et a!., 1987). However, an engineered virus with a neuropeptide (neurohormone) gene placed behind a strong non-essential viral promoter could well produce a devastating neurohormonal deregulation in the insect host. An oecologically acceptable strategy for the use of a genetically engineered

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baculovirus has recently been discovered (Wood et at., 1990). Understanding the neuroendocrine basis of locust phase changes may well reveal new target systems for such non-conventional methods. In conclusion, locust phase change with its extraordinary phenotypic plasticity is an extremely interesting biological problem and there is enough evidence to conclude that it is at least influenced, and perhaps completely regulated, by endocrine and/or neuroendocrine factors. A reassessment of the subject using modern methods and novel considerations may lead to interesting and important findings in the fields of both basic and applied science.

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Metabolism and Mode of Action of Invertebrate Hormones” (Eds J. Hoffmann and M. Porchet), pp. 373-383. Springer-Verlag, Berlin. Sellier, R. (1955). Recherches sur la morphogknese et le polymorphisme alaires chez les Orthoptkres Gryllides. Annls Sci. nat. (Zool.) Sir. I 1 16 (1954), 595-739. Siegert, K., Morgan, P. and Mordue, W. (1985). Primary structures of locust adipokinetic hormones 11. Biol. Chem. Hoppe-Seyler 366, 723-727. Simmonds, M. S. J. and Blaney, W. M. (1986). Effects of rearing density on development and feeding behaviour in larvae of Spodoptera exempta. J. Insect Physiol. 32, 1043-1045. Staal, G. B. (1961). “Studies on the Physiology of Phase Induction in Locusta migratoria migratorioides R. & F.” H. Veenman & Zonen N.V., Wageningen, The Netherlands. (Also as Meded. Landbouwhugesch. Wageningen No. 72 and Publikatie Fonds Landbouw Export Bureau 1916-1918 No. 40, 1-125.) Staal, G. B. and De Wilde, J. (1962). Endocrine influences on the development of phase characters in Locusta. Colloq. int. CNRS (“Physiologie, Comportement et Ecologie des Acridiens en Rapport avec la Phase”), No. 114, 89-105. Stone, J. V. and Mordue, W. (1980). Adipokinetic hormone. Zn “Neurohormonal Techniques in Insects” (Ed. T. A. Miller), pp. 31-80. Springer, New York. Stower, W. J. (1959). The colour patterns of hoppers of the desert locust (Schistocerca gregaria Forskbl). Anti-Locust Bull. 32, 1-75. Stower, W. J. (1963). Photographic techniques for the analysis of locust “hopper” behaviour. Anim. Behav. 11, 198-205. Stower, W. J., Davies, D. E. and Jones, I. B. (1960). Morphometric studies of the desert locust, Schistocerca gregaria. J. Anim. Ecol. 29, 309-339. Strich, M. C. (1955). Etude de la glande ventrale chez Locusta migratoria migratorioides L. (Orth. Acridoidea). Annls Sci. nat. (Zool.) Sir. I I 16 (1954), 399-411. Strich-Halbwachs, M. C. (1954). R81e de la glande ventrale chez Locusta migratoria (L.). C.r. Sianc. SOC.Biol. 148, 2087-2091. Strich-Halbwachs, M. C. (1958). Action de la glande ventrale sur le developpement ovarien de Locusta migratoria L. (Orthoptera). J. Insect Physiol. 1, 34G351. Strich-Halbwachs, M. C. (1959). Controle de la mue chez Locusta migratoria. Annls Sci. nat. (Zool.) Sir. 12 1, 483-570. Strong, L. (1965). The relationships between the brain, corpora allata, and oocyte growth in the Central American locust, Schistocerca sp.-I. The cerebral neurosecretory system, the corpora allata, and oocyte growth. J. Insect Physiol. 11, 135146. Strong, L. (1968). The effect of enforced locomotor activity on lipid content in allatectomized males of Locusta migratoria migratorioides. J . exp. Biol.48, 625630. Symmons, P. M. (1969). A morphometric measure of phase in the desert locust, Schistocerca gregaria (Forsk.). Bull. ent. Res. 58, 803-809. Tobe, S. S. and Stay, B. (1985). Structure and regulation of the corpus allatum. Adv. Insect Physiol. 18, 305432. Tojo, S., Morita, M. and Hiruma, K. (1985a). Effects ofjuvenile hormone on some phase characteristics in the common cutworm, Spodoptera litura. J. Insect Physiol. 31,243-249. Tojo, S., Morita, M., Agui, N. and Hiruma, K. (1985b). Hormonal regulation of phase polymorphism and storage-protein fluctuation in the common cutworm, Spodoptera litura. J. Insect Physiol. 31, 283-292.

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Truman, J. W., Riddiford, L. M. and Safranek, L. (1973). Hormonal control of cuticle coloration in the tobacco hornworm, Manduca sexta: basis of an ultrasensitive bioassay for juvenile hormone. J. Insect Physiol. 19, 195-203. Uvarov, B. P. (1921). A revision of the genus Locusta, L. (= Pachytylus, Fieb.), with a new theory as to periodicity and migrations of locusts. Bull. ent. Res. 12, 135-163. Uvarov, B. P. (1966). “Grasshoppers and Locusts”, Vol. 1. Cambridge University Press, Cambridge. Uvarov, B. P. (1977). “Grasshoppers and Locusts”, Vol. 2. Centre for Overseas Pest Research, London. Verdier, M. (1958). Modifications pigmentaires lie& a la densite chez les Tettigonides. Bull. SOC.zool. Fr. 83, 252-253. Vincent, J. F. V. (1972). The dynamics of release and the possible identity of bursicon in Locusta migratoria migratorioides. J. Insect Physiol. 18, 757-780. Wajc, E. (1973). “The Effect of the Corpora Allata on Flight Activity of Locusta migratoria migratorioides (R. & F.).” PhD Thesis, London. Wajc, E. and Pener, M. P. (1969). The effect of the corpora allata on the mating behavior of the male migratory locust, Locusta migratoria migratorioides [R. & F.]. Israel J. Zool. 18, 179-192. Wajc, E. and Pener, M. P. (1971). The effect of the corpora allata on the flight activity of the male African migratory locust, Locusta migratoria migratorioides (R. & F.). Gen. comp. Endocr. 17, 327-333. Walloff, Z. (1972). Orientation of flying locusts, Schistocerca gregaria (Forsk.) in migrating swarms. Bull. ent. Res. 62, 1-72. Warren, J. T., Sakurai, S., Rountree, D. B., Gilbert, L. I., Lee, S.-S. and Nakanishi, K. (1988). Regulation of the ecdysteroid titer of Manduca sexta: reappraisal of the role of the prothoracic glands. Proc. nntl. Acad. Sci. USA 85, 958-962. Weaver, P. (1983). Pheromones and behavior. In “Invertebrate Endocrinology”, Vol. 1, Endocrinology of Insects (Eds R. G. H. Downer and H. Laufer), pp. 543555. Alan R. Liss Inc., New York. Wigglesworth, V. B. (1954). “The Physiology of Insect Metamorphosis.” Cambridge University Press, Cambridge. Wilson, E. 0. and Bossert, W. H. (1963). Chemical communication among animals. Recent Progr. Horm. Res. 19, 673-716. Wilson, I. D. and Morgan, E. D. (1978). Variations in ecdysteroid levels in 5th instar larvae of Schistocerca gregaria in gregarious and solitary phases. J . Insect Physiol. 24, 751-756. Wood, H. A., Hughes, P. R., Van Beek, N. and Hamblin, M. (1990). An ecologically acceptable strategy for the use of genetically engineered baculovirus pesticides. In “Insect Neurochemistry and Neurophysiology, 1989” (Eds A. B. Borkovec and E. P. Masler) pp. 285-288. Humana Press, Clifton, New Jersey. Wyatt, G. R., Cook, K. E., Firko, H. and Dhadialla, T. S. (1987). Juvenile hormone action on locust fat body. Insect Biochem. 17, 1071-1073. Yagi, S. (1976). The role of juvenile hormone in diapause and phase variation in some lepidopterous insects. In “The Juvenile Hormones” (Ed. L. I. Gilbert), pp. 288300. Plenum Press, New York. Yagi, S. and Kuramochi, K. (1976). The role ofjuvenile hormone in larval duration and spermiogenesis in relation to phase variation in the tobacco cutworm, Spodoptera litura. Appl. Ent. Zool. 11, 133-138.

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Addenda

Some unforeseen delays occurred in the publication of the present review. Meanwhile many relevant new articles have been published. This situation led to the following addenda, completed before the proofs became available. The development of the recent plague of the desert locust, Schistocerca greguriu (see Section 6), was summarized by Skaf (1990). In 1989 this plague declined, presumably partly because of the massive pesticide spraying in Africa and adjacent semi-arid zones (Arabian Peninsula, etc.) and partly because of the dry weather in 1989 in these areas. There is now much argument about whether the massive employment of pesticides was a suitable and economic strategy (cf. Gibbons, 1990). An interesting event in the recent plague is that in autumn 1988 gregarious swarms of the desert locust escaping from West Africa crossed the Atlantic and reached the Caribbean region, implying an uninterrupted flight of some 5000 km (Kevan, 1989; Ritchie and Pedgley, 1989). Although displacement of locust swarms depends mostly on atmospheric air currents (see Rainey, 1989), crossing of the Atlantic means that physiological factors, including the endocrine control of flight fuel mobilization, transport and utilization by adipokinetic hormones (see Sections 3.7.2,4.3 and 6), must ensure the ability of the gregarious adults to fly, at least in order to remain airborne, for much longer periods than was previously suspected. The recent locust plague probably played a major role in the resumption of locust research, perhaps best reflected by the currently increasing number of reviews devoted to various subjects on locusts. The article of Waloff and Popov (1990), dealing with the vast contributions of Uvarov to acridology, presents excellent historical perspectives and a summary of concepts in relation to locust phases (see Section 2.1). Two recent reviews (Ferenz, 1990; Loher, 1990) are devoted to locust pheromones. They detail this subject much more widely than the relevant section of the present review (see Section 5) and emphasize the pheromonal aspects of locust phase changes. The major conclusions, however, are rather similar; more basic research is urgently needed for understanding the roles of pheromones in locust phase transformation and for evaluating the applied potential of pheromone-related manipulations in locust control. Pheromonal-hormonal interrelations should also be studied. Endocrine effects on locust phase changes were summarized in a smallscale review (Pener, 1990) with a section on applied aspects. A major review was published by Dale and Tobe (1990) on the endocrine basis of locust phase polymorphism. Like the present article (see Section 4. l), these authors also conclude that knowledge regarding the importance of JH titre and

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biosynthesis in phase differentiation in locusts is unsatisfactory. They outline three major reasons for this situation: (1) the complexity of the relationship between JH biosynthesis and JH titre is not fully understood even in gregarious locusts; (2) the effects of factors other than the rate of JH biosynthesis (e.g. haemolymph JH esterases, JH binding proteins, etc.) on JH titre have not been investigated sufficiently in locusts and no data comparing crowded and isolated locusts are available in this respect; (3) phase differentiation involves causative factors which are independent of JH titre. According to Dale and Tobe (1990, p. 409), “. . . no very startling evidence has yet been yielded by the comparative study of the action of endocrine agents in locusts of different phases” and consequently, “this leaves an understanding of the fundamental nature of phase polymorphism still to be sought”. Adipokinetic hormones and their mode of action in relation to mobilization, transport and utilization of lipids as flight fuel were again reviewed recently (Goldsworthy and Mordue, 1989; Wheeler, 1989; Gade, 1990). Although these reviews extend the subject to insects other than locusts and to metabolic hormones other than AKHs (for example, hypertrehalosaemic hormones), AKHs in locusts still constitute the basic and best investigated case. Within the last few years considerable advancement has been made in research on insect neurohormones (review by Holman et al., 1990; see also several articles in the book edited by Borkovec and Masler, 1990), but no studies have been reported on the role of neurohormones in locust phase changes, or on phase-dependent differences in neurohormones and/or in their mode of action. The only notable exception comes from our laboratory. We injected graded doses of synthetic AKHI, or of CC extracts, to 10-19and 24-30-day-old isolated and crowded adult males of Locusta migratoria migratorioides and assessed haemolymph lipid levels (Ayali and Pener, 1991 and submitted). We found that: (1) the resting lipid level (before injection) was considerably higher in crowded than in isolated locusts, and (2) the increase of haemolymph lipids 90-100 min after injection of either AKHI or CC extracts was again markedly higher in crowded locusts. Appropriate calculations have shown that the adipokinetic response of the isolated locusts, as reflected by elevation of haemolymph lipid level, is only 3040% of that of the crowded locusts. These findings correlate well with the more active flight behaviour of the gregarious locusts in comparison to that of the less active solitary ones (see Section 3.7.2). The recent locust plague led to the establishment of the “Emergency Centre for Locust Operation” within the Plant Production and Protection Division of the Food and Agricultural Organization (FAO) of the United Nations. This Centre now produces a register (Food and Agricultural Organization, 1989, 1990) comprising information on currently con-

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ducted locust research projects, including projects on locust physiology and endocrinology, all around the world. Density-dependent phase polymorphism in lepidopteran larvae (see Section 2.2) has received some further attention. Most important is the review by Hammond and Fescemyer (1987) which, although focused on one species, Anticarsia gemmatalis, presents a reasonable account of phase polymorphism in noctuids. A further r,elevant publication is that of Fescemyer and Hammond (1988). Recently, Morita et al. (1988) have shown that cuticular melanization in crowded larvae of Spodoptera litura is caused by a melanization-and-reddish-colouration hormone (MRCH) in the absence (or in the presence of low levels) of JH. Primary structure analysis of this MRCH isolated and purified from head extracts of adult Bombyx mori (Matsumoto et al., 1990) revealed that it is the same molecule as the pheromone biosynthesis activating neuropeptide (PBAN), previously purified by the same group, from the same source, through monitoring its pheromonotropic activity in B. mori (Kitamura et al., 1989). Interestingly, extracts of brain, CC, CA, suboesophageal ganglion and thoracic ganglia from adult Locusta migratoria induced pheromonotropic responses in the European corn borer moth, Ostrinia nubialis (Sreng et al., 1990). M. Altstein, Y. Gazit, A. Ayah and M. P. Pener (in preparation) also found by a pheromonotropic bioassay employing the noctuid Heliothis peltigera and quantification of the pheromone produced (for the method see Gazit et al., 1990), as well as by an immunochemical analysis using a PBAN antiserum and ELISA, that the brain, CC, CA and suboesophageal ganglion of young fifth-instar male nymphs of Locusta comprise a PBAN-like peptide. Considering the identity of the MRCH and the PBAN (see above), it is tempting to speculate that the PBAN-like neuropeptide found in Locusta may induce melanization in locusts and may be identical to the NSC-CC neuroendocrine factor which promotes the black patterns characteristic to gregarious hoppers (see Sections 3.2.1, 3.2.3 and 4.3). However, the situation may not be so simple. Like Sreng et al. (1990), we found this PBAN-like neuropeptide also in the CA of Locusta, but CA implantations to gregarious hoppers lead to green colour which is accompanied by reduction or disappearance of the black patterns (see Section 3.2.1). Although the effects of the implanted CA are undoubtedly caused by the JH because they are induced also by JH-analogues, it is difficult to reconcile the reduction of the black patterns with the assumption that the PBAN-like neuropeptide (which is present and perhaps even originates in the CA-cf. Sreng et al., 1990) promotes melanization and black pattern formation in locusts.

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References t o the addenda Ayah, A. and Pener, M. P. (1991). Differences in response to adipokinetic hormone between the solitary and gregarious phases of Locusta migratoria. Gen comp. Endocr. 82, 254 (Abstracts of papers presented at the Fifteenth Conference of European Comparative Endocrinologists, abstract no. 123. Borkovec, A. B. and Masler, E. P. (Eds) (1990). “Insect Neurochemistry and Neurophysiology, 1989”. Humana Press, Clifton, New Jersey. Dale, J. F. and Tobe, S. S. (1990). The endocrine basis of locust phase polymorphism. In “Biology of Grasshoppers” (Eds R. F. Chapman and A. Joern), pp. 393414. John Wiley & Sons, New York. Ferenz, H. J. (1990). Locust pheromones-basic and applied aspects. Bol. Sanidad Veg., Fuera de Serie, No. 20 (also as “Proceedings of the 5th International Meeting of the Orthopterists’ Society, 17-20 July 1989, Valsain (Segovia), Spain”), 29-37. Fescemyer, H. W. and Hammond, A. M. (1988). Effect of larval density and plant age on size and biochemical composition of adult migrant moths, Anticarsia gemmatalis Hubner (Lepidoptera: Noctuidae). Env. Ent. 17, 213-219. Food and Agricultural Organization (1989). “The Desert Locust Research and Development Register, No. 1, July 1989”. Emergency Centre for Locust Operations, Food and Agricultural Organization of the United Nations, Rome, Italy. Food and Agricultural Organization (1990). “The Desert Locust Research and Development Register, No. 2, March 1990”. Emergency Centre for Locust Operations, Food and Agricultural Organization of the United Nations, Rome, Italy. Gade, G. (1990). The adipokinetic hormone/red pigment-concentrating hormone peptide family: structures, interrelationships and functions. J. Insect Physiol. 36, 1-12. Gazit, Y., Dunkelblum, E., Benichis, M. and Altstein, M. (1990). Effect of synthetic PBAN and derived peptides on sex pheromone biosynthesis in Heliothis peltigera (Lepidoptera: Noctuidae). Insect Biochem. 20, 853-858. Gibbons, A. (1990). Overkilling the insect enemy. Science 249, 621. Goldsworthy, G. and Mordue, W. (1989). Adipokinetic hormones: functions and structures. Biol. Bull. 177, 218-224. Hamrnond, A. M. and Fescemyer, H. W. (1987). Physiological correlates in migratory noctuids: the velvetbean caterpillar as a model. Insect Sci.Appl. 8, 581-589. Holman, G.M., Nachman, R.J. and Wright, M. S. (1990). Insect neuropeptides. Annu. Rev. Ent. 35,201-217. Kevan, D. K. McE. (1989). Transatlantic travellers. Antenna 13, 12-15. Kitamura, A., Nagasawa, H., Kataoka, H., Inoue, T., Matsumoto, S., Ando, T. and Suzuki, A. (1989). Amino acid sequence of pheromone-biosynthesis-activating neuropeptide (PBAN) of the silkworm, Bombyx mori. Biochem. biophys. Res. Commun. 163, 520-526. Loher, W. (1990). Pheromones and phase transformation in locusts. In “Biology of Grasshoppers” (Eds R. F. Chapman and A. Joern), pp. 337-355. John Wiley & Sons, New York. Matsumoto, S., Kitamura, A., Nagasawa, H., Kataoka, H., Orikasa, C., Mitsui, T. and Suzuki, A. (1990). Functional diversity of a neurohormone produced by the suboesophageal ganglion: molecular identity of melanization and reddish

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colouration hormone and pheromone biosynthesis activating neuropeptide. f. Insect Physiol. 36, 427432. Morita, M., Hatakoshi, M. and Tojo, S. (1988). Hormonal control of cuticular melanization in the common cutworm, Spodoptera litura. f. Insect Physiol. 34, 751-758. Pener, M. P. (1990). Endocrine effects on locust phase changes; basic and applied aspects. Bol. Sanidad Veg., Fuera de Serie, No. 20 (also as “Proceedings of the 5th International Meeting of the Orthopterists’ Society, 17-20 July 1989, Valsain (Segovia), Spain”), 39-55. Rainey, R. C. (1989). “Migration and Meteorology. Flight Behaviour and the Atmospheric Environment of Locusts and other Migrant Pests.” Clarendon Press, Oxford. Ritchie, M. and Pedgley, D. (1989). Desert locusts cross the Atlantic. Antenna 13, 10-1 2. Skaf, R. (1990). The development of a new plague of the desert locust Schistocerca gregariu (Forskgl) (Orthoptera; Acrididae) 1985-1989. Bol. Sanidad Veg.,Fuere de Serie, No. 20 (also as “Proceedings of the 5th International Meeting of the Orthopterists’ Society, 17-20 July 1989, Valsain (Segovia), Spain”), 59-66. Sreng, L., Moreau, R. and Girardie, A. (1990). Locust neuropeptides stimulating sex pheromone production in female European corn borer moth, Ostrinia nubialis. f. Insect Physiol. 36, 719-726. Waloff, N. and Popov, G. B. (1990). Sir Boris Uvarov (1889-1970): The father of acridology. Annu. Rev. Ent. 35, 1-24. Wheeler, C. H. (1989). Mobilization and transport of fuels to the flight muscles. In “Insect Flight” (Eds G. J. Goldsworthy and C. H. Wheeler), pp. 273-303. CRC Press, Boca Raton, Florida.

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A Fresh Look at the Arousal Syndrome of Insects Sarah A. Corbet Department of Zoology, Downing Street, Cambridge CB2 3fJ, UK

1 Introduction 81 2 Endocrine control 83 3 An extended arousal syndrome 85 3.1 Effects on nerve and muscle 90 3.2 Metabolic substrates, water and ions 92 4 Physiological correlates of flight, paralysis and insecticide poisoning 99 4.1 Flight 99 4.2 Paralysis and insecticide poisoning 100 5 Discussion 101 5.1 Fight or flight response-or activation continuum? 101 5.2 Quantifying the activation continuum 102 5.3 Voluntary cessation of activity 103 5.4 Insecticide design 104 Acknowledgements 106 References 106

1

Introduction

The “fight or flight” syndrome of vertebrates comprises an assemblage of linked responses to arousal or stress, which prepare the body for intense activity. Physical exercise leads to similar endocrine and physiological responses (Gorbman et al., 1983). It has been proposed that insects exhibit a comparable assemblage of responses-the “generalized stress syndrome” of Heslop and Ray (1959), “general arousal syndrome” of Evans (1980a,b) and Evans and Siegler (1982), the EXIT (excitatory hypertrehalosaemic), “stress” or “excitation” response of Downer (1979, 1980), or the “fight or flight” syndrome of Orchard et al. (1981) and Davenport and Evans (1984a). The responses embraced by these proposals can be regarded as preparation for vigorous activity, including increased mobilization of metabolic substrates, cardioacceleration, and neurophysiological changes from a condition ADVANCES IN INSECT PHYSIOLOGY VOL. 23 ISBN &-12-024223-0

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favouring maintenance of posture towards a condition favouring locomotion (Evans and Siegler, 1982). The vertebrate syndrome is controlled by the sympathetic nervous system via noradrenaline, and by adrenaline released from the adrenal medulla. If the insects have a functional equivalent of the vertebrate autonomic nervous system, it probably includes the dorsal unpaired median system with the perisympathetic neurohaemal organs, the caudal sympathetic system associated with the terminal abdominal ganglion, and the stomatogastric nervous system. Major neurotransmitters and neuromodulators in this system are octopamine and neuropeptides (Orchard and Lange, 1988). Octopaminergic neurons have been claimed to control the release of peptide neurohormones into the haemolymph (Section 2). Although the term fight or flight response is appropriate for certain manifestations, the term arousal is more generally applicable and is used below. I will examine the possibility that arousal involves changes in a wide range of physiological systems, and may be manifested over a wide range of intensities and time periods, its effects persisting for hours or days. Thus, the acute fight or flight response would represent an extreme position on an activation continuum, at the other end of which would be less obvious responses of lower intensity which must concern experimentalists because they affect numerous aspects of physiology and behaviour. Beament (1958) drew attention to the parallels between the responses elicited in cockroaches by stress, by immobilization, by enforced sustained activity, and by poisoning with DDT. Many subsequent studies have also shown that arousal, intense activity, poisoning by certain insecticides, and perhaps feeding result in common physiological changes. As in vertebrates, physiological changes triggered by “alarm” stimuli can also be elicited by vigorous activity (which also causes other changes appropriate for recovery from activity rather than preparation for activity). Subsequent work has revealed common features in the endocrine control of these arousal responses, and Hoyle (1975), Evans (1980a), Downer (1980) and Sombati and Hoyle (1984a,b) proposed syndromes characterized by their common mediation by octopamine. Neuropeptides are also involved in at least some of the same responses (e.g. Orchard, 1987). The following discussion focuses on a single much-studied species, the American cockroach Periplaneta americana, selected because of its peculiar response to immobilization or stress (Beament, 1958), and the clues that this may give about the effects of insecticides. Because of the special features of cockroaches (including their large size, the storage of glycogen in the fat body and its use as the major fuel for flight, the inability to undertake sustained flight, and the remarkable tolerance of a wide range of states of hydration) research done on other species is not necessarily applicable

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directly to this one, but locusts and other insects are mentioned below where they offer informative parallels or contrasts. The situation in locusts is rather different, particularly because of their use of lipid as fuel for sustained flight and their ability to maintain a water balance in sustained flight for many hours (Weis-Fogh, 1967b). Here, I propose that the concept of an insect arousal syndrome be extended to include some responses (including the ionic regulation and maintenance of the water balance) not previously regarded as part of it and I consider which of the postulated responses have been shown to occur in appropriate circumstances, and which are governed by neuroactive substances known to be associated with arousal. Water balance seems to be more critical for insects than for most vertebrates, and if vigorous activity in insects tends to disturb the water balance, preparation to compensate for that imbalance may be expected to play a larger part in the arousal response of insects than in that of vertebrates. I therefore pay particular attention to aspects of water balance.

2 Endocrine control In general, it appears that the physiological responses that comprise the insect arousal syndrome are mediated by octopamine in cockroaches and locusts and in some other groups (Hoyle, 1975; Evans, 1980a,b;Sombati and Hoyle, 1984a,b) and, at least when elicited by exercise and insecticide treatment, by neuropeptides (see below). Other compounds such as 5hydroxytryptamine (5-HT), which may be important in other groups of insects, are given less attention here. Octopamine, a biogenic amine which is the phenolic analogue of noradrenaline, can act as a neurotransmitter, a neuromodulator or a neurohormone (Orchard, 1982; Evans, 1985a). As a component of the arousal syndrome it may operate in more than one of these modes, and may originate from multiple sources in the body (Evans, 1985a). It has been claimed that release of octopamine at specific sites within the metathoracic ganglion, by iontophoresis or by stimulation of a dorsal unpaired median (DUM) neuron, can dishabituate or potentiate input to motor neurons (Sombati and Hoyle, 1984a) or even initiate some motor activities (Sombati and Hoyle, 1984b). In their orchestration hypothesis, Sombati and Hoyle (1984b) suggested that during arousal octopamine might function both generally, to heighten the excitatory state of the insect, and also locally, to modulate specific elements of behaviour, enhancing those relevant to escape and inhibiting others. The probability that any behaviour would take place would be increased by the

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general release of octopamine, raising the general excitatory state. The probability that a particular element of behaviour would take place would depend on activation of the appropriate subset of DUM neurons, with local release of octopamine at appropriate sites within the neuropile. When the general excitatory state is high, further activation of even one of the several DUM neurons responsible for modulation of a particular behaviour could raise the probability of that behaviour occurring to the point at which it is performed ‘‘spontaneously”. Sombati and Hoyle (1984b) proposed that this modulatory action of particular DUM neurons was mimicked by the iontophoretic release of octopamine at specific sites within the neuropile. That octopamine raises the general level of excitability of an insect is widely accepted, but the local modulatory effects proposed by Sombati and Hoyle (1984b) require further investigation before alternative interpretations can be eliminated (P. D. Evans, personal communication). The important behavioural role of octopamine in lobsters (reviewed by Kravitz, 1988) provides an intriguing parallel but is not considered further here. In Periplaneta, the titre of haemolymph octopamine is elevated by handling or mechanical stress (Bailey et al., 1983; Davenport and Evans, 1984a), by flight (Bailey et al., 1983; King et al., 1986) or by treatment with certain insecticides (Davenport and Evans, 1984a). In locusts, the titre of haemolymph octopamine is elevated by handling or mechanical stress (Davenport and Evans, 1984a), by flight (Goosey and Candy, 1980; Orchard, 1987) or by treatment with certain insecticides (Davenport and Evans, 1984a); it rises quickly when flight begins and peaks about 10min later (Goosey and Candy, 1980; Orchard, 1987). Since the early finding that blood-borne factors from stressed or insecticide-poisoned cockroaches could cause paralysis and associated symptoms (Sternberg and Kearns, 1952; Beament, 1958), much progress has been made in elucidating the function and structure of the neuropeptide hormones involved in stress and flight, notably the cockroach hyperglycaemic hormones and the locust adipokinetic hormones (AKH) (Orchard, 1987). Many have now been sequenced, revealing the AKH family of octapeptides, nonapeptides or decapeptides (listed by Holman et al., 1990). Although these neuropeptides were named according to the source species and the response of the target organ in the particular bioassays with which they were tested (Goldsworthy et al., 1986b; Gade, 1988; see also Raina and Gade, 1988), their species- and function-specificity has proved to be low. Thus, Gade (1988) showed that hyperlipaemia in locusts and hypertrehalosaemia in cockroaches can be elicited by many different members of the AKH peptide family, originating from different genera of insects; and a peptide that elicits hypertrehalosaemia in cockroaches also causes cardioacceleration in cockroaches and shows myotropic activity in locusts (O’Shea

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et al., 1984; Holman et al., 1990). Furthermore a given function in a given species may be performed by several neuropeptides of the same or different families; 1 1 myotropic neuropeptides, of which eight are structurally similar leucokinins, have been isolated and characterized in Leucophaea maderae (Holman et al., 1990). In Periplaneta, release of hyperglycaemic neuropeptides is said to be stimulated by octopamine (Downer et al., 1984), and haemolymph titres of neuropeptides are probably elevated by flight (King et al., 1986). In locusts the hormones AKH I and I1 are synthesized in and released from the glandular lobes of the corpora cardiaca (Rademakers and Beenakkers, 1977). It has been claimed that their release is mediated by octopamine (Orchard et al., 1983; but see Konings et al., 1988). Adipokinetic hormones are released in locusts during flight (Cheeseman et al., 1976; Goldsworthy, 1983), and their titres in the haemolymph peak later than that of octopamine, about 30 min after the beginning of flight (Orchard, 1987). Release of neuropeptides is stimulated by treatment with certain insecticides in Locusta in vivo and in vitro (Singh and Orchard, 1982) and in Schistocerca (Samaranayaka, 1974). When octopamine and neuropeptides act as neuromodulators, they modify the response of muscle or nerve to an existing neural message. It appears that they can exert a similar modulatory action in other components of the arousal syndrome. Thus octopamine modifies the response of a moth to a pheromone (Linn and Roelofs, 1986; Section 3.1), and neuropeptides modify the response of rectal epithelium to an osmotic gradient (Phillips and Lechleitner, 1988).

3 An extended arousal syndrome

By analogy with vertebrates, and taking into account some special characteristics of insects, it is possible to compile a list of responses that might be expected to be involved in preparation for activity. Table 1 lists the expected responses and summarizes some evidence relevant to their control. Table 2 shows which of these responses have been shown to follow stress, flight or insecticide treatment, and also feeding. [It is not clear whether feeding can result in arousal, but it leads to release of neurosecretory material from the corpora cardiaca in Schistocerca (Highnam et al., 1966; see also Coast, 1988) and Locusta (Bernays and Chapman, 1972) and of diuretic hormone and 5-HT in Rhodnius (Maddrell, 1964; Lange et al., 1989). Starvation can also lead to increased titres of octopamine in Schistocerca (Davenport and Evans, 1984b).] Tables 1 and 2 focus on Periplaneta americana, mentioning locusts or other insects where appropriate. In both

TABLE 1 References implicating octopamine or neuropeptides in the control of elements of the postulated arousal response. All references relate to Periplaneta americana except those in square brackets. A question mark indicates cases where the involvement of the substance is not clear Feature

Octopamine

Neuropeptide

Expose sensilla Modify peripheral sensory threshold Facilitate efferent pathway

[Mosquito: Nijhout (1977)l [Moths: ?Linn and Reolofs (1986)l

[Locust: Bernays and Chapman (1972)l

Cardioacceleration Gut muscle Oviduct muscle Activate glycogen phosphorylase in fat body Modify trehalase activity Carbohydrate utilization by muscle

[Locust: flight muscle, Whim and Evans (1988); extensor tibiae, Evans and O’Shea (1977), O’Shea and Evans (1979)l Miller (1985) Huddart (1985) Stoya and Penzlin (1988); [locust: Orchard and Lange (1987)l Gole and Downer (1979); [locust, no effect: Carlisle et al. (1988)] Jahagirdar et al. (1984); [not in locust: Beenakkers et al. (1985)l [Locust: Goosey and Candy (1980)]

~

[Locust: O’Shea et al. (1984), Evans and Myers (1986b)l Miller (1989, [locust: Cuthbert and Evans (1989); Manduca: Tublitz and Truman (1985)l Huddart (1 985) Proctolin: Stoya and Penzlin (1988); [locust, proctolin: Orchard and Lange (1987)] Steele (1963), McClure and Steele (1981), Hanaoka and Takahashi (1976); [locust: Goldsworthy et al. (1986a,b), Carlisle et al. (198811 [Not in locust: Beenakkers et al. (1985)] [Locust: see Robinson and Goldsworthy (1976)l

Thermogenesis Malpighian tubule secretion

[?Locust: Carlisle et al. (1988)l [Acheta:Coast (1989); not in locust: Morgan and Mordue (1984)]

Malpighian tubule contraction Reabsorption in hindgut Salivary fluid secretion Cuticular transpiration Tracheal ventilation and spiracle closure Sodium release from fat body Sodium transport across gut wall Malpighian tubule sodium transport Octopamine release Neuropeptide release

[Nauphoeta: House and Ginsborg (1985)l -

[Locust: Sombati and Hoyle (1984b), Evans, personal communication and see Evans (1978), Myers and Evans (1 988)] -

~

Mills and Whitehead (1970), Penzlin (1971); [locust: Proux et al. (1982), Proux et al. (1988a), Acheta: Coast (1988, 1989)] Cameron (1953), Crowder and Shankland (1972) Tolman and Steele (1980), Steele and Tolman (1980); [Locusta: Herault and Proux (1987)l [Locust: Baines and Tyrer (1989)l ?Noble-Nesbitt and A1 Shukur (1987, 1988a,b) ?Bhatia and Tonapi (1968), [locust: see Myers and Evans (1988)l Spring et al. (1986) [Locust: Audsley and Phillips (1988), Phillips and Lechleitner (1988)l [Acheta: Spring (1990)l

Downer et al. (1984), [locust: Orchard et al. (19831

TABLE 2 References that associate elements of the postulated arousal response (as in Table 1) with stress, flight, insecticide treatment or feeding. All references relate to Periplaneta americana except those in square brackets. A question mark indicates cases where the association is uncertain Feature

Stress

Flight

Expose sensilla Modify peripheral sensory threshold Facilitate efferent pathways Cardioacceleration

Gut muscle Oviduct muscle Activate glycogen phosphorylase in fat body

Insecticides -

-

e.g. DDT, Haynes (1988)

[Locust: Bernays and Chapman (1972)l

~

~

-

Hypertrehalosaemia: ?Downer (1 98 1b)

Modify trehalase activity Carbohydrate utilization by muscle

Hayakawa

Thermogenesis

[Bees: e.g. Chappell (198211

el

al. (1988)

-

Muscle: Jahagirdar et al. (1985), hypertrehalosaemia: ?King et al. (1986) [Locust: Beenakkers et al. (1985)l Downer (1979) [locust: Jutsum and Goldsworthy (1976), Goldsworthy (1983)l ~

Feeding

Glycogen depletion: Orr and Downer (1982, 1983)

Orr and Downer (1982, 1983)

Davey (1962), [Rhodnius: S. H. P. Maddrell, personal communication] Davey (1 962) -

-

Malpighian tubule secretion

Malpighian tubule contraction Reabsorption in hindgut

-

[Rhodnius: Casida and

Maddrell (1971)l

[Locust: Mordue (1969); Rhodnius: Maddrell (1964); Acheta: Coast (1988); Libellula: Nicholls (1985)l ~

~

?Beament (1958)

Salivary fluid secretion Cuticular transpiration Tracheal ventilation and spiracle closure

[Locust: Mordue cited in Goldsworthy (1976)l

[Locust: Goldsworthy (197611

-

Machin et al. (1986), Miller (1981)

Sodium release from fat body Sodium transport across gut wall Malpighian tubule sodium transport Octopamine release

Bailey et al. (1983)

Bailey et al. (1983)

Neuropeptide release

?Beament (1958)

?King et al. (1986); [locust: Rademakers and Beenakkers (1977), Cheeseman et al. (1976), Orchard (19871

[Locust: Samaranayaka (197711 -

?Chapman (1985), [locust: Bernays and Chapman (1974)l

?Ingram (1955) [Lindane, Schistocerca: ?Samaranayaka (197411

~

~

-

~

Davenport and Evans (1984a) Cook and Holman (1985)

[Locust: Davenport and Evans (1984b)l [Locust: Highnam et al. (1966), Bernays and Chapman (1972)l

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90

tables, interest focuses on the gaps; these represent effects which might be expected but which do not seem to have been found, in many cases perhaps because they have not been sought.

3.1

EFFECTS ON NERVE AND MUSCLE

Arousal is expected to enhance the insect’s responsiveness to relevant sensory stimuli. Behavioural changes are well known. A startled cockroach that raises its antennae presumably increases its opportunities for perception of visual or chemical stimuli. Andersen (1 968) showed that a “sleeping” flour moth, Ephestia kuehniella, lays its antennae down over its back, where the chemosensilla are shielded by the row of scales along the upper surface of the antenna, and tucks the tips of the antennae under the wings. When alerted, the moth raises its antennae, exposing the chemosensilla, and then holds them forward in front of the head. The sensitivity of males of the mosquito Anopheles stephensi to the female wingbeat frequency depends on the position of antennal hairs, the erection of which varies on a die1 cycle and is stimulated in isolated antennae by octopamine and other a-adrenergic agonists (Nijhout, 1977; Nijhout and Sheffield, 1979). The sensitivity of Periplaneta cercal mechanoreceptors also depends on the position of the cerci (Libersat and Camhi, 1988). In honeybees, mechanical disturbance during “sleep” increases the responsiveness of an optomotor interneuron to pattern movement (Kaiser and Steiner-Kaiser, 1983). Arousal is also expected to affect peripheral sensory thresholds. The evidence that it does so is indirect. Octopamine applied topically or by injection evidently lowered the threshold for a response to pheromone by male moths (Linn and Roelofs, 1986), and octopamine injected into the brain increased responsiveness to olfactory stimuli in honeybees (Mercer and Menzel, 1982). It was assumed that the octopamine acted on some part of the afferent pathway. In Periplaneta, the presence of octopamine in neurohaemal areas in the antennal hearts led to the suggestion that octopamine may modulate chemoreceptor sensitivity (Pass et al., 1988). Chemoreceptor sensitivity is under the influence of a factor from the corpus cardiacum in locusts (Bernays and Chapman, 1972), and endocrine control of chemoreceptor sensitivity has also been suggested in other species (Blaney et al., 1986). In the locust flight system, proprioceptor sensitivity seems to be modulated by AKH (Reichert, 1988) and by 5-HT (M. D. Whim and P. D. Evans, personal communication). Proctolin, octopamine and 5-HT modulate proprioceptor sensitivity in lobsters (Pasztor and Bush, 1989), and octopamine has been implicated in the control of sensitivity in the eyes of Limulus (Battelle et a/., 1982).

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In Periplaneta, haemolymph titres of octopamine show circadian fluctuations, peaking at the time of maximum activity (Davenport and Evans, 1984a). Die1 changes in responsiveness of moths to pheromones have been shown to be under central control in some species (Worster and Seabrook, 1988), and it would be interesting to know whether die1 fluctuations in octopamine titre are involved. Schistocerca flight muscles are innervated by octopaminergic neurons from DUM cells. Stimulating these neurons, or applying octopamine, reduces twitch duration by increasing the rate of relaxation. These and other effects of octopamine prime flight and other muscles for activation (e.g. Candy, 1978; O’Shea and Evans, 1979; Whim and Evans, 1988). Both octopamine and neuropeptides of the AKH family act as neurotransmitters and neuromodulators in the extensor tibiae muscle of Schistocerca (see Evans and Myers, 1986a). In the skeletal component of the muscle, octopamine increases the rate of relaxation of twitch tension generated by fast and slow motoneurons (O’Shea and Evans, 1979; Evans, 1980b, 1985b; Evans and Siegler, 1982). Evidently octopamine increases behavioural responsiveness by changing the response of the muscle “from a postural to a dynamic mode” (Evans and Siegler, 1982). Octopamine reduces the frequency of the rhythm in the myogenic component of the extensor tibiae of Schistocerca gregaria (Hoyle, 1975; Evans and O’Shea, 1978). Neuropeptides from the corpora cardiaca of Periplaneta americana [myoactive factors I and I1 (MF I and II), subsequently shown to be identical with cockroach hypertrehalosaemic hormones (Goldsworthy et al., 1986b)l affected both skeletal and myogenic components of the extensor tibiae of Schistocerca nitens (O’Shea et al., 1984). By analogy with vertebrates, the arousal syndrome might be expected to influence the contraction of visceral muscles, including those involved in haemolymph circulation, and those of the gut, Malpighian tubules (Section 3.2) and oviduct. The myogenic fibre bundle, in the extensor tibiae muscle of the hindleg, in which the effects of octopamine and neuropeptides have been explored in locusts (Evans and O’Shea, 1978; O’Shea et al., 1984; Evans and Myers, 1986a), is not found in Periplaneta. Among many factors known to stimulate Periplaneta heartbeat i n vitro are octopamine and neuropeptides, including neurohonnone D, now known to be identical with cockroach hypertrehalosaemic factors (O’Shea et al., 1984; Miller, 1985; Goldsworthy et al., 1986b). Whether or not these factors are functioning as hormones, cardioacceleration is implicated as a component of the arousal syndrome. Periplaneta hindgut muscle has been shown to respond to octopamine and to cockroach hypertrehalosaemic factor, as well as to proctolin (Huddart, 1985). Periplaneta oviduct muscle contains octopamine and proctolin (Orchard and Lange, 1987, 1988). The frequency and

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amplitude of phasic contractions of the oviduct muscle are increased or decreased by octopamine, depending on concentration, and are affected by proctolin, which also induces their tonic contraction (Stoya and Penzlin, 1988).

3.2

METABOLIC SUBSTRATES, WATER AND IONS

Excitation, exercise or insecticides cause an array of interconnected changes involving mobilization and utilization of metabolic substrates and the redistribution of ions and water (Tables 1 and 2). These changes are outlined in Fig. 1, in which pathways thought to be associated with the postulated arousal syndrome are shown as heavy lines.

3.2.1 Metabolic substrates An increase in the concentration of haemolymph trehalose is the most obvious feature of the EXIT response (excitation-induced hypertrehalosaemia) of Downer (1979), which shifts the resting insect “from a state of carbohydrate flux that favours the synthesis and deposition of glycogen to a trehalogenic condition in which glycogenolysis and trehalogenesis predominate” (Downer, 1981b). In Periplaneta, changes in the concentration of haemolymph trehalose follow stress (Matthews and Downer, 1974; Downer, 1979, 1980), flight (King et al., 1986) or insecticide poisoning (Granett and Leeling, 1972; Orr and Downer, 1982). Hypertrehalosaemic effects have been found for both octopamine (Downer, 1980) and neuropeptides of the adipokinetic hormone/ red pigment concentrating hormone family, notably M F I and I1 of O’Shea et al. (1984) (Gade, 1985, 1988; Orchard, 1987). The absolute quantity of trehalose in the haemolymph of Periplaneta would be increased by fat body glycogenolysis and decreased by utilization in the muscles. Haemolymph trehalose concentrations therefore depend on the relative rates of these two processes and also on the haemolymph water content, which may also change in response to exercise (King et al., 1986) or (in another cockroach, Blatella germanica) insecticide poisoning (Mansingh, 1965). Thus, haemolymph trehalose concentrations are increased by stimuli which elicit glycogenolysis and/or trehalogenesis in the absence of vigorous activity, i.e. stress, or treatment with octopamine or neuropeptides. Flight or running, or insecticide treatment with associated hyperactivity, affect substrate utilization and water content as well as substrate mobilization, and the net effect on haemolymph trehalose concentration is less predictable. Similar considerations apply to lipid concentrations in the haemolymph of locusts,

93

THE AROUSAL SYNDROME

fat body

salivary haemolymph gland + gut

WATER

IONS

METABOLIC

SUBSTRATES

carbon dioxide

muscle FIG. 1 Diagrammatic representation of pathways of transfer of water, ions and metabolic substrates between fat body, haemolymph, gut, salivary glands and muscle. Transfers thought to be associated with arousal are shown as continuous lines. Mt, Malpighian tubules; gw, gut wall.

although haemolymph volume changes little during sustained flight in locusts (Goldsworthy, 1976). Stress, exercise and insecticide treatment have similar effects, accelerating the flux of metabolic substrates, notably carbohydrates and lipids. At least some of these effects seem to be mediated by octopamine and neuropeptides (Tables 1 and 2). In Periplaneta at room temperature, elevated body temperature is a consequence of, but not a prerequisite for, flight (Downer, 1981b). In species or situations where warm-up is a prerequisite for flight, arousal might be

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expected to initiate warm-up. Carpenter bees, Xylocopn, initiate warm-up when prodded (Chappell, 1982), and bumble bees interrupt their torpor at night with brief periods of thermogenesis, and warm up in preparation for activity just before the light phase begins (Surholt et al., 1988). If thermogenesis is stimulated by octopamine, elevation of body temperature, which is relatively easily quantified, might be useful in preliminary screening of compounds which would activate the response either directly or by lowering the threshold for other stimuli. 3.2.2

Water

The effects of exercise on insect water balance are likely to depend on the type of fuel used for flight. Energy substrates differ from one another, first, in the amount of metabolic water released by oxidation per unit weight of fuel, or per unit of energy generated, and secondly in the amount of water associated with them in storage. One gram dry weight of stored glycogen has associated with it about 2.5 g water, whereas lipid requires no associated water (WeisFogh, 1967a). To produce 10 kJ by oxidation of carbohydrate requires 0.57 g fuel and releases 0.31 g metabolic water and about 1.43g associated water (1.74 g water altogether), whereas to produce the same amount of energy by oxidation of lipid requires 0.24 g fuel and releases 0.26 g metabolic water and no associated water (Weis-Fogh, 1967a; Edney, 1977). Some insects, notably flies and bees, derive fuel for flight from sugars stored in the crop (WeisFogh, 1967a; Keeley, 1985; Surholt et a/., 1988). In such cases, the amount of associated water depends on the concentration of the crop contents. Thus, exercise can increase the rate at which water is released in muscle or into the haemolymph, either metabolically or from fuel-associated storage, and glycogen yields more than six times as much water as lipid does. Exercise is also expected to increase the rate of evaporative water loss. Evaporation loss during sustained flight in locusts, when they are metabolizing fat, can balance calculated metabolic gain (Weis-Fogh, 1967b). In other insects, notably large bees, the extra water produced during exercise can exceed concurrent losses. In male bumble bees fed on a 50% sugar solution, metabolic and fuel-associated water released within the haemolymph and gut during flight exceeds that lost by evaporation and excretion (Bertsch, 1984). Similarly, in the large carpenter bee, Xylocopa capitata, flight produces a positive water balance, and the bees urinate just before and during flight (Nicolson and Louw, 1982). Aphisfabae releases drops of urine in the early glycogen-fuelled phase of tethered flight, but not during the subsequent lipidfuelled phase (Cockbain, 1961a,b). If a flight-associated water burden cannot be discharged during flight, it must be sequestered or eliminated during a period of recovery afterwards.

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95

Nectar in flowers is often particularly dilute in the mornings, after equilibrating with the high ambient humidity of night (Corbet et al., 1979), and male insects preparing for reproductive routines that require sustained flight or hovering sometimes take the more concentrated nectar available later in the day and store it in the crop overnight, perhaps first concentrating their crop contents by tongue-lashing (Bombus: Bertsch, 1984; Surholt et al., 1988) or by other means (Syrphus: Gilbert, 1984). If the concentration of sugar in the crop contents exceeds about 30% by weight, nectar will yield less associated water per unit energy than will glycogen. Coevolution between flowers and bees is likely to involve nectar solute concentrations compatible with maintenance of bee water balance (Willmer, 1988). In Periplaneta, the water gain associated with flight can be calculated in two ways. The immediate source of most of the fuel for flight is the glycogen stored in the flight muscles. During flight to exhaustion (12-19 min), the flight muscles supply 91% of the glycogen used (Downer and Parker, 1979). The total glycogen depletion (1.84 mg) would yield 1.03 mg metabolic water with about 4.60 mg associated water, or about 5.63 mg water altogether. A larger water gain is estimated if water production in flight is calculated from measurements of oxygen consumption rates. If a metabolic rate of 38 ml O,/g/h (Polacek and Kubista, 1960) were sustained for 15 min, a 750mg cockroach metabolizing carbohydrate and producing 0.67 mg water/ml 0, (Edney, 1977) would use about 4.5 mg carbohydrate and produce about 2.5 mg metabolic water plus about 11.3 mg associated water or 13.8 mg water altogether. These gains may be offset to some extent by evaporative loss during flight. Redistribution of water may continue during recovery from flight if the glycogen reserves in the flight muscles are replenished from those in the fat body. Glycogen-associated water released from the fat body into the haemolymph during post-flight glycogenolysis might then be taken up in the muscles during glycogen resynthesis there. Table 3 shows the estimated water content of some parts of the body of Periplaneta. It has been suggested that the fat body can act as a reservoir of water in Periplaneta americana (Verrett and Mills, 1975) and in Leucophaea maderae, in which fat body water content falls from about 65% to about 40% as the water content of the egg case increases (Scheurer and Leuthold, 1969). If glycogenolysis releases glycogen-associated water, the arousal response is likely to involve compensation for water transfer from fat body to haemolymph. The gut can act as a water reservoir storing imbibed water which later passes to the haemolymph (Verrett and Mills, 1975; Mullins, 1981). King et al. (1986) suggested the hindgut as a possible source for some of the extra water that entered the haemolymph after flight in Periplaneta, but pointed

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TABLE 3 Estimates of water content (11)of body parts of Periplaneta americana Haemolymph

Salivary glands

Up to 230

Gut

Fat body

Up to 130

60

28 u p to 100 160 Up to 146

u p to 200 in crop 96

Ovarioles References, comments Up to 30 l000mg adult female (total water up to 770 pl): Verrett and Mills (1975) Adult male: Downer (1981a), Downer and Parker (1979) Sutherland and Chillseyzn (1968) Adult male: King et al. (1986) Wall (1970) Bignell (1 98 1)

out that the rectum contains only 3 or 4p1 of fluid. The salivary glands in Periplaneta are another possible source for the extra water that appears in the haemolymph after flight (King et a/., 1986). Those of Leucophaea provide a source of water which may replenish haemolymph water during desiccation; removal of the salivary glands enhanced the rise in haemolymph osmolarity during desiccation (Laird et al., 1972). Transfer of water from haemolymph to gut on feeding in Locusta is attributed to secretion of hypotonic saliva (Bernays and Chapman, 1974). Some of these reservoirs (salivary glands, foregut and hindgut) are lined with cuticle and can accommodate a hypotonic solution without deleterious consequences (Dow, 1986). In others, notably the haemolymph, osmotic adjustment is necessary, and haemolymph osmolarity may be regulated by activation of the rectum-Malpighian tubule system or by transfer of cations between the fat body and the haemolymph (Section 3.2.3). In cockroaches, therefore, the arousal response might include preparation for transfer of water into internal reservoirs such as the fat body, the salivary glands or the gut; or adjustment of haemolymph osmolarity, perhaps involving changes in the pattern of movement of water and ions through the Malpighian tubule-rectum system or ion transfer between haemolymph and a reservoir such as the fat body; or transfer of water to the exterior as liquid or in the vapour phase; or behavioural changes such as changes in frequency of drinking or changes in preferred food osmolarity or ambient humidity.

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Adjustment of haemolymph water content in relation to osmolarity is brought about largely by the Malpighian tubule-rectal system (Maddrell, 1980, 1981) in conjunction with ionic adjustment mediated by the fat body (Spring et al., 1986). Adjustment in relation to volume may be achieved, in part, by modulating liquid-vapour mass transfer in the tracheal system (Corbet, 1988). If these regulatory systems were activated by excitation, they might be capable of compensating for flight-induced changes in haemolymph composition, so averting sudden changes in osmolarity or volume. In the absence of studies of flight-induced changes in the time-course of water flux in cockroaches, these ideas cannot be tested rigorously, but they can be examined for compatibility with existing evidence. In Peripluneta, King et al. (1986) found that haemolymph volume changed little during a brief flight, but over the next 6 h the volume increased by 26% and the osmolarity decreased correspondingly. By 24 h, both features had recovered to their initial values. The water balance of cockroaches in flight is considered in Section 5. Although some early claims should be treated with caution (Phillips et al., 1982), there is evidence both for diuretic factors, which stimulate the transfer of water from haemolymph to gut via the Malpighian tubules, and for antidiuretic factors, which stimulate fluid reabsorption across the hindgut wall, from gut lumen to haemolymph (Table l), and some insects release both a diuretic factor and an antidiuretic factor at the same time (Spring, 1990). Accelerated fluid cycling through the Malpighian tubule-rectal system might speed up the removal of unwanted solutes from the haemolymph (Nicolson and Hanrahan, 1986; Spring, 1990). In Peripluneta, fluid secretion by Malpighian tubules is stimulated by a peptide from the terminal abdominal ganglion (Mills, 1967; Mills and Whitehead, 1970), and Penzlin (1971) found a diuretic factor in the brain and corpora cardiaca. A diuretic factor from Periplaneta is effective on Malpighian tubules of locusts (Aston and Hughes, 1980) and crickets (Coast, 1989). Tubule fluid secretion is stimulated by neuropeptides in Locusta (Morgan and Mordue, 1981; Proux et al., 1988a), and by factors from the corpus cardiacum in Acheta domesticus (Coast, 1988, 1989; Spring, 1990). In Acheta, octopamine produced a significant but very small stimulation of secretion (Coast, 1989). In the hindgut of Periplaneta, extracts of the corpus cardiacum/corpus allatum complex promoted flow from lumen to haemolymph (Steele and Tolman, 1980) and stimulated oxygen consumption and glycogen phosphorylase activation (Tolman and Steele, 1980). Fluid transfer across the wall of the Peripluneta midgut is also influenced by peptide hormones (Sauer and Mills, 1971). A neuropeptide from the corpus cardiacum of Locusta stimulated rectal reabsorption in both locusts and cockroaches, and a similar

98

S. A. CORBET

neuropeptide from cockroaches also affected both species (Proux et al., 1988b). The effect of octopamine on the passage of water across the gut wall does not seem to have been explored. Discharge of water in the vapour phase may take place by cuticular transpiration or via the tracheal system. With a ventilated capsule applied to a small area of cuticle, cuticular transpiration can be distinguished from spiracular water loss (Machin et al., 1986). Using this technique, Toolson and Hadley (1987) showed that in the desert cicada, Diceroprocta apache, cuticular water loss via tracts of pores is under the insect’s control, and is adaptively enhanced at high temperatures. The possibility that water loss in the vapour phase (“integumentary” water loss) may be under hormonal control in Periplaneta has been explored by Noble-Nesbitt and A1 Shukur (1987, 1988a,b; see Machin et al., 1986). At least part of the stress-induced vapour loss in cockroaches may take place via the spiracles (Kestler, 1985; Machin et al., 1986). The possibility that tracheal water vapour loss is significant and capable of adaptive regulation has been considered elsewhere (Corbet, 1988). It might involve ventilation movements, producing cyclical changes of intratracheal pressure, in the whole abdomen or sometimes perhaps in other cavities including air sacs. Whether abdominal ventilation movements function in water regulation, or for oxygen supply, or both, arousal might be expected to lead to an increase in their frequency or intensity coordinated with control of spiracular closing. Release of octopamine by iontophoresis within the metathoracic ganglion of Schistocerca increased the intensity and frequency of ventilation movements (Sombati and Hoyle, 1984a,b). The spiracle muscles of Gromphadorhina portentosa contain octopamine (Evans, 1978), and those of Schistocerca are sensitive to octopamine (P. D. Evans, personal communication). The possibility that peripheral neurosecretory cells associated with the spiracles of Schistocerca may contain octopamine and neuropeptides is discussed by Myers and Evans (1988). A neurohormone has been implicated in the regulation of spiracular closing patterns in Periplaneta (Bhatia and Tonapi, 1968). 3.2.3 Ions The major cation in cockroach haemolymph is sodium (Spring et al., 1986). When Periplaneta haemolymph volume changes as a result of dehydration and rehydration, haemolymph sodium and potassium concentration and osmolarity change relatively little, because water loss is compensated for by removal of sodium from the haemolymph. Only about 4% of the sodium removed is excreted (Hyatt and Marshall, 1985); most is sequestered in the

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fat body, probably in urate cells. Studies on uptake and release of sodium by the fat body in vitro gave evidence for hormonal control (Hyatt and Marshall, 1985; Spring et al., 1986).

4 Physiological correlates of flight, paralysis and insecticide poisoning

4.1

FLIGHT

King et al. (1986) monitored changes in the volume and composition of the haemolymph of Periplaneta americana during and after brief flights lasting up to 1Omin. After an inferred initial steep rise, haemolymph octopamine concentration fell steadily over the next 24 h. The concentration of hyperglycaemic neuropeptides was not measured, but is likely to have risen because release is stimulated by octopamine (Downer et al., 1984). The amount of trehalose in the haemolymph peaked briefly early in flight, fell, then rose steadily during rest to reach a second, higher peak about 6 h later, and fell again towards 24 h. Trehalose is released from the fat body of Periplaneta under the influence of octopamine (Gole and Downer, 1979) and hypertrehalosaemic neuropeptides (Steele, 1961; Hanaoka and Takahashi, 1976). Because the amount of trehalose in the haemolymph pool depends on the relative rates of supply and removal, flux studies would be necessary for a full interpretation. While the amount of trehalose in the haemolymph increased by about 65%, its concentration rose by only about 30%, because the haemolymph water content was augmented by about 30 p1(26%) over the first 6 h. Not all of this can be accounted for by metabolic or fuel-associated water (Section 3.2.2); at least half is likely to have come from another source. Accelerated fluid reabsorption in the hindgut, stimulated by neuropeptides (Table l), may have facilitated water transfer between the gut lumen and the haemolymph. Changes in haemolymph trehalose concentration were to some extent mirrored by changes in the major cation, sodium. Despite this weak compensation, haemolymph osmolarity did not remain constant. It peaked early in flight, decreased over the following 6 h in proportion to the increase in haemolymph volume, and had recovered to pre-flight values by 24 h. King et al. (1986) showed that species can differ markedly in the physiological consequences of flight. Disturbance of water balance is associated with flight in locusts and in Rhodnius, as well as in Periplaneta, but haemolymph volume increases only in Periplaneta. In locusts, diuretic hormone is released during flight (Goldsworthy, 1976), and water loss by enhanced Malpighian tubule activity during flight is presumably balanced by

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gain of metabolic water and increased rectal reabsorption (Goldsworthy, 1976). Diuretic hormone is also released during flight in Rhodnius (Gringorten and Friend, 1979), in which haemolymph volume falls by up to 50% during flight because any gain of metabolic water is more than compensated by diuresis and urine discharge (Gringorten and Friend, 1979). Evidently enhanced secretion by Malpighian tubules is not balanced by accelerated rectal reabsorption in this case. The finding that the percentage of water in Rhodnius haemolymph did not fall correspondingly implies some osmotic regulation.

4.2

PARALYSIS AND INSECTICIDE POISONING

Beament (1958) immobilized cockroaches for long periods, so that they could struggle but not escape. In the hours following release, they developed symptoms of paralysis, with loss of movement, decreased haemolymph volume, increased fluid in the gut (Beament, 1958), weight loss, and loss of muscular response to neural stimulation (Cook and Holt, 1974). They eventually died. Paralysis was a consequence of struggling or enforced movement; the individuals that subsequently became paralysed were those that showed an increased rate of oxygen uptake during immobilization (Heslop and Ray, 1959), and paralysis could be averted by cutting off or denervating the struggling limbs or engendered by enforcing sustained activity beyond the voluntary stopping point by means of electrical or mechanical stresses (Beament, 1958) or, in Nauphoeta, by forcing subordinate cockroaches to continue fighting (Ewing, 1967). Symptoms of DDT poisoning resemble those of paralysis induced by enforced activity (Beament, 1958). Both types of paralysis depend on a similar initial burst of respiration (Heslop and Ray, 1959), and the stressinduced decrease in haemolymph volume and increase in hindgut fluid volume (Beament, 1958) and weight loss (Cook and Holt, 1974) find parallels in locusts (Samaranayaka, 1974, 1977) and other species (Casida and Maddrell, 197 1) poisoned with various insecticides. Cockroaches paralysed after stress or by DDT poisoning contained a blood-borne factor capable of inducing symptoms of paralysis when injected into untreated recipients (Sternberg and Kearns, 1952; Beament, 1958; Sternberg et al., 1959). The blood-borne factor produced by DDT poisoning in Periplaneta was thought to be a peptide with hyperglycaemic effects (Granett and Leeling, 1972). Recent studies implicate tamine as a modulator of aminergic function in stressed and insecticide-poisoned cockroaches (Hayakawa et al., 1987, 1988). Enforced activity and insecticide poisoning both raise the haemolymph concentration of taurine (Jabbar and Strang,

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1985), which inhibits octopamine release from the central nervous system, octopamine-stimulated cyclic AMP production in haemocytes and haemolymph trehalase activity (Hayakawa et ul., 1987, 1988). In Schistocerca topical application of lindane causes glycogen depletion in the fat body and the elevation of haemolymph lipid concentration (Samaranayaka, 1974). These effects are prevented by decapitation and so are probably mediated by AKH released from the glandular lobes of the corpus cardiacum. Samaranayaka (1974) showed that lindane poisoning causes depletion of neurosecretory material in the glandular and storage lobes of the corpora cardiaca. Several insecticides applied topically (not including DDT) mimic stress in that they trigger the release of octopamine into the haemolymph in locusts (Davenport and Evans, 1984a). Taurine concentrations in the haemolymph are increased by enforced activity and by insecticide poisoning in locusts, as in cockroaches (Jabbar and Strang, 1985). In Schistocercu, treatment with lindane increases gut water content and decreases haemolymph volume, even when a neck ligature prevents the involvement of the mouth (Samaranayaka, 1977). Treatment of Rhodnius with various insecticides, topically or by injection, releases from the mesothoracic ganglionic mass a diuretic factor which stimulates fluid secretion by the Malpighian tubules in an in vitro assay (Casida and Maddrell, 1971). The frequency of contraction of locust Malpighian tubule muscle is increased by an “autoneurotoxin” from the haemolymph of DDT-poisoned Periplaneta (Flattum et al., 1973), and contraction of Malpighian tubule muscles in Periplaneta is stimulated by a factor from the insecticide-treated central nervous system (Crowder and Shankland, 1972). In cockroaches stressed by immobilization, an early symptom is failure to absorb droplets of water placed on the surface of the cuticle (Beament, 1958). Further evidence that evaporative water loss is enhanced by paralysis comes from the observation that condensate formed on the lids of pots containing paralysed cockroaches, but not in pots containing unparalysed controls (Cook and Holt, 1974). In cockroaches treated with pyrethrum, topically or by injection, small droplets of fluid appear on the surface of the cuticle (Ingram, 1955).

5 5.1

Discussion FIGHT OR FLIGHT RESPONSE-OR

ACTIVATION CONTINUUM?

A stimulus of low intensity, such as a moving shadow, a gentle puff of air or a transient whiff of a noxious odour, might lead to a behavioural responsealerting the sensory system; producing crypsis or threat behaviour; or

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preparing for flight. Octopamine release, locally within the nervous system or more generally into the haemolymph, may accompany and perhaps potentiate and modulate these early responses. A more intense or prolonged stimulus might cause further release of octopamine, accentuating these responses and resulting in neuromodulation, cardioacceleration and other effects on visceral muscle, accelerated secretion by the Malpighian tubules and the mobilization of metabolic substrates. In most natural situations the insect escapes from this positive feedback loop by removing itself from the stimulus. The response can reach pathological proportions when the insect is prevented from doing this, as by persistent attack by a dominant aggressor in a confined space, or by immobilization, confinement in a rotating vessel, or persistent excitation of peripheral receptors or of the central nervous system by insecticides. Increased concentrations of octopamine, or the locomotion itself, or both, may stimulate release of neuropeptides from the corpora cardiaca and other sources. These neuropeptides would in turn elicit longer-term effects including perhaps further stimulation of the Malpighian tubule-rectal system and transfer of sodium and other ions between the haemolymph and the fat body or gut lumen. Redistribution of ions and water, accelerated by the neuropeptides, would tend to compensate for activity-induced changes in water status or in haemolymph composition. When the stimulation and the activity stopped, rapid uptake might quickly reduce the haemolymph titre of octopamine (Evans, 1985a); neuropeptide titres might also fall. Rates of transfer of water and ions would wane and osmotic and ionic regulation would slow down, so that the insect could compensate only slowly for any osmotic or ionic imbalance resulting from continuing redistribution of water or metabolic reserves, for instance, those required for full recovery to prepare for the next flight.

5.2

QUANTIFYING THE ACTIVATION CONTINUUM

The intensity and duration of a stimulus would affect the outcome of arousal. In Periplaneta, a transient stimulus of low intensity, such as an encounter with another individual, elicits a gentle startle response; a transient stimulus of high intensity elicits the acute fight or flight response, and rapid escape ends the stimulation. Sustained low-intensity stimulation, as produced by crowding, may raise the general level of activity of each individual. Sustained and inescapable high-intensity stimulation may elicit activity sustained beyond the voluntary stopping point (hyperactivity), and the activity itself may be a cause of paralysis and death.

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How can the state of arousal be expressed quantitatively? It is desirable to have an index that is readily quantified and generally applicable among different species (see Tobler, 1983). A cockroach’s state of arousal is to some extent reflected in its posture and behaviour. Surholt et al. (1988) recognize grades of rest in male bumble bees, and Kaiser (1988) describes changes in posture and responsiveness associated with “sleep” in honey bees. Andersen (1968) has suggested that some measure of responsiveness to a standard stimulus might give a useful index of the depth of “sleep”, over a range of states from thanatosis or “death feigning” in which repeated stimulation decreases the probability of a response, through “sleep” in which repeated stimulation may be required to elicit a response, to an aroused state where the probability of a response is so high that some activity appears to be spontaneous. Posture and behaviour might contribute to a quantitative index of the state of arousal if a consistent relationship with the titres of octopamine and neuropeptides could be demonstrated. The possible relationship between octopamine levels and activity is discussed by Davenport and Evans (1984a).

5.3

VOLUNTARY CESSATION OF ACTIVITY

If insecticides were to trigger the orchestrated hormone release that characterizes the arousal response in a healthy insect, rather than the release of a cocktail of functionally unrelated hormones, pathological consequences might follow if insecticide-induced neural or neuromuscular hyperactivity were to continue beyond the insect’s voluntary stopping point. Voluntary cessation of activity is not always attributable to exhaustion of metabolic substrates (Downer and Parker, 1979), and its causation deserves further attention, because one effect of insecticides seems to be to interfere with this cessation. Activity stops very suddenly after a quick dash in Periplaneta, but might dwindle more gradually in an insect capable of sustaining flight, and maintaining water balance, over a period of hours, such as Aphis fabae (Cockbain, 1961a,b). One well-documented example of the processes involved in the voluntary cessation of activity may be the change of “mood” in Aphis fabae towards the end of a migratory flight, when its tendency to settle increases (Kennedy, 1966). Failure to stop activity might result in a mismatch between the physiological consequences of activity and the hormone-controlled processes that normally compensate for these and assist recovery from them. Thus, insecticide-induced neural stimulation might release either too much or too little octopamine or neuropeptide in relation to activity. It might induce octopamine or neuropeptide release persisting long after activity has ended,

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producing pathological overcompensation, or it might deplete, or inhibit release of, octopamine or neuropeptide, so that the consequences of sustained activity are not fully compensated. The high levels of taurine in the haemolymph of stressed and insecticide-poisoned cockroaches, which can inhibit octopamine release (Jabbar and Strang, 1985; Hayakawa et al., 1987, 1988), raise the possibility that the effect of these treatments on octopamine titre is chemically mediated. A cockroach’s previous history of activity or hydration influences its physiological responses to elements of the arousal syndrome such as ionic compensation for changed haemolymph volume (Spring et al., 1986). It might therefore be expected that the effectiveness of insecticides could be enhanced by careful timing of application in relation to the water status or die1 periodicity of activity of the insect, or by manipulation of water status by a component of the treatment.

5.4

INSECTICIDE DESIGN

Arousal involves a chain of processes, beginning with stimulation of peripheral sense organs and including afferent neural messages leading to the release of octopamine and neuropeptides and effects on metabolism and water and ion balance. It has been suggested that new insecticides might be designed to mimic hormones involved in this process (Samaranayaka, 1974; Hollingworth et al., 1984). The formamidine insecticides target octopamine receptors (Evans, 1985a), and a new octopaminergic insecticide has been designed (Jennings et al., 1988). In view of the difficulties and uncertainties about the penetration of insecticides into insects, rational design of an insecticide might aim to mimic the initial effect on peripheral sense organs rather than some later stage in the chain, and to produce a sustained effect in place of the transient stimulus in relation to which the insect’s acute fight or flight response has evolved. Low doses of DDT affect the nervous system peripherally; only at higher doses are central nervous effects seen (Haynes, 1988). In this context Periplanera may not be a very suitable model, because the startle response in cockroaches is largely mediated by cercal mechanoreceptors, stimulation of which causes the insect to turn away from the source of air disturbance (Ritzmann, 1984), rather than by chemoreceptors. Chemical stimuli are more relevant to the design of insecticides. Preliminary screening might aim to identify compounds that elicit readilymonitored elements of the arousal response such as thermogenesis or conspicuous startle behaviour. Further studies would attempt to see which of these compounds would produce persistent rather than transient effects.

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If the aim is to produce lasting effects on peripheral sense organs, it might be possible to design candidate compounds chemically related to odours known to be effective at low concentration for the species concerned. In honey bees many related compounds mimic alarm pheromones in eliciting an arousal response, and several established insecticides (including DDT, permethrin and some other pyrethroids) are behaviourally active. A study of structure-activity relationships revealed common features among the alarm pheromones and compounds that mimic them. The study was made as a basis for the design of honey bee repellents (Melksham et al., 1988), but might also give clues useful more generally in the design of insecticides capable of eliciting pathological arousal. Hormones associated with arousal may influence other components of the endocrine system and so lead to longer-term changes. Octopamine is present in the corpora allata of the cockroach, Diploptera punctata, and acts to decrease rates of juvenile hormone (JH)synthesis (Thompson et al., 1988). Allatostatins, which are neuropeptides, have been found in the brain of Diploptera (Khan et al., 1988; Woodhead et al., 1989). Crowding in locusts can elevate haemolymph octopamine levels (Davenport and Evans, 1984a) and can lead to long-term physiological changes in which JH is implicated (Hardie and Lees, 1985). It has been suggested that this and other crowding effects on insects are mediated by sustained arousal induced by encounters (Davenport and Evans, 1984a). The apparent effect of AKH in Locusta of suppressing vitellogenin production (Applebaum and Moshitzky, 1988) also implies a long-term effect of a neurohormone associated with arousal. Changes involving the corpus allatum could ultimately have far-reaching effects, for example on reproduction or migration. Thus, arousal may affect an insect’s physiological condition for minutes via octopamine, for hours or days via neuropeptides, or even perhaps for weeks via JH. Locomotion is an expected component of the response to moderate or intense arousing stimuli, and repellency at sublethal doses has been recorded for many insecticides (Haynes, 1988). Since this locomotion would often remove the insect from the stimulus, the sustained stimulation necessary for an insecticide to kill the insect would require treatment of an area from which the insect could not escape, or use of an insecticide whose effect persisted away from the source, perhaps because of irreversible changes in peripheral sensilla or adsorption on to the insect’s cuticle. The titre of haemolymph octopamine in Periplaneta fluctuates through the light/dark cycle, with peaks coincident with those of locomotory activity (Davenport and Evans, 1984a, 1985), and circadian fluctuations in male responsiveness to female pheromones in the flour moth Ephestia kuehniella (Gordon, 1980) may represent a similar phenomenon. Circadian variation in

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susceptibility to insecticides is thus to be expected, and has been reported (e.g. Shipp and Otton, 1976). The constellation of physiological and behavioural responses, which together constitute an adaptive programme which defines a transient or longlasting state of arousal, makes it necessary to specify the position on the activation continuum of insects involved in physiological experiments. Davenport and Evans (1984b) draw attention to this in relation to “Monday morning” locusts which are hyperactive after a weekend of food deprivation. Further, as several authors have pointed out (e.g. Davenport and Evans, 1984a; King et al., 1986; Machin et al., 1986; Lange et al., 1989), insects are liable to move abruptly into a state of arousal in response to stress such as handling in preparation for an experiment. The state of arousal can be exploited naturally, as by fighting Nuuphoeta (Ewing, 1967), and also artificially, in so far as some insecticides kill by eliciting exaggerated symptoms of arousal. If the chain of effects involved in arousal and recovery, perhaps lasting for up to several days, can be explored in detail, it may be possible to design insecticides that mimic specific links in the chain, or to potentiate the effects of an insecticide by selecting or manipulating conditions of application so that the insecticide reinforces a pre-existing condition of stress.

Acknowledgements

I thank Philip Corbet, Peter Evans, William Kirk, Peter Lawrence and Simon Maddrell for reading drafts of sections of this paper; their suggestions and comments, and those of two referees, have improved it greatly. I am also grateful to Sir James Beament, who has illuminated various aspects of the problem in discussion.

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The Genetics of Division of Labour in Honey Bee Colonies Robert E. Page, Jraand Gene E. Robinsod “Departmentof Entomology, University of California, Davis, CA 95616, USA bDepartmentof Entomology, University of Illinois, Urbana, IL 61801, USA

1 Introduction 118 2 Genetics of the honey bee 119 2.1 Haplodiploidy 119 2.2 Sex determination 119 2.3 Mating behaviour 120 2.4 Polyandry and sex determination 121 2.5 Genotypic composition of colonies 124 2.6 Polyandry and genotypic variability 126 2.7 Summary 126 3 Division of labour 128 3.1 Patterns of division of labour 128 3.2 Plasticity in division of labour 130 3.3 Hormonal regulation of division of labour 131 3.4 Hormonal regulation of plasticity in division of labour 134 4 Genetic basis for division of labour 136 4.1 Components of division of labour 137 4.2 Summary 143 5 Colony-level integration of individual behaviour 143 5.1 Behavioural variability within a subfamily 144 5.2 Plasticity in division of labour independent of age polyethism 144 5.3 Behavioural dominance 146 5.4 Genetic basis for “idiosyncratic”, “elite” and “reserve” workers 148 5.5 Summary 149 6 The evolution of division of labour 149 6.1 Self-organization 149 6.2 Natural selection operates on parameters of dynamic systems 154 6.3 The organizational structure of honey bee societies 156 6.4 Genotypic variability and adaptation 157 7 Conclusions 162 Acknowledgements 163 References 163 ADVANCES IN INSECT PHYSIOLOGY VOL. 23 ISBN &12424223-0

Copyright 0 1991 Academic Press Limited All rights of reprodurnon in any form reserved

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G. E. ROBINSON

Introduction

The social organization of insect colonies has fascinated natural historians for thousands of years. Aristotle wrote in History of Animals about a division of labour among honey bees that is based on age. He observed that individuals that flew into and out of a beehive, some with loads of pollen, had less “hair” on their bodies than bees that worked in the hive. Because of his anthropocentric view of development, Aristotle erroneously concluded that the hairy, hive bees are older than the foragers. The phenomenon of “age polyethism”, the age-dependent performance of tasks, is now well documented; we know that field bees are older than hive bees and lose their body hair with age. The genetic basis of division of labour is a relatively new area of investigation. It has developed only recently through studies of the honey bee, Apis mellifera L., and is the subject of this review. Advances in this field have come first from studies of honey bees, rather than other social insects, because more is known about the genetics of the honey bee than any other species. Scientific investigation of honey bee genetics spans more than a century, beginning with the discovery of Dzierzon (1 845) that male honey bees develop from unfertilized eggs. Controlled mating of queens and drones was attempted unsuccessfully by Gregor Mendel and many others before it became possible with the development of instrumental insemination techniques about 50 years ago (see Laidlaw, 1987, for review). Comparable techniques of instrumental insemination do not exist for any other social insect. This technology was motivated partly because of the economic value of honey bees as honey producers and pollinators, but also because of the fascination that biologists have with the organization of honey bee societies and the seemingly selfless behaviour of individual workers. We begin this review by describing genetic characteristics that are unique to the Hymenoptera, including honey bees. We then show how these characteristics, when combined with the mating behaviour of queens, affect the genetic “structure” of honey bee colonies and populations of colonies. We follow with results that demonstrate the fundamental elements of division of labour among workers and suggest how colony-level natural selection adapts populations of colonies to their environment via changes in the behaviour of individual, effectively-sterile workers. Finally, we present theoretical models that suggest that some properties of division of labour, such as the occurrence of labour specialists and the ability to reallocate labour in the face of a changing environment, are a consequence of selforganization that may be intrinsic to many types of complex systems, including insect colonies.

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2 Genetics of the honey bee 2.1

HAPLODIPLOIDY

Haplodiploidy is an important genetic characteristic of the Hymenoptera. Males are haploid with just one set of chromosomes, while females are diploid with two sets. Dzierzon (1845) first hypothesized that male honey bees are derived from unfertilized eggs. He proposed that a drone has a mother, but no father. Nachtsheim (1913) verified this hypothesis by demonstrating that female bees have 32 chromosomes (16 pairs) while drones have only 16. Subsequently, it has been shown that this form of parthenogenesis, called arrhenotoky, occurs throughout the Hymenoptera (see Crozier, 1977).

2.2

SEX DETERMINATION

In the honey bee, and indeed many other Hymenoptera (see Page, 1986; Page and Kerr, 1990), genic level determination of sex is apparently under the control of a single gene with multiple alleles. Individuals that are heterozygous at this locus develop into females while homozygous individuals develop into diploid males. Normal, haploid males are effectively homozygous because they have just one set of chromosomes. Whiting and associates discovered the genic mechanism of sex determination in the parasitic wasp Habrobracon juglandis and first demonstrated the existence of biparental males (Whiting, 1943). These males were derived from fertilized eggs of inbred stocks. The ability to instrumentally inseminate queens enabled scientists to address the question of sex determination in honey bees. Mackensen (1951) observed that many of the larvae derived from queens that had been inseminated with semen from their own sons disappeared from the wax cells used by bees to raise immature workers. He proposed that this was a consequence of lethal homozygosity at a sex locus analogous to the one described by Whiting. Mackensen (1955) went on to estimate that there were 1 1 different sex alleles in the North American population that he studied. Other estimates for different populations followed: Laidlaw et al. (1956) estimated 12 sex alleles in a population in Piracicaba, Brazil, Woyke (1976) estimated only six sex alleles in the honey bee sanctuary of Kangaroo Island off the coast of Australia, and Adams et al. (1977) estimated a minimum of 18.9 sex alleles in Rio Claro, Brazil.

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The Habrobracon model of sex determination was further supported by the studies of Rothenbuhler (1957). Rothenbuhler found wild-type patches of eye tissue in mosaic drones that were derived from queens that were homozygous for recessive eye mutations. He argued that the wild-type male eye facets must have been biparental in origin, and were diploid. Woyke (reviewed by Woyke, 1986) demonstrated that larvae that hatched from viable eggs, but were removed from their cells by workers within about 6 h of hatching, were diploid drones and proposed that they were homozygous at the sex locus (Woyke, 1963a,b). Woyke (1963~)developed a technique for raising diploid males to the adult stage and provided genetic proof that they arise from fertilized eggs (Woyke, 1965). Woyke and Skowronek (1974) studied spermatogenesis in diploid males. They found that it was similar to that in haploid males and, consequentIy, resulted in the production of diploid spermatozoa. Diploid males, however, have testes that are much smaller than haploid males and produce fewer sperm cells (Woyke, 1973).

2.3

MATING BEHAVIOUR

Queen honey bees mate on average with a large number of drones. Estimates range from about 7 to 17 (reviewed by Page, 1986). Matings take place while queens and drones are in flight, within a few days after queens emerge as adults. Each queen takes a series of mating flights over several days and mates with up to 17 different males on any given flight (Woyke, 1962). Each male mates just once, deposits about 6 million spermatozoa (Kerr et al., 1962) into the oviducts of the queen, and then dies. Queens return to the hive after the mating flight. A total of about 6 million (Kerr et al., 1962) of the sperm deposited by all the drones into the oviducts of the queen migrate by active and passive processes (Ruttner and Koeniger, 1971) into the spermatheca (the sperm storage organ) over a period of about 40 h (Woyke, 1983). The sperm that enter the spermatheca are fairly well mixed, representative of most of the drones, and remain viable throughout the life of a queen. This was demonstrated by instrumentally inseminating queens that were homozygous for two recessive, visible mutations, cordovan integument and tan eye colour (Laidlaw and Page, 1984). Homozygous expression of the cordovan gene (cd) turns areas of the integument that are normally black into a brown colour. The tan gene (sf) is an allele at the locus for snow (s) eye colour. The snow (s) and tan (sf) alleles are recessive to the wild-type, black eye colour ( + ) while snow and tan alleles are co-dominant and produce red-coloured eyes in combination. Therefore, double homozygous (cdlcd; sr/sr)queens produced six classes of visibly distinguishable worker progeny when they are

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inseminated with the semen of six drones carrying different combinations of markers (see Table 1). TABLE 1 Test genotypes

Progeny Queens

Drones

Integument

Eye colour

+,+

Wild type Wild type Wild type

Tan Red

Cordovan Cordovan Cordovan

Wild type Tan Red

+ , s'

+,S

cd/cd,sl/sl

+

cd, cd, s' cd, s

Wild type

Worker progeny of each of the six drones emerged from the same brood combs during the same period of time for each queen (see Fig. 1). The proportional representation of each drone was not equal, probably because males produced different quantities of sperm and the proportions of progeny of each male fluctuated over time. These results demonstrated, however, that some sperm of each male reached the spermatheca, were mixed, although not completely, and were used for fertilizing eggs during the same relatively short period of time.

2.4

POLYANDRY AND SEX DETERMINATION

Page (1980) proposed that the extreme expression of polyandry demonstrated by queen honey bees may have evolved as a consequence of the genic mechanism of sex determination. The actual number and distribution of sex alleles is determined in natural populations by the rate of mutation at the sex locus, selection intensity and the effective size of the breeding population (Yokoyama and Nei, 1979). If we assume that all heteroallelic combinations have equal fitness, and make the usual Hardy-Weinberg population assumptions of infinite population size, no migration or mutation, and random mating, then, at equilibrium, all alleles are expected to be at equal frequency, l/k, where k is the number of alleles (Wright, 1939, 1965). With these assumptions, the probability for each mating that a queen will mate with a male that has an allele identical to one of her two alleles is 2/k. If a queen mates with n males, the probability P,, that she will mate with exactly y males

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PROGENY COUNTS

8K .20 I I

1

2

I

I

3

4

5

SAMPLE FIG. 1 Pattern of sperm use during a 14-month period for a single queen inseminated with the semen of six different males. Each line represents the proportion of worker offspring belonging to each of the six subfamilies that emerged from a brood comb during a single sample period. (Data from Laidlaw and Page, 1984.)

with alleles matching one of hers is, from the binomial probability density function,

If each male contributes equal numbers of spermatozoa to the queen’s spermatheca, and the sperm of the different males are used randomly, then the expected proportion of diploid males produced by a colony as a consequence of mating with n males is y/2n. Therefore, the expected production of functional females (workers and queens) is 1 -y/2n. The expected proportion of functional females (F) in the population is determined only by the number of sex alleles that are segregating, F = 1 - l/k (Shaskol’skii, 1968; Woyke, 1976; Adams et al., 1977; Page and Marks, 1982). The number of matings by individual queens affects the distribution of female production among colonies (see Page and Metcalf, 1982; Page and Marks, 1982; see Fig. 2). If the sperm of different mates are used at random by queens, and if each male contributes an equal number of spermatozoa, then the variance in functional female production among colonies in a population with k alleles and n matings per queen is: 1/2n (l/k)(l - 2/k) (Page and Marks, 1982). The effect of the proportion of diploid males produced by a colony on the fitness of a queen is not known. However, models that use biologically realistic non-linear relationships between diploid drone production and

123

GENETICS OF DIVISION OF LABOUR IN HONEY BEE 1.00 0.90

‘i

0.00 0.70 0.60 0.50

0.40 0.30

0.20

,“ 0.10 al

3

0-1.00

k=IO,n=IO

0.70

I

E/? 0.20

0. I 0

.50 0.60

T

1.00 0.50 0.60 0.70

0.00 0.90

1.00

Viability (V)

FIG. 2 Frequency distribution of brood viability among queens in different populations. Each population has a constant number of sex alleles ( k = 10). The number of matings (n) varies among the populations but is constant within populations. (From Page and Metcalf, 1982, with the permission of the University of Chicago Press.)

fitness, such as those described by concave, convex and sigmoid functions (Page, 1980; Crozier and Page, 1985), result in differential fitness among queen genotypes that specify more or fewer matings (Fig. 3). Concave fitness functions favour polyandry while convex functions favour monandry. Page (1980) argued that honey bees are most likely to have a concave relationship, because they produce reproductives relatively late in the life cycle of a colony. The worker population at this time is large and every additional potential worker lost as a consequence of homozygosity at the sex locus “costs” the colony less than if the colony was in an earlier stage of development. Crozier and Page (1985) generalized this model to the social Hymenoptera and suggested that it explains the association of polyandry with species that have large worker populations (see also Cole, 1983). Crozier and Page (1985) evaluated eight different hypotheses for the evolution of polyandry in the social Hymenoptera on the basis of their

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FEMALES FIG. 3 Possible relationships between fitness of queens and brood viability (females) due to the production of diploid males. I is concave, I1 is sigmoid, and I11 is convex.

generality, plausibility and how well they explained the observed relationship between colony worker population and mating behaviour. Although they favoured the one presented here, there are others that remain plausible (see also Sherman et al., 1988, and Section 6.4).

2.5

GENOTYPIC COMPOSITION OF COLONIES

The mating behaviour of a queen honey bee profoundly affects the genotypic “structure” of a colony. A honey bee colony typically consists of a single long-lived queen, anywhere from zero to several thousand drones (depending on the time of year), and usually tens of thousands of workers. Colonies consist of at least several different subfamilies of workers because of polyandry and the mixing of sperm from different males within the spermathecae of queens. Members of the same subfamily are called “super sisters” (Page and Laidlaw, 1988). They share both a queen mother and a drone father and, assuming random mating of queens and drones, have on average 75% of their genes in common by descent (Fig. 4 and Table 2). Individuals that belong to different subfamilies are half sisters, they are derived from different, unrelated drone fathers and share an average of 25% of their genes in common. Use of the proper term “super sister” is important because, under haplodiploidy, true full-sister relationships can also exist if two “brother”

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n

/' /

FIG. 4 A hypothetical pedigree demonstrating different relationships possible within and among hymenopteran families. Female symbols, A-D, reproductives; uncrossed female symbols, a and b, their female offspring; male symbols, x-z, haploid male reproductives; solid lines, egg gametes; dashed lines, sperm gametes.

TABLE 2 Relationships of individuals in Fig. 4 Individuals

Relationship"

Gb

bl,b2 bl,b3 or b2,b3 a, b l a, b3 b3,b4

Super sister Full sister Genomic half sister Paternal half sister Maternal half sister

0.75 0.50 0.50 0.25 0.25

OBased on the genetic-pairing terminology of Page and Laidlaw (1988). hGis the pedigree coefficient of relationship (Pamilo and Crozier, 1982).

drones (derived from the gametes of the same mother queen) inseminate a queen, Since drones are haploid and have no father, all of the sperm they produce contain identical genomes (with the exception of random mutations) that are derived from the egg gamete produced by the queen. However, any two eggs laid by a queen contain different genomes due to recombination, therefore, the sperm produced by males derived from them will differ. This results in a genetic relationship between individual progeny of two brother drones of 0.50, the equivalent of full sisters in diploid systems. This genetic relationship is also equivalent to that between diploid full sisters with respect to the origin of the genomes (Page and Laidlaw, 1988).

126

2.6

R.E. PAGE JR AND G. E. ROBINSON POLYANDRY A N D GENOTYPIC VARIABILITY

Polyandry affects the distribution of additive genetic variance within populiations. The average genetic relationship between individuals within colonies decreases with an increase in the number of times queens mate (assuming random mating). At the same time, the relationships of individuals thlat belong to different colonies increase. This relationship can be expressed by: V,=(l-r)V,

where V, = the total additive genetic variance in the population; V , = the variance between individuals within colonies; and r = the coefficient of genetic relationship between individuals within colonies (Crow and Kimura, 1970, p. 141). When considering how the genetic structure of a colony influences its social structure, the effect of polyandry on the distribution of genotypes. is more important than its effect on additive genetic variance (Crozier and Page, 1985). This can be demonstrated by considering the case for a shgle locus with two alleles. A colony where all workers are heterozygous at this locus has maximum genetic diversity but zero genotypic diversity. Crozier and Page (1985) showed how genotypic diversity increases within colonies of a population as a function of increasing numbers of segregating alleles a t a single locus (all alleles are assumed to be at equal frequencies) and increasiing numbers of matings. They used for their measure of genotypic diversity ithe probability that two individuals selected at random from the colony have different genotypes. Assuming that there are two equally frequent alleles at each locus, we show how genotypic diversity is distributed within and among colonies as a function of both the number of loci and the number of matings (Fig. 5).

2.7

SUMMARY

Haplodiploidy is a genetic system that results in the production of haploid, parthenogenic males and diploid sexually-produced females. The genic mechanism of sex determination that allows haplodiploidy to function results in a severe genetic load in honey bees when matings occur betwleen related individuals. This genic mechanism may have been one of the important factors in the evolution of the polyandrous mating system. Polyandry results in genotypic diversity within colonies with colonies composed of a large number of subfamilies. Different subfamilies have different

127

GENETICS OF DIVISION OF LABOUR I N HONEY BEE 0.8

,

&

213 LOCI LOCUS

-0-

0.0

0

4LOCl 1

2

3

4

3

4

MATINGS

00 0

1

2

MATlNGS

FIG. 5 The distribution of genotypes within and among colonies of populations as a consequence of increasing numbers of loci and matings. Values were derived from computer-generated mating tables assuming two, equally frequent alleles at e:ach locus, and that the progeny frequencies of all male mates are equal. It is also assumed that colonies have infinitely large worker populations and there are an infinite number of colonies within the population. (Top) The average probability for the population that two individuals drawn at random from any single colony will have identical genotypes; (Bottom) the average for the population of the proportion of the total possible number of genotypes represented within colonies.

genotypic distributions resulting from the genomic contributions of the different drone fathers and recombination in the queen. This subfamily genotypic structure has been used to study the evolutionary genetics of division of labour and will be discussed in Section 4.

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3 Division of labour In the more advanced eusocial species there is a reproductive division of labour between anatomically distinguishable primary reproductives and usually less reproductively-capable workers (see Wilson, 1971; Oster and Wilson, 1978). In the termites, members of both the reproductive and worker castes are both male and female while in the matrifilial social Hymenoptera, the worker caste consists solely of females. Within the worker caste there is usually a further division of labour such that individuals vary in their probabilities of performing the different tasks associated with social living (see Wilson, 1971; Oster and Wilson, 1978). As a consequence, workers become specialized in particular activities. Division of labour among workers is fundamental to the organization of complex insect societies. It is proposed to be more efficient for colonies to accomplish tasks with collections of specialized individuals than with undifferentiated workers (Oster and Wilson, 1978; Wilson, 1985a,b; Jeanne, 1986a,b). The activities that are performed by an individual often change with age, a form of behavioural development known as age polyethism. In a minority of ant species and nearly all termites, the division of labour among workers is also associated with morphological differences. Individuals belonging to different physical subcastes behave differently, thus increasing the amount of behavioural variability within colonies.

3.1

PATTERNS OF DIVISION OF LABOUR

Worker honey bees pass through distinct behavioural phases during their 4- to 7-week life. Each phase is marked by the performance of one or more tasks (Fig. 6). Young bees labour in the nest, while older individuals forage (reviewed by Free, 1965; Seeley, 1985; Winston, 1987), a pattern that is thought to be universal among species of highly evolved social insects (Wilson, 1971). During each behavioural phase a worker may be said to belong to a particular “age caste”, a group of similar-aged individuals, located in a distinct region of the nest, that performs more or less the same kinds of jobs for a sustained period of time (Oster and Wilson, 1978). Four worker age castes have been suggested: cell cleaning, brood and queen care, food storage, and forager (Seeley, 1982). These results are consistent with earlier findings of age-dependent changes in behaviour (Gerstung, 1891-1921; Rosch, 1925; Lindauer, 1952; Ribbands, 1952; Sakagami, 1953a). In contrast, Kolmes (1985) reported that bees do not exhibit distinct ontogenetic

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TASKS FIG. 6 Diagram of a model of division of labour. (From Page et al., 1989b, with the permission of Westview Press.)

behavioural phases prior to the onset of foraging. Recently Seeley and Kolmes (1991) demonstrated that age polyethism for nest duties does indeed exist, and was not evident in the earlier study of Kolmes (1985) due to a difference in the way that bees were marked for observation. Transitions between age castes prior to foraging involve age-dependent changes in the relative frequencies with which tasks are performed. The final shift is relatively more pronounced and marks the onset of a behavioural phase devoted to specific tasks associated with foraging. There are also age-related changes in exocrine gland development that are associated with age polyethism (reviewed by Winston, 1987). For example, maximum amounts of larval food are produced early in life by bees in the brood- and queen-care phase, while highest levels of alarm pheromones are found at older ages when bees may be involved in nest defence. Superimposed on the age-based system of division of labour is a pattern of individual behavioural variability among workers. There are differences in the rate at which workers pass through age castes. Some show precocious behavioural development, while others mature more slowly (e.g. Sekiguchi and Sakagami, 1966; Nowogrodzki, 1983). There is also inter-individual variation in the degree of task specialization within an age caste. For example, only a few per cent of a colony’s workers ever guard the nest entrance (Lindauer, 1952; Moore et al., 1987) or remove corpses from the

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nest (Sakagami, 1953a; Visscher, 1983). In addition, some bees guard the nest entrance (Moore et al., 1987) or remove dead individuals from the nest (Sakagami, 1953a; Visscher, 1983) continually for several days, while others perform these activities only infrequently, perhaps on just a single day. An example of extreme specialization within an age caste, or “idiosyncratic” behaviour (Oster and Wilson, 1978), is water collection. Many foragers gather water facultatively in response to colony conditions (Lindauer, 1955), but there are a small number of bees that specialize exclusively in water collection throughout their foraging careers (Lindauer, 1952; Robinson et al., 1984). Highly specialized individuals are thought to contribute significantly to the growth and development of a colony by working more efficiently than less specialized workers (Oster and Wilson, 1978; Jeanne, 1986a,b). In some cases, a small group of “elite” workers (Oster and Wilson, 1978) performs a unique function that influences the behaviour of the entire colony. For example, about 5% of a colony’s population play a pivotal role in the reproductive process of colony fissioning (swarming); they scout for potential nest sites and then direct the entire swarm to the new location (Seeley et al., 1979). A similarly-small fraction of a colony’s workers exert great influence over the colonial response to a foreign queen. They initiate aggression towards a foreign queen and continue to behave aggressively long after other workers become habituated to her presence (Robinson, 1984).

3.2

PLASTICITY IN DIVISION OF LABOUR

A key feature of division of labour is its flexibility (Oster and Wilson, 1978). Division of labour in honey bee colonies is highly structured, but colonies cope with constant variation in age demography (Fukuda, 1983) and resource availability (Visscher and Seeley, 1982) via ongoing adjustments in the proportions of individual workers engaged in various tasks. This is accomplished via the behavioural plasticity of individual workers. Previous studies suggest three mechanisms of behavioural plasticity: (1) atypical agedependent behaviour (reviewed by Winston, 1987); (2) increases in the frequency with which workers perform a needed task that is within their typical age-specific repertoire (Kolmes, 1985; Kolmes and Winston, 1988); and (3) changes in the overall activity levels of workers (Sekiguchi and Sakagami, 1966; Kolmes, 1985; Winston and Fergusson, 1985). Changes in age polyethism result in variability in the ages at which tasks are performed (reviewed by Winston, 1987). For example, experimental perturbations of colony age demography can lead to: (1) accelerated behavioural development, with the appearance of “precocious foragers” that begin foraging as early as 7 days of age, which is approximately 2 weeks before the

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typical onset of this behaviour occurs (Nelson, 1927; Rosch, 1927; Himmer, 1930; Haydak, 1932; Sakagami, 1953b; Robinson et al., 1989); (2) arrested behavioural development, i.e. the appearance of “overaged nurses” that continue to care for the larvae and queen for 4 or 5 weeks (Milojtvic, 1940); and (3) behavioural reversion, i.e. the appearance of bees that shift from foraging to brood care (Rosch, 1930; Milojevic, 1940; G. E. Robinson, R. E. Page, C . Strambi and A. Strambi, unpublished data; R. E. Page, G. E. Robinson, D. S. Britton and M. K. Fondrk, unpublished data). Other environmental and colony factors known to affect age polyethism in addition to colony age demography are food availability (Sekiguchi and Sakagami, 1966), colony population (Winston and Punnett, 1982), amount of storage comb (Rosch, 1930) and amount of brood (Winston and Fergusson, 1985). Plasticity in colony division of labour independent of changes in individual age polyethism apparently can also occur. Kolmes (1985) and Kolmes and Winston (1988) have reported increases in the frequency with which some tasks are performed in a colony, without changes in the age distributions of workers performing these tasks. These results were obtained in response to changes in colony and environmental conditions that are considered less severe than the changes that induce changes in age polyethism. An increase in comb-building behaviour was observed after the removal of 10% of the wax combs from a colony (Kolmes, 1985). Moderate changes in colony age-caste demography have been observed to be associated with increases in the overall levels of brood care (Kolmes and Winston, 1988). However, it is not known whether plasticity in division of labour independent of changes in age polyethism is actually based on increased activity on the part of workers already engaged in these tasks, shifts from one task to another by workers within an age caste, or changes in the proportion of colony members that are working. The existence of an uncommitted pool of “reserve” bees is suggested by the observations of Sekiguchi and Sakagami (1966), Kolmes (1985) and Winston and Fergusson (1985). 3.3

HORMONAL REGULATION OF DIVISION OF LABOUR

Juvenile hormone (JH), a major insect developmental hormone (Riddiford, 1985), is involved in the control of age polyethism in adult worker honey bees (reviewed by Robinson, 1987a). JH is synthesized and released by the corpora allata, paired endocrine glands that are regulated by neurosecretory cells in the brain. JH I11 is the only homologue found in worker bees (Hagenguth and Rembold, 1978), and its titre increases as the adult bee ages (Fluri et al., 1982; Robinson et al., 1989). Low titres are associated with behaviour in the nest such as brood care, during the first 1-3 weeks of the bee’s adult life, whereas a

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R. E. PAGE JR A N D G. E. ROBINSON

higher titre at about 3 weeks of age is associated with the onset of foraging. Treatment with JH (Jaycox, 1976), JH mimic (Jaycox et al., 1974), or JII analogue (Robinson, 1985, 1987b; Sasagawa et al., 1985) induces precocious foraging. Bees treated at 1 day of age with 250pg of the JH analogue methoprene began foraging on average about 8 days earlier than untreated individuals (Robinson, 1985), a strong effect because the lifespan of adult bees is only 4-7 weeks. Subsequent experiments (Robinson, 1987b) have demonstrated that premature foraging can be induced in a dose-dependent manner with applications of 50-250 pg. In contrast, methoprene treatments do not affect the intensity of foraging once it begins (Robinson, 1985) and supports the hypothesis that JH affects only the timing of behavioural development. Robinson (1987b) presented results that support the hypothesis that JH is involved in the regulation of age polyethism throughout the life of the bee, not only during the shift to foraging. Observations were made of groups of individually labelled bees that were treated at 1 day of age with different doses of methoprene. Hormonally distinct workers displayed distinct patterns of age polyethism, despite their identical ages. Dose-dependent treatment effects support the hypothesis of an association between naturally rising endogenous JH titres and changes in the relative frequency with which age-dependent behaviours are performed. Applications of JH or JH analogues also induce several changes in worker bee physiology that are associated with age polyethism (reviewed by Robinson, 1987a). Injections of JH (Rutz et al., 1974, 1976), JH mimic (Jaycox et al., 1974), or orally administered JH analogues (Beetsma and Ten Houtem, 1974) cause a premature degeneration of the brood-food producing hypopharyngeal glands, a process that usually accompanies the shift from nest to field activities. The role of JH in regulating hypopharyngeal gland development was confirmed by Imboden and Liischer (1975), who found that removal of the corpora allata blocked the degeneration of the hypopharymgeal glands, but that exogenous application of JH again caused it. Methoprene treatment also induces premature production of the alarm pheromones, 2-heptanone and isopentyl acetate (Robinson, 1985). These results demonstrate the involvement of JH in coordinating exocrine and behavioural development, processes which are closely associated in the worker bee. Robinson (1987b) suggested the following model to explain how JH affects bee behaviour (Fig. 7). Bees of all ages live together in an enclosed, densely populated colony, and thus encounter throughout their lives a plethora of stimuli that can elicit the performance of many different tasks. It is thus likely that behavioural development in the bee is subserved by age-dependent changes in sensory perception and/or behavioural responses to environmental stimuli. The probability that a bee will perform a task is determined by two factors: (1) the magnitude of the task stimulus, which affects the

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probability of being “exposed” to it; and (2) the response threshold to the stimuli associated with the task, i.e. the probability of responding, given exposure to the task. As a bee ages, she may undergo a programmed change in central nervous system (CNS) response thresholds to task-associated stimuli, mediated by changes in JH titre. For example, a young individual, with a low JH titre, may be more sensitive to stimuli that elicit brood care than to stimuli associated with foraging (and vice versa for an older bee, with a higher hormone titre).

FIG. 7 Model explaining the role of juvenile hormone (JH) in regulating honey bee age polyethism. The probability that a worker bee will perform a given task is determined by (1) JH-dependent response thresholds that determine the probability of responding to task-related stimuli, and (2) environmental and colony conditiqns that determine the probability of encountering a task, by shaping colony needs qnd determining the relative magnitude of the tasks (depicted here as the relative size of “Task A” and “Task B” boxes). According to the model, the JH titre increases with worker age due to a genetically determined pattern of development. In addition, the JH titre (and therefore response thresholds) may be modulated by environmental and colony conditions, enabling individual workers to respond to changing colony conditions. (From Robinson, 1987b, with permission from Springer-Verlag.)

Hormonal modulation of behavioural response thresholds in bees was reported by Robinson (1 987c). Methoprene treatment prematurely redwed the behavioural threshold sensitivity to alarm pheromones, stimuli that play a role in the task of nest defence (Collins et al., 1980). Treatment did not affect peripheral perception, measured by electroantennograms. These results suggest that JH affects the age-dependent response to alarm pheromone in the CNS, which supports the hypothesis that response thresholds are hormonally regulated.

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3.4

HORMONAL REGULATION OF PLASTICITY IN DIVISION OF LABOUR

Robinson et al. (1989) demonstrated that environmental cues modulate tho: intrinsic rise in JH, resulting in changes in age-caste membership of indi. viduals. This provides a mechanism for the ability of colonies to reallocat(: labour resources in response to changing environmental and colony conditions. Workers were exposed to conditions that uncoupled the usually closely associated factors of worker age and behavioural status in order t o test for an association between behavioural status and J H titre that is independent of age. Two colonies, each initially consisting of 2000, 1-3-dayold bees, were established to induce division of labour independent of worker age. Within 1 week these “single-cohort colonies” contained some bees that cared for larvae (“nurses”) and others that foraged precociously. The emergence of new adults was prevented by replacing combs of developing pupae with combs of eggs and young larvae from other colonies; the ageing experimental colonies then contained overaged nurses and normal-aged foragers. Groups of precocious foragers and normal-aged nurses were collected when they were 7-10 days old, and normal-aged foragers and overaged nurses when they were 21-24 days old. Radioimmunoassay (Strambi et al., 1981) revealed that in each single-cohort colony, pooled samples of foragers had significantly higher haemolymph levels of JH than pooled samples of nurse bees, at both ages (Table 3). Similar results were obtained for demographic changes occurring naturally during colony fission that also result in skewed age distributions. A reproTABLE 3 Mean JH titres f S.E. for honey bees experiencing experimentally induced changes in demography in single-cohort colonies ( n =number of worker groups, 4-1 6 workers/group; n = 5, except n = 4 for colony 444G3 normal-aged nurses; n = 6 for colony 4450-3 precocious foragers, normal aged foragers, external control nurses, and external control foragers). External controls (n = 500) were taken from the same source colonies used to establish single-cohortcolonies, marked at one day of age, reintroduced to their respective colonies, and sampled as nurses at 7-10 days of age and foragers at 21-24 days of age. P values shown are results of t-tests (for samples with unequal variances). From Robinson et al. (1989) Colony

Normal aged nurses

Precocious Overaged foragers nurses

4440-3

5.4 2.4 27.3 f 6,3 P<0.01

2.8 0.8 17.4f 2.9 P<0-01

5.8 f 1.3 36.0 f 8.3 P<0-01

4450-3

2.7 f 0.9 72.6 f 10.5 P
8.6 f 1.9 24.8 f 6.2 P < 0.05

2.9 f0.7 35.5 f 6.11 P<0.01

*

Normal aged foragers

*

External control nurses

External control foragers

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135

ductive swarm of honey bees contains workers of all ages (Butler, 1940) but develops an age structure dominated by older individuals soon after establishing itself in a new nest site. This is because new adults do not emerge until 3 weeks after the first eggs are laid by the queen. As the worker population ages in a newly swarm-founded colony, some of its youngest workers continue to care for larvae as overaged nurses, while similar-aged individuals in a swarm’s parent colony switch from nursing to foraging as a consequerice of a continual emergence of young bees. Robinson et al. (1989) reported that levels of JH were significantly lower for nurse bees from swarms than for foragers from parent colonies, despite the fact that both groups of workers were 20-30 days old (Fig. 8). Plasticity in JH titre also underlies the reversal of normal behavioural development from foraging to nursing (Robinson et al., unpublished data). Observations were made on nine cohorts of bees, each originally consisting of 200 1-day-old individually identifiable adult workers that were placed irito an observation hive every 3 days. Foragers were identified when the focal bees were 22-50 days old. Observations were then made to determine which bees showed behavioural reversion after most young, nursing-age bees were removed from the colony. The hormonal basis for reversion was determined by comparing the JH titre of foragers collected before young bees were removed with that of bees that reverted from foraging to nursing. Haemolymph levels of JH were significantly higher in foragers than in individuals that reverted from foraging to nursing (22.4 f4.3 [S.E.] vs. 7.2 f 1.3 pmol/ Colony 1

r 20

P

Colony 2

r

0.01

P < 0.01

-

E!

c .-

I-

I

7

1010

FIG. 8 Mean JH titres (pmol JH III/lOOpl haemolymph) f S.E. for honey bees experiencing naturally induced changes in demography associated with colony parent-colony foragers. All bees were 20-30 days old fission. W , Swarm nurses; 0, when sampled. P values based on results of t-tests (for samples with unequal variances). The number of worker groups sampled is indicated on graph (4-16 workers/group). (From Robinson et al., 1989, with permission from the American Association for the Advancement of Science.)

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100 PI, [P
4 Genetic basis for division of labour Darwin (1859, pp. 253-254) proposed an explanation for the evolution of behavioural and anatomical differentiation among non-reproductive workers. Darwin considered the occurrence of adaptive traits in nonreproductive individuals to be a serious difficulty for his theory of evolution by natural selection. He proposed a model of how colony-level selection can change the distribution of workers with certain traits within colonies of a population. A modern interpretation (see Page et al., 1989b) of this model requires: (1) heritable variation in worker behaviour within colonies, due to either worker and/or queen genotype; (2) variation in reproductive success between colonies due to different distributions of worker traits; and (3) changes in the distributions of worker traits within colonies as a consequence of colony-level selection. Although it is assumed that worker differentiation is a product of colony-level selection (Oster and Wilson, 1978), the underlying genetic mechanisms have received relatively little attention (Crozier and Consul, 1976; Owen, 1986). Consequently, the proximal determinants of division of labour among workers in insect colonies had been thought to be primarily, if not exclusively, environmental and ontogenetic (Wilson, 1985a,b). In this section we present evidence for genetic determinants of behavioural differences among worker nestmates.

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COMPONENTS OF DIVISION OF LABOUR

Genetic information regarding division of labour must reside within individuals-there is no separate colony genotype shared by all. This information may be expressed by the individual adult workers, among which the division of labour is formed. It may also be expressed by other colony members, such as the queen or the immature brood, in a way that regulates worker behaviour. Therefore, genetic variability for the traits of individuals that ultimately affect colony behaviour constitutes the “grist” upon which the “mill” of natural selection can operate. A challenge for evolutionary biologists is to identify those characteristics of individuals that show genetic variability, for they represent the components of division of labour that are currently selectable. 4. I. 1 Genotypic variability for performing tusks

Genotypic variability among individuals for performing specific tasks has been demonstrated with two experimental approaches. Some studies have examined differences in behaviour among members of different subfamilies within a colony, while others have compared the behaviour of unrelated co- or cross-fostered workers that were derived from either naturally or artificially selected strains of bees. 4.1.1.1 Behavioural variability among members of diferent subfamilies Robinson and Page (1988, 1989a) studied 10 colonies for the tasks guarding the nest entrance (G), removing dead bees from the hive (“undertaking”, U), foraging for nectar (N), foraging for pollen (P), and dancing on the cluster of a swarm (“scouting”, S ) . Each colony consisted of a single queen and three subfamilies of workers derived from three different drone fathers. Matings were controlled by instrumental insemination to assure that each subfamily was distinguishable on the basis of the enzyme marker, Mulate dehydrogenuse (Mdh). Mdh has three alleles that normally segregate in populations of North American bees. The markers have no visible phenotypic effects, appear to be neutral with respect to the behaviour studied, and allowed behavioural data to be collected blindly. Workers from these colonies were sampled while performing the specific tasks and then were analysed by protein electrophoresis to determine their subfamily membership. Differences in subfamily representation were observed, demonstrating genotypic variability for performing tasks. Results for one representative colony are presented (Fig. 9). Each of the 10 colonies involved in these studies showed similar, statistically significant, heterogeneous patterns of

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behaviour among subfamilies and demonstrate genotypic variability for tasks that are performed at similar ages: undertaking or guarding (performed just prior to the onset of foraging; Sakagami, 1953a; Moore et al., 1987) and foraging for pollen or nectar (Free, 1965; Sekiguchi and Sakagami, 1966). These results also demonstrate the genotypic “structure” of the division of labour in these colonies that is attributable to the genotypic differentiation of subfamilies. ,.

G

U

N

P

S

FIG. 9 The relative likelihood that individuals belonging to each of three subfamilies within a representative colony perform given tasks. The width of each box represents the likelihood that an individual sampled while performing that task belongs to a particular subfamily. Likelihoods have been adjusted for unequal numbers of members of each subfamily. G, guarding; U, undertaking; N, collecting nectar; P, collecting pollen; S, scouting from a swarm. (From Page, 1991.)

Breed et al. (1990) used a similar approach to study the division of labour associated with colony defence. They found that bees that respond to major disturbances of the colony by flying around the intruder, and possibly stinging, were similar in age to foragers but formed a distinct group of older workers. Behaviourally, defenders had less worn wings than foragers, suggesting less flight activity. Genetically, defenders differed in Mdh frequencies from foragers in the same colony, demonstrating different subfamily composition. Defenders also differed in Mdh frequencies from guards in the same colony, providing further evidence for division of labour associated with colony defence. They proposed that this newly discovered group be called “soldiers”, due to their important role in colony defence. These results highlight the fact that genetic analyses of colony behaviour can be used to probe for previously unidentified elements of division of labour. Additional studies using the same or different methodology have demonstrated genotypic variability among subfamily members within colonies for queen larval care (Page et al., 1989a; G. E. Robinson and R. E. Page, unpublished data) and allogrooming (Frumhoff and Baker, 1988). Within queenless colonies, differences have been found among workers belonging to

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different subfamilies engaged in the exchange of food (trophallaxis), oviposition behaviour, oophagy and drone larval care (Moritz and Hillesheim, 1985; Hillesheim et al., 1989; Robinson et al., 1990). The studies reviewed thus far in this section involved colonies derived from queens that were instrumentally inseminated with semen from two or three males. However, queens naturally mate with 7-17 different males (see Section 2.3), suggesting that the genotypic composition of these experimental colonies was simpler than that of colonies derived from naturally mated queens. Calderone et al. (1989) studied behavioural differentiation among subfamilies in colonies with naturally mated queens. Two colonies were selected with queens that were homozygous at the Mdh locus and that had mated with at least one drone carrying each of the three different Mdh alleles. Thus, there were three classes of heterozygous workers in these colonies, each class possibly composed of multiple subfamilies. It was less likely, but still possible, to detect subfamily differences using this method than with the controlled-mating method discussed above. Nevertheless, significant differences were found in allozyme frequencies between guards and workers collected at random from the honey storage area of the hive (and assumed to be of similar age; see Seeley, 1982), and between nectar and pollen foragers. These results suggest that genotypic differences in worker behaviour probably are also present in colonies derived from naturally mated queens. 4.1.1.2 Behavioural variability among co- and cross-fostered cohorts The behaviour of co- and cross-fostered individual workers from artificially selected strains of bees was studied (Calderone and Page, 1988) in order to determine more precisely the effects of the environment and genotype, and their interaction, on the expression of worker behavioural variation. Cohorts came from strains of honey bees that were artificially selected for high and low quantities of stored pollen (Hellmich et al., 1985). Eggs laid by queens of each strain were raised in the same colony at the same time to separate the effects of rearing environment from genotype in explaining behavioural variability. Combs were transferred into an incubator when the immature workers were pupae; adults that emerged in the incubator during the same time interval were individually tagged for identification and placed into the same hive for observation. One-hundred-and-seventy-five tagged workers from each of the high and low strains were placed into each of two observation hives that contained about 10 000 wild-type workers each. In this common environment, the combined proportion of observed pollen-foraging trips for workers of the high strain in both colonies was significantly higher, 0.65, than for the low strain, 0.04. In addition, workers of these two strains differed significantly in behaviour within the nest prior to the initiation of foraging, suggesting

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genotypic differences in nest tasks as well. A follow-up experiment (Calderone and Page, 1991) demonstrated significant differences between co-fostered workers of high and low pollen hoarding strains for the nest tasks of fanning, trimming the wax cappings on cells containing brood or honey, larval care, nest care and grooming nestmates. Genotypic differences in behaviour were also reported by Rothenbuhler and Page (1989). Two colonies were initiated, each consisted initially of a single queen and 400 marked workers that had emerged as adults within the previous 48-h period. The workers came from two different strains of bees that had been selected for high and low hygienic behaviour (the Brown and Van Scoy strains, respectively), a behavioural trait that results in workers uncapping and removing diseased larvae from cells on the wax combs (see Rothenbuhler, 1964). Workers from the Brown strain were also known to be more defensive than workers from the Van Scoy strain. Each single-cohort colony received 200 young workers from each strain. Four to six weeks later these similar-aged, but genetically different, workers were performing different tasks in both hives. Workers of the Brown strain engaged primarily in cell cleaning and defensive behaviour while workers belonging to the Van Scoy strain foraged. 4.1.2 Genotypic variability for rate of behavioural development The most pronounced age-dependent change in the behaviour of worker honey bees is the shift from performing tasks within the nest to foraging outside. In the study of Calderone and Page (1988) discussed in Section 4.1.1, there were significant genotypic differences in the age at which workers began foraging. There were also significant genotypic differences in the age at which several nest tasks were performed: brood care, nest care and construction, and grooming nestmates (Calderone and Page, 1988, 1991). The allocation of effort to a particular task by an individual worker depends on both the probability of performing that task during a given behavioural phase and the amount of time that an individual belongs to that particular phase. These two components of behaviour were shown by Calderone and Page (199 1) to be independent. Individually-marked cohorts of 300 1-day-old workers from high and low pollen hoarding strains were cofostered in an observation hive containing about 18 OOCL20 000 unrelated, “wild-type’’ workers. Three hundred 1-day-old wild-type workers from the resident colony were also marked. Individuals of these three cohorts were observed for 21 days and their behaviour recorded. Significant differences were found among cohorts for the age at which they performed six of the nine activities observed. The average proportion of time that workers spent

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engaged in each activity differed among cohorts for three activities, but only two of these activities were also performed at different ages by members of different cohorts. There were no differences among cohorts in the proportion of time engaged in four of the six activities that were performed at different ages. 4.1.3 Genotypic variability for response to changing colony conditions Changes in colony age demography result in altered patterns of age polyethism among some worker honey bees, as described in Section 3.2. A genetic component for plasticity of age polyethism has been demonstrated (Robinson et al., 1989, unpublished data; Page et al., unpublished data). In the experiments of Robinson et al. (1989), nine single-cohort colonies, each initially consisting of 2000, 1- to 3-day-old bees, from colonies consisting of three Mdh marked subfamilies, were established to induce division of labour among workers of similar ages. Significant differences in subfamily frequencies were detected in samples of normal-aged nurses and precocious foragers in seven out of nine colonies, and in samples of overaged nurses and normalaged foragers in four out of eight colonies. Genotypic differences in responses to changing colony conditions were also observed by Page et al. (unpublished data). Three “composite” single cohort colonies were established in observation hives, each containing a queen, a small quantity of worker eggs and larvae, and about 2000 individually marked, similar-aged workers. The workers in each colony were the progeny of two different, unrelated queens that were each instrumentally inseminated with the semen of a single, unrelated drone. All workers were within 5 days of each other in age in one colony and within 48 h of age in the other two colonies. Hive entrances were observed daily to determine the identity of precocious foragers. Both genotypic and ontogenetic determinants of precocious foraging were detected. In each colony, one of the two genetically distinct cohorts was significantly over-represented in the group of observed precocious foragers. In addition, the oldest bees in each cohort were relatively more likely to become precocious foragers in all three colonies. To study the effects of genotype and age on the initiation of foraging behaviour of older-aged workers, an additional 2000 newiy-emerged, unmarked, workers (less than 24 h old) were placed into each hive after the marked workers were 1&17 days old. These younger bees performed tasks within the nest, which resulted in the marked bees shifting to foraging. As with precocious foragers, there were significant genotype and age effects in each colony for the likelihood of engaging in foraging. All the young bees were removed from each hive 3-5 days after their

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introduction to study the effects of genotype and age on behavioural reversion. By this time, no older tagged bees were observed feeding larvae, however, in the absence of younger bees, some foragers reverted to larval care (as described in Section 3.4). There was no genotypic bias associated with behavioural reversion, and likewise, worker age had no independent effect on the likelihood of reverting to larval care behaviour. The amount of foraging experience was a significant determinant: those individuals who foraged for a longer period of time were significantly less likely to revert to nursing, regardless of age or genotype. Similarly, no genotypic differences in the likelihood of showing behavioural reversion were found in a companion study, using a different experimental colony (Robinson et al., unpublished data; discussed in Section 3.4). Kolmes et al. (1989) also demonstrated genetic variability for responses to changing environmental conditions. They established a colony with six genotypically distinct groups of workers and recorded the ages at which individually-tagged workers took their first foraging trips. They set up an additional 15 colonies into which they added 50 newly-emerged, tagged workers from each of two phenotypically distinguishable subfamilies from one queen, and again recorded the age at which the individuals began foraging. These 15 colonies varied with respect to quantities of wax comb in the hive. Subfamily differences were detected in the age a t which workers began foraging, but only in the colonies that were relatively deprived of wax. These results demonstrate that individuals of some genotypes are more likely to respond to changes in colony conditions by altering their agedependent trajectories of behavioural development. The observed behavioural differences may be a consequence of genetic differences in the sensitivity of individuals to environmental change. Alternatively, they may reflect the existence of slight genotypic differences in rates of behavioural development, already known to exist in bees (Winston and Katz, 1982; Calderone and Page, 1988, 1991), that are magnified by experimental changes in social environment. For example, genotypes that under normal conditions result in slightly more rapid development, with an earlier onset of foraging than other genotypes, may be relatively more likely to become precocious foragers in the absence of normal-aged foragers. 4.1.4 Possible efsects of the queen and brood Genotypic components of division of labour discussed in the preceding sections are intrinsic to the colony’s individual adult workers. However, it is possible that the allocation of workers to various tasks is also affected by the queen and the immature workers. Queens influence worker foraging activity, behaviourally in Polistes wasp colonies (Reeve and Gamboa, 1987), and via

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pheromones in colonies of honey bees (Free, 1967; Jaycox, 1970; Free et al., 1984), but there is presently no evidence of genetic variation among queens for these traits.

4.2

SUMMARY

These results demonstrate that colony-level selection can change the behaviour of workers by changing their genotypes. Properties of individual workers may be adjusted by natural selection and adapt colonies to particular environments. Changes in thresholds of response to specific task-eliciting stimuli may alter the probabilities that individuals engage in different tasks. This would also change the relative numbers of individuals in a colony performing those tasks. Changes in rates of development in workers that lead to changes in behaviour may also affect the distribution of workers among tasks by altering the amount of time individuals are in a particular state of behavioural development. If individuals belong to a particular behavioural state for longer periods of time, then there will be more individuals available for tasks performed during that behavioural state. Regulation of the rate of behavioural development in response to colony needs also may be important for integrating behaviour at the colony level by providing appropriate levels of responses to changing environmental conditions.

5 Colony-level integration of individual behaviour Some of the most conspicuous traits of social insects are a consequence of the collective behaviour of individuals. However, the mechanisms that integrate worker behaviour into coordinated colony patterns are poorly understood. Intracolonial genetic variation in behaviour may underlie some behavioural phenomena that are properties of the whole colony. In this section we discuss how genetic variability within colonies may affect the regulation of colony behaviour. We must first issue the caveat that many of the studies discussed here and above were conducted using honey bees from commercial stocks in North America where honey bees are not native. Overall levels of observed genetic variability may be greater in colonies of North America, due to population mixing resulting from commercial beekeeping practices. Alternatively, variability may be less in North American populations than those found in populations in Europe, their source of origin, due to bottleneck introductions of the original stocks. However, these are likely to be only differences of degree and affect the ability of investigators to detect the variability. It is

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obvious that genetic variability for behavioural traits exists in natural populations and that they are subject to natural selection. This is demonstrated by the behavioural differences found among geographical races of honey bees found in Europe and Africa (see Ruttner, 1988). 5.1

BEHAVIOURAL VARIABILITY WITHIN A SUBFAMILY

Genetic variability for components of division of labour were demonstrated in the studies discussed in Section 4 by showing behavioural differences between individuals belonging to different subfamilies. Genotypic variability no doubt also exists among individuals of a subfamily due to recombination in queens. This within-subfamily genotypic variability should also generate behavioural variability among individual colony members, although it has not been demonstrated.

5.2

PLASTICITY IN DIVISION OF LABOUR INDEPENDENT OF AGE POLYETHISM

As discussed in Section 3.2, a key feature of the division of labour is its flexibility. The ability of colonies to respond to changes in environmental and social conditions by altering the ratio of individuals performing various tasks within a given age caste may be in part a consequence of intracolonial genetic variation in worker behaviour (Robinson and Page, 1989b). For example, under ‘‘normal’’ conditions (Fig. lOA), a task is performed by workers with the lowest response thresholds. These workers may represent a specialized subset of the colony, in both a behavioural and a genetic sense. If the need for this task increases due to changes in colony and/or environmental conditions, and there is a concomitant rise in the levels of associated stimuli (Fig. lOB),

__

-

CHANGES IN ENVlRONYENT INCREASE COLON” NEEDS

@ .

8

MOST SENSITWE

-

RESPONSETHRESHOLD LOW HlGH

SENSITIVE LEAPT

FIG. 10 Effects of intracolonial variation in behavioural response thresholds on colony plasticity for division of labour. This hypothetical colony is composed of three subfamilies (distributions 1-3). A, “Normal” conditions, with task performed by workers with lowest response thresholds; B, high stimulus conditions due to increased colony needs; workers with relatively higher response thresholds are now involved. Shaded portions indicate the proportion of each subfamily’s workers performing the task. (From Robinson and Page, 1989b, with the permission of Westview Press.)

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then more and more of a colony’s workers whose genotypes result in relatively higher response thresholds will perform the task. Through this mechanism, moderate, transient increases in the need for a particular task would elicit a graded colony response based on differential recruitment among genotypes, as more individuals shift from one job to another within an age caste. Recent results, however, do not support this hypothesis for all tasks (G. E. Robinson and R. E. Page, unpublished data). Corpse-removal behaviour was studied because it is possible to vary the level of corpse-removing stimuli in a precise way. Electrophoretic analyses of bees with allozyme subfamily markers revealed genotypic differentiation for undertakers and samples representing the subfamily composition of the whole colony under conditions of “low” need for corpse removal (I 5 corpses added every 15 min), as in earlier studies (Robinson and Page, 1988). The subfamily composition of undertakers deviated significantly from expected in seven out of 12 cases (six colonies tested, twice each). However, the genotypic composition of samples of undertakers collected in response to a “high” need for corpse removal (1000 corpses added at one time) was no less deviant with respect to subfamily composition than the composition of those that responded to the presumed low stimulus. Even more surprising was the finding that there was a 30-50% decrease in corpse-removal activity in the second trial relative to the first in all six experimental colonies, even though the second trial occurred 7-10 days after the removal of only a few per cent of the colony’s workers in Trial 1. Results of a second experiment confirmed that removing some undertakers from a colony can cause a decrease in subsequent undertaker activity for several days. These results suggest a lack of individual and colony-level plasticity for corpse-removal behaviour. A decrease in corpse-removal efficiency seems to have occurred because there was a depletion of individuals with specific genotypes, amounting to no more than 1-5% of each colony’s population, and no measurable shift to undertaker activity by other colony members with different genotypes. In contrast, Calderone and Page (1991) demonstrated plasticity among genotypes for the task of pollen foraging. They reported that co-fostered workers from artificially selected strains of bees that collect and store either high or low amounts of pollen responded differently to changes in foragingrelated stimuli in a common colony environment. Similarly, N. W. Calderone and R. E. Page (unpublished data; see Calderone, 1988) found that crossfostered workers from these strains collected different proportions of pollen and nectar, depending on colony environment. These results suggest that depletion of a colony’s pollen foragers, or an increase in colony demand for

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pollen, should result in individual workers from different subfamilies switching from nectar to pollen foraging, but this has not yet been demonstrated. The role of genotypic variability in coordinating a colony’s response to changing conditions may vary with task. One possible determinant of behavioural plasticity may be whether the task is “rare”, performed by only a fraction of a colony’s workers, or “common”, performed by most workers at some point in their life. Pollen foraging is not as rare a task as undertaking, but apparently is a more restricted activity than nectar collection. Extensive observations of individually marked workers indicate that a sizeable proportion of a colony’s foraging force (28-33%) never collect pollen (Ribbands, 1952, 1953; Sekiguchi and Sakagami, 1966), while virtually all workers are thought to gather nectar (Sekiguchi and Sakagami, 1966). Therefore, it is possible that workers performing a rare task are in a state of behavioural development that is “harder to enter into” than a behavioural state associated with a common task. Differences in “access to a behavioural state” may occur because some behavioural states are accessible only via ontogeny (development from one age caste to another), and different individuals vary in their competence to develop into that behavioural state because they have different genotypes or different environmental experiences. Because of inter-individual differences in the relative accessibility of a behavioural state, response thresholds for a particular task may not vary continuously among worker genotypes. Rather, individuals may either perform the task or not, with threshold levels for individuals that perform it set very low, at or below normal stimulus levels, while the response thresholds of those that do not perform it are set very high, at stimulus levels not actually attainable under normal conditions. Another possible determinant of behavioural plasticity may be the nature of other tasks that are performed by workers during the time in their lives when they are performing the task in question. Individuals may be competent to switch to a needed task, but do not, because they are more sensitive to the stimuli eliciting the performance of a different task. If this is the case, then we should not consider the response thresholds of workers to single tasks independently of response thresholds to other tasks. Each may affect the other.

5.3

BEHAVIOURAL DOMINANCE

The “phenotype” of a colony for a given behavioural trait is determined by the behaviour of its workers. If each individual worker’s phenotype is a consequence of the additive effects of its genotype and environment, and each individual contributes equally and additively to the colony phenotype,

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then, in a given environment, the additive effects of all worker genotypes will define the colony phenotype. However, if the interactions of individuals with different genotypes are affecting the phenotype of the colony, then the additive genotypes of the workers of a colony will not be a good predictor of the colony phenotype. For tasks performed by only a small number of a colony’s workers, interactions between individuals may be very important and result in non-additive inter-individual effects on the colony phenotype, a kind of “behavioural dominance” (one might equally argue for the term “behavioural epistasis”). Performance of a relatively rare task by individuals with the lowest response thresholds for that task, i.e. those most sensitive to the task-related stimuli (Fig. 11A), may help maintain the stimulus level below the thresholds of less sensitive individuals, further diminishing the probability that individuals less sensitive to these stimuli will perform that task (Fig. 11B). The actions of a small subset of workers may thus determine the colony behavioural phenotype, via behavioural dominance and negative feedback regulation of task performance. A

B

FIG. 1 1 Effects of behavioural dominance and negative feedback loops of task regulation on the genotypic composition of a group of task performers in a hypothetical colony composed of three subfamilies (distributions 1-3). A, Initial stimulus level associated with a particular task in a colony; B, actual stimulus level in the colony as a consequence of negative feedback. Shaded portions indicate the proportion of each subfamily’s workers performing the task. (From Robinson and Page, 1989b, with the permission of Westview Press.)

The effects of behavioural dominance, coupled with negative feedback, may be such that the variance in a colony’s worker genotypes is an important determinant of the colony phenotype for some tasks. N. W. Calderone and R. E. Page (unpublished data; see Calderone, 1988) reported evidence for behavioural dominance in a study of the pollen-foraging behaviour of crossfostered workers from artificially selected high and low pollen-hoarding strains of bees. Cohorts of 200 individuals from each of the strains were cofostered in each of four high-strain and four low-strain resident colonies. Foragers from the high strain returned with pollen loads an average of 54% of the time in high-strain resident colonies and 75% of the time in low-strain colonies. Low-strain cohort members returned with pollen 3% of the time

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compared with 35% in high- and low-strain colonies, respectively. Lowstrain workers collected less pollen in high-strain colonies apparently because the relatively greater amount of pollen in the high-strain colonies maintained the stimulus level close to or below their thresholds of response for pollen foraging behaviour. 5.4

GENETIC BASIS FOR

“IDIOSYNCRATIC”,

“ELITE” AND “RESERVE”

WORKERS

Genotypic differences in response thresholds, coupled with the effects of behavioural dominance, provide a mechanistic explanation for the occurrence of “idiosyncratic” and “elite” workers that perform tasks that most colony members do not perform, like guarding, undertaking and scouting. Individuals performing rare tasks may have rare genotypes that result in low response thresholds (see Fig. 12). This model also predicts the existence of inactive “reserve” workers, whose presence in honey bee colonies has previously been suggested (Sekiguchi and Sakagami, 1966; Kolmes, 1985; Fergusson and Winston, 1985). Inactive workers may have response thresholds so high that they are usually insensitive to the task-related stimuli they encounter. Differences between subfamilies in the proportion of individuals with extreme genotypes may be a consequence of differences in either the response threshold mean or variance for each subfamily. Behavioural dominance and negative feedback may result in greater behavioural differentiation among workers than would be expected on the basis of differences in response thresholds alone.

h

THAT PERFORM THATPERF A RARE TASK

NACTNE WORKERS

z

MOST SENSITIVE

RESPONSE THRESHOLD LOW f HIGH

LEAST SENSITIVE

FIG. 12 Hypothetical distribution of behavioural response thresholds for a subfamily. The behaviour of bees that perform rare tasks or remain inactive may be a consequence of their possessing rare genotypes that result in extreme behavioural response thresholds. (From Robinson and Page, 1989b, with the permission of Westview Press.)

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149

SUMMARY

In this section we have shown how genotypic differences in the behaviour of individual workers may affect the integration of colony behaviour. Worker behaviour changes as the needs of the colony change, but the genotypes of workers affect their capacity to change. We proposed that the genotypic composition of colonies affects social organization through behavioural interactions of individuals. Behavioural interactions among individuals with different thresholds of response, due to differences in experience or genotype, lead to interesting colony-level behavioural phenomena such as behavioural dominance and the occurrence of idiosyncratic, elite and reserve workers. In the next section we show how integrated social behaviour such as that discussed in this and previous sections can emerge through self-organization. 6 The evolution of division of labour

A honey bee society is a collection of tens of thousands of individual workers that carry out coordinated activities without any centralized control centre to direct them (see Seeley, 1989). At any instant, colonies exist in organizational states that can be described by the number of individual workers engaged in each of the possible tasks in a worker’s behavioural repertoire. Colonies also display dynamic behaviour, changing organizational states according to changes in the internal nest and external environments. In the previous sections we have presented what we believe are the behavioural components of individuals that are subject to natural selection. In this section we attempt to demonstrate how those properties of individuals relate to parameters of colony-level behaviour and how natural selection can operate on dynamic changes in colony-level behaviour. We use an ensemble modelling approach based on binary switching networks (see Kauffman, 1984; Gelfand and Walker, 1984) that has been used to model the behaviour of many different kinds of dynamic systems. We show how many of the typical features of insect societies, like those of other complex systems, may be self-organized. We then suggest how natural selection can operate on components of division of labour to “fine tune” social organization. 6.1

SELF-ORGANIZATION

Following the approach of Page and Mitchell (1991; unpublished data), assume that worker honey bees are analogous to elements of a binary switching network of size N . Each individual can be either on or of with respect to performing a particular task while belonging to a particular task

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FIG. 13 Diagram of a switching net. Open circles, elements; arrows, connections among elements. Values (on or o f n carried by arrows are initiated by elements at the tails of the arrows and are inputs into the elements located at the arrows’ heads. This particular net has N = 5 and K = N - 1 (elements are not connected to themselves).

group. Assume that K, the “connectance” of individual elements within the network, is large, equal or close to N , with each element receiving directed inputs from and sending outputs to all other elements (Fig. 13). Inputs carry on or ofsignals according to the state of the element that initiates it. Each element switches on or ofaccording to the number of on or ofinputs coming from all N individuals. Thus, each element has a threshold of response that comes from the subset, {F’), of the set of Boolean switching functions, { F } , that is restricted to those functions that switch the element on or ofwhen the number of on or of input lines exceed some specific value. To make this model correspond to a honey bee colony, assume that the number of elements in the network, N , is the number of individuals that belong to a particular behavioural state (age caste) that are behaviourally competent to perform a particular task. Individuals receive information of the state status (on or of) of other individuals via a common perceived stimulus, rather than by individual connections. Each on individual performs a given task and, by doing so, decrements the stimulus for that task one stimulus unit. When an individual “switches of”, she stops performing the task and the stimulus level is incremented one stimulus unit. For example, one stimulus in a honey bee colony may be the number of empty food storage cells that stimulate individuals to forage. As more individuals forage, storage cells fill up and, with a constant food consumption rate and foraging success, the storage cells provide a statistical record of foraging activities of individuals (Fig. 14).

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. 0 . 0 0

. 0 . 0 0

0 0 0 . 0

0 0 0 . 0

0 0 . 0 0

@ 0 . 0 0

. 0 0 0 .

Individual Worker

0

0

0

0

0 0 . 0 0

0 0 . 0 0

0 0 0 . 0

0 0 0 . 0

~

.

~

Input Record N = 30

FIG. 14 A diagram illustrating how a switching net such as that shown in Fig. 13 can be transformed to represent a network of honey bees sharing information through cues provided by the amount of food, say pollen, stored in a comb. The left box repi:esents a section of comb with 30 cells. The 22 open circles are empty while the 8 closed circles are full. An individual worker assesses the number of empty cells and compares it with its threshold function, f,. Because the number of empty cells exceeds the threshold value of 7, the worker forages and fills the stippled cell represented in the right-hand box. The record now changes as a consequence of the behaviour of the worker. (From Page and Mitchell 1991, with permission from the Philosophy of Science Association.)

For the computer simulations presented here, N = 100, K = N , and {F')is the set of thresholds with integer values I , 2, .. ., N . The initial stimulus level is set equal to N and all individuals are initially ofl. The order of events for the simulations is as follows: 1 . Each individual is assigned a threshold of response randomly from the set, {F'}. 2 . An individual is randomly selected. 3. The individual is checked to determine if it is currently on. If it is on, it is the stimulus turned offand the stimulus incremented one unit. If it is 08, is not changed. 4. The threshold value for that individual is then compared with the current residual stimulus level (the residual is the initial stimulus level minus the number of individuals currently engaged in that task because each individual decrements the stimulus one unit while they perform the task). If the stimulus level exceeds the randomly assigned threshold value, then the individual is turned on and the residual stimulus level is decremented one unit. The individual is then recorded as being on or off for that sampling event.

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Extremely ordered behaviour emerges from this model system (Fig. 15). An initial homeostatic, negative feedback phase occurs drawing the stimulus level down near an equilibrium. This occurs as a consequence of individuals turning on, so an increase in the proportion of the individuals on is observed. The system then “searches” stochastically until it finds a “steady state attractor” where each element is frozen either on or of. This system behaviour is similar to that suggested in Section 5.3 for honey bee colonies where behavioural dominance leads to an extreme division of labour and specialization. In a real honey bee society of individuals may engage in different tasks or, perhaps, develop into different age-castes (Fig. 15).

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Page and Mitchell (1991; unpublished data) compared the dynamic behaviour of models with thresholds drawn randomly from uniform distributions with ranges 1-50, or 51-100 (Fig. 16). Each model showed the characteristic negative feedback phase followed by a searching phase near an equilibrium point then locked into a steady state attractor. Systems with higher mean thresholds ended up in attractors with smaller proportions of elements turned on (Fig. 16). Therefore, changing the mean of the distribution of thresholds changes the steady state conditions of the system. In a honey bee colony, changes in threshold distribution of this kind among workers may result in similar changes of dynamic behaviour with more or fewer individuals specializing in that task.

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Changing the variance in thresholds around a given mean affected both the rate at which the system located an attractor and the stability of the system to external perturbation. A network was constructed where all individual elements had the same mean threshold value of 50.5. Systems with no variance in thresholds located their steady state attractors directly and avoided the search phase. The variance in thresholds among elements of model systems affected dynamic behaviour resulting from external perturbations to the stimulus level. Networks of N = 100 with thresholds drawn at random from a uniform distribution with range 1-100 were compared with networks where all elements had thresholds of 50.5. The initial stimulus level was set equal to N , then increased 20 units every 500 random individual samples. The residual stimulus was then measured at each of 3000 sample events. Networks with no variance in thresholds showed tremendous homeostatic properties; they returned the residual stimulus to predisturbance levels following each increase in the stimulus until all individuals were on, then they lost their ability to regulate. Networks with variable thresholds did not regulate the stimulus level as well, but did regulate the number of individuals turned on in response to each increase in stimulus and provided a more integrated system response to external changes in conditions (Fig. 17). The variance in thresholds of individual workers within honey bee colonies may affect colony-level responses to changing environmental conditions.

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FIG. 17 Regulation of stimulus level by model networks containing N=100 elements with thresholds drawn at random from a uniform distribution with integer values of 1-100 (upper graph) and fixed threshold values of 50.5 (lower graph). (From Page and Mitchell, 1991.)

These particular network models are admittedly simple, but more complex models with multiple tasks, thresholds and stimuli yield similar behaviour (see Page, 1991; Page and Mitchell, unpublished data). This modelling approach allows the exploration of complex dynamic systems, such as insect societies, with relatively simple but powerful methods. Hopefully, it will yield a better understanding of the social dynamics of insect societies and provide direction for further research. 6.2 NATURAL SELECTION OPERATES ON PARAMETERS OF DYNAMIC SYSTEMS The results of the computer simulations discussed above, and also those presented by Page (1991), suggest that division of labour can spontaneously occur among groups of cohabitating and mutually tolerant individuals. Complex behaviour emerges if individuals vary with respect to their likelihood to perform a given task during any given period of time. Differences in task performance likelihoods may be a consequence of individual differences in thresholds of response for task-eliciting stimuli. There are many factors, both genetic and epigenetic, that may cause individuals to vary with respect to their response thresholds, including: 1. Individuals may vary in age and state of development that affects behavioural competency.

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2. Individuals may be similar in age and state of development but differ in their probabilities of performing tasks as a consequence of prior experiences that affect thresholds of response. 3 . Individuals may be the same age and differ in developmental state because of different endogenous rates of development. 4. Individuals may be of the same age and differ in developmental states due to environmental effects on rates of development. 5. Individuals may be of the same age and differ in developmental state due to differences in sensitivity to environmentally induced developmental changes. 6. Individuals may be of the same age and differ in developmental state as a consequence of having a different ontogenetic sequence of behavioural states. The role of natural selection in shaping the organization of complex insect societies may be one of “fine tuning” a self-organized dynamic system. Natural selection “sees” the dynamic properties of the complex society, not the individual worker components of behaviour. The dynamic parameters N, K and (F’)can be affected by changing behavioural components of individual workers and characteristic of queens, hence affecting change in the dynamic properties of the social system. We list below potentially selectable characteristics of individuals that may affect each of the dynamic systems parameters.

6.2.1 Selection on N Selection for colony size may affect N by increasing the number of individuals in all age castes. Selection for queen fecundity or worker life expectancy could increase the colony worker population. An increase in N for a specific age caste also may be achieved by altering behavioural development rates so that individuals belong to a given age caste for a longer or shorter period of time.

6.2.2 Selection on K K can be altered by adjusting the cues and/or signals used as stimuli and by changing the way individuals sample the stimulus environment. Kcan also be adjusted spatially. Individuals located close to each other are more likely to share information-are more connected-than those located in different parts of the nest. The location in the nest where a task is performed may be selectable (Oster and Wilson, 1978; Seeley, 1982).

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Selection on

IF'}

Selection on threshold functions may occur by selecting for higher or lower mean thresholds or by selecting for an increase or decrease in the variances in thresholds within colonies. Changes in mean thresholds result in changes in the likelihood that individuals belonging to specific age castes will perform specific tasks, resulting in concomitant changes in the numbers of individuals engaged in those tasks. Selection on the variance in thresholds may affect the colony's response to changing environments. Selection on the variance may occur by selecting for mechanisms that affect genetic variability within colonies such as polyandry and polygyny or by selection favouring heterozygous or homozygous queens. Behavioural modification through learning can also affect the distribution of thresholds among individuals within colonies. Individuals may be selected to alter their thresholds on the basis of prior experience. For example, an individual's threshold may decrement after performing a specific task, making it more likely that she will perform that task again the next time she encounters the stimulus. The models presented here deal with networks of elements that are restricted to on or ofstates representing the performance of only one or two tasks (assuming that offor one task may be on for another). Networks, however, may also be coupled. Individuals may have multiple states with respect to which tasks of a set they perform while they belong to a given age caste. These states may be determined by multiple stimulus-threshold complexes with various decision rules (Page and Mitchell, 1991). Natural selection may organize these complexes and determine the relative likelihood that an individual will perform one of several tasks at a given age.

6.3

THE ORGANIZATIONAL STRUCTURE OF HONEY BEE SOCIETIES

A honey bee society has to satisfy three organizational needs simultaneously: (1) specialization, presumably because specialists work with greater efficiency (Oster and Wilson, 1978; Jeanne, 1986a,b), (2) phenotypic plasticity in order to respond to changing environments, and (3) phenotypic integrity to ensure that changes in colony phenotype are appropriate. Specialization occurs when specific individuals perform specific activities at a greater frequency than expected if activities are performed by individuals drawn at random. One way to achieve specialization is for each individual to be competent to perform only one particular task at a time. However, the cost of this specialization is a decrease in the amount of time each individual works. This may be due to chance, by not encountering appropriate stimuli

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that elicit the performance of the task, due to low existing stimulus levels associated with that task as a consequence of the environment, or due to low stimulus levels that are a consequence of negative feedback resulting from other workers reducing the stimulus through their activities. Response efficiency is the average proportion of times that individuals perform any task upon encountering appropriate stimuli. Ways to increase response efficiency include reducing thresholds, which may result in an inappropriate number of responses for those tasks, and making individuals competent to respond to multiple stimuli associated with thresholds for multiple tasks. Responding to multiple stimuli may increase the probability of encountering a stimulus that releases a response, hence increases the response efficiency, but may reduce the proportion of work that is performed by specialists because it reduces the number of times given tasks are performed by specific individuals. This represents a trade-off between specialization and response efficiency. Colony-level phenotypic plasticity is the ability of a colony to change the numbers of individuals engaged in each of the possible tasks in response to changing colony or external environments. It can occur as a consequence of individuals changing the frequencies with which they perform particular tasks, perhaps by changing thresholds of response, or as a consequence of ontogenetic changes that result in changes in the proportions of individuals in different age castes. However, inappropriate responses may occur if individuals are too sensitive to stimuli that result in changes of rates of development or have thresholds too low for particular stimuli. Colonies need to be responsive and phenotypically plastic but only within appropriate constraints. This constrained plasticity is colony integrity.

6.3.1 Behuviourul modularity Colony integrity is maintained in the face of fluctuating stimulus environments by behavioural modularity of individuals. Modularity may be achieved by several methods. Behavioural development may result in behavioural modularity because individuals are apparently only competent to perform certain tasks during specific developmental phases. A developmental lag may occur before individuals shift from one set of tasks to another, often with accompanying changes in the development of the appropriate exocrine glands. This developmental lag could buffer the effects of ephemeral changes in colony, or external, stimulus conditions. Evidence for a lag in behavioural development is suggested by the observed delay of about 7 days before individuals in colonies of similar-aged bees become precocious foragers (see Section 3.4).

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Behavioural modularity may be achieved spatially. Individuals tend to perform sets of tasks that correspond with particular regions of the nest. This increases the efficiency with which individuals encounter appropriate stimuli to which they are competent to respond and decreases the likelihood that they will encounter other inappropriate stimuli (Wilson, 1976; Seeley, 1982). An example of spatial modularity may be the observation of Lindauer (1961) that workers that forage on the same floral resources tend to aggregate together in the same regions of the nest. Seeley (1982) demonstrated a correlation between the tasks performed by a given age caste and location in the nest where those tasks are performed. Behavioural modularity may also occur if the genotype of an individual sets limits on its responsiveness to specific stimuli. Some individuals may change the likelihood that they will perform certain activities within a set of bounds determined by their genotype. Within a colony there probably is a distribution of genotypes associated with each activity that is determined by the genomes of the drones with whom the queen mated, and by recombination in the queen. Thus, a colony may have genotypes capable of responding over a wide range of possible stimulus conditions but each individual may have a restricted range. Behavioural modularity may be integrated with individual response efficiency by building task sets with genetic covariance. If there is no genetic covariance and thresholds of response for tasks performed at the same time during an individual’s life are independent of each other, then random assortment of genes relating to those tasks could result in some individuals having low thresholds for all tasks and, as a consequence, being extremely active task performers. Other individuals that have genotypically high thresholds may be relatively inactive. There is some evidence for genetic covariance for at least two tasks, i.e. pollen and nectar foraging. The results of N. W. Calderone and R. E. Page (unpublished data; Calderone, 1988) discussed in Section 5.3 suggest that pollen and nectar collecting are negatively correlated both genetically and environmentally. High-strain bees collected large quantities of pollen and little nectar while low-strain bees collected low quantities of pollen and more nectar. When placed in an environment with small quantities of hoarded pollen (fostered in a low-strain colony) workers of both strains increased their pollen collecting and decreased nectar collecting relative to their sisters fostered in high-strain colonies. Further studies support the hypothesis that pollen and nectar foraging are negatively correlated. R. E. Page and M. K. Fondrk (unpublished data) repeated the two-way selection of Hellmich et af. (1985). One hundred and twenty-five commercial colonies with naturally-mated queens were evaluated for the amount of pollen stored in their combs. Ten high- and 10 low-strain

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parent queens were selected to constitute the foundation stock. Drones were raised from five queens of each group and virgin daughter queens were raised from the remaining five. Several daughters of each queen were instrumentally inseminated with semen from a single, different drone of another selected queen, for example L1 xL2, L3 x L4 , L5 xL 6 , L 7 x L 8 , L9xL10, and likewise for crosses of the high strains. Fifty-one resultant daughter colonies were then examined for quantities of stored pollen. After a single generation of selection, colonies of the high strain had about 1.8 times more stored pollen than colonies of the low strain 127 cm2, N = 27 for the low line; X = 233 cm2, N = 24 for the high line; P < 0.01, Mann-Whitney U test; Sokal and Rohlf, 1981; Fig. 18).

(x=

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FIG. 18 Frequency distribution of colonies for the amount of pollen stored in combs. Measurements (cmZ)were taken on colonies following a single generation of selection for high (a)and low (m) quantities of pollen stored in combs. (From R. E. Page and M. K. Fondrk, unpublished data.)

Two sets of 5-min observations were made of foragers at all 51 colony entrances. Observations were taken randomly and blindly on a single day. Six observers were used simultaneously so that each round of entrance counts only required approximately 90 min to complete. Rounds of counts were taken between 09:30 and 12:30. There was no significant difference between colonies of high and low strains for the total number of foraging trips observed for both observation periods combined 555 for low strains, 543 for high strains). However, the ratio of incoming pollen foragers to non-pollen foragers did differ significantly between the two strains (X=O.30 and 0.39 for low and high strains, respectively; P
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There were no differences between high and low strains for the proportion of nectar foragers, water foragers, or “empty” bees, returning without pollen. In summary, a single generation of selection significantly changed the ratio of pollen and nectar collectors within the foraging age caste of colonies but did not change the colony’s allocation of labour to foraging.

6.3.2 Behavioural canalization and heritability In order to maintain genotype-based behavioural modularity within age castes, it is necessary for individuals to avoid, or at least reduce, behavioural canalization. Canalization occurs if prior activities of an individual influences the probabilities of performing current activities. Early experience may not be good predictors of colony needs at later stages of individual behavioural development. If early experiences determine the likelihood of individual activities later in life, then the common colony environment will canalize individuals towards similar behaviour, reducing behavioural modularity, and disrupting colony integrity. To avoid behavioural canalization, subsequent behavioural states can be “walled off” from previous experiences. Each individual’s initial response thresholds could then be based on genotypically programmed information. After the initial setting of threshold conditions, each individual may vary as environmental conditions change within bounds set by its genotype. If behavioural canalization is not avoided, the heritability of traits expressed late in life, like foraging for pollen, would be greatly reduced due to all of the possible environmental effects on individual behaviour. However, the heritability for pollen collecting behaviour is apparently quite high (see Section 4.1.2). In addition, bees that have been selected for specific traits, like pollen collecting, hygienic behaviour and defence, demonstrate these behavioural characteristics even when raised in common or extremely atypical colony environments. In other words, they don’t demonstrate the effects of behavioural canalization.

6.4

GENOTYPIC VARIABILITY AND ADAPTATION

As we have seen above, polyandry generates genotypic variability within colonies and this variability may lead to functional modularity of behaviour. It is tempting to ask if genotypic variability is itself an adaptation with respect to its possibly-functional role in division of labour. Crozier and Consul (1976) demonstrated that genetic polymorphism for worker traits can be maintained at a single locus by colony-level selection if heterozygous workers make colonies more “fit” than homozygous workers.

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Moritz and Southwick (1987) demonstrated that experimental groups of workers that were genotypically more heterogeneous responded more to alarm pheromone than did homogeneous groups. They proposed that this constitutes a colony-level analogue of genetic overdominance and that it may be selectively advantageous. If so, then polyandry may be selectively favoured in order to generate the necessary genotypic variability. Kolmes et al. (1989) suggested that genotypic variability is important when colonies are under environmental stress. Colonies with greater genotypic diversity are better able to adjust their work force in response to changing colony needs. They propose that genotypic variability may be crucial to colony behaviour. Page et al. (1989b) proposed a model for how genetic polymorphism for individual worker behavioural traits can be maintained within populations if the genotype of an individual results in her becoming a “genetic specialist” for an activity. The model is based on the assumption that specialization by workers increases the survival and/or reproductive success of queens through greater efficiency of the colony workforce. They assume that the activities of pollen and nectar collecting are under the control of two gene loci, each with two alleles. These assumed loci are dosage compensated so that all individuals forage for pollen or nectar with the same expected frequency and success, only the probability of foraging for each is affected. Locus A controls the likelihood of foraging for pollen while locus B controls nectar foraging. An individual with the genotype AAbb forages only for pollen while an individual aaBB only for nectar; they are extreme specialists. AABB, aabb and AaBb genotypes forage half of the time for pollen and half for nectar; they are generalists. AABb and Aabb forage 75% of the time pollen and AaBB and aaBb 75% of the time for nectar; they are “quasispecialists”. Assuming that the optimal, “most fit”, queens (colonies) have half their worker force foraging for nectar and the other half foraging for pollen, then there are several ways this optimum allocation can be met. For example, colonies may consist of all AaBb, AABB or aabb individuals; or colonies may have half their workers with AAbb or AaBB genotypes and half with aaBB or AABb, respectively. If optimal colonies with genetic specialists and/ or quasi-specialists are sufficiently more fit (produce more reproductive offspring), then these loci will be maintained in a polymorphic state by colony-level selection and polyandry results in higher average fitness of polymorphic populations. If not, then the population should become fixed for one of two alternative states, all AABB or aabb genotypes. The question remains of whether the model makes realistic assumptions. It seems likely that specialists improve colony efficiency which is related to

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colony survival and reproductive success (see Wilson, 1985b). If, however, workers become nectar or pollen foragers on the basis of a single event and then, through positive reinforcement, remain constant to that activity for the rest of their lives, then all workers are extreme specialists, regardless of genotype, and the model fails to represent the system. In contrast, if workers switch between pollen and nectar foraging, and the likelihood that they switch is determined in some way by their genotype, then the model may have some relevance. Some workers apparently do change between pollen and nectar foraging activities at some time during their lives (see Ribbands, 1953). Fewell and Winston (1990) showed that colonies alter their ratio of pollen and nectar collectors, but do not alter their total forager population, in response to changes in the amount of pollen stored in the hive. R. E. Page and M. K. Fondrk (unpublished data) found that individual workers from colonies confined to flight cages switch between nectar and pollen collecting in response to changes in quantities of stored pollen and brood. The contrived genetic system of two alleles and two loci is probably unrealistic. However, the selection response of populations studied by Hellmich et al. (1985) suggests that pollen foraging is a polygenic trait with high heritability (expressed genetic variability). The study of R. E. Page and M. K. Fondrk (unpublished) discussed above suggests that the kind of dosage compensation assumed in the model of Page er al. (1989b) may be reasonable. It is important to note that simply demonstrating the consequences of genotypic variability does not necessarily imply that genotypic variability is an adaptive trait facilitating division of labour. Polyandry increases genotypic diversity within colonies but isn’t necessary; recombination within heterozygous queens also generates diversity. Polyandry may have evolved in honey bees and other social insects in response to selection unrelated to its effects on division of labour (see Section 2.4). Colony-level selection may now operate on the consequences of the genotypic variability that polyandry generates. It is also possible that the observed intracolonial genotypic variability is present only because of the inability of directional selection to eliminate it in favour of the “best” genotype. The inability to fix an optimal genotype in a population may be due to unpredictable changes in the environment or complicated population structure. 7

Conclusions

Honey bee colonies are dynamic biological entities. They are composed of a diverse mixture of genotypes due to recombination and the polyandrous mating behaviour of queens which results in a rich diversity of behaviour, both among individual workers within colonies and among colonies within

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populations. They are constantly changing states due t o colony- and individual-level developmental processes and often change reproductive states in association with queen replacement or loss. They are hierarchically organized biological units upon which natural selection can potentially operate at several levels: the gene, the individual and the colony (Wilson and Sober, 1989). It is this theoretical framework that should guide our quest for understanding the evolutionary genetics of insect sociality.

Acknowledgements This work was funded by National Science Foundation grants BNS 8719283 and BNS 9096139 to R.E.P. and by an Ohio State University Postdoctoral Fellowship and National Science Foundation grant BSR 8800227 to G.E.R.

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Aerodynamics and the Origin of Insect Flight C. P. Ellington Department of Zoology, University of Cambridge, Downing Street, Cambridge C623EJ, UK

1 Introduction 171 2 Early insects 172 2.1 Fossil record 172 2.2 Protopterygotes 174 2.3 The first flights 175 3 Basic aerodynamics 178 3.1 Fluid forces 178 3.2 Reynolds number 180 3.3 A miscellany of force coefficients 182 3.4 Mechanics of gliding 186 4 Gliding cylindrical bodies 188 4.1 Resolved-flow analysis 189 4.2 Lift and drag at constant Reynolds numbers 4.3 Glide characteristics of cylinders 194 4.4 Stability and control 198 5 Aerodynamic function of winglets 200 5.1 Improvement of glide angle 200 5.2 Reduction of glide speed 203 6 Concluding remarks 206 Acknowledgements 208 References 208

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Introduction

The extraordinary diversity and success of the insects is largely due to their flying ability and the scope it offers for mobility, dispersal and migration. They are such accomplished fliers that it is difficult to imagine them falling out of trees and descending to the ground in a rather unprofessional and uncontrolled manner. But how else did flight begin? The origins of insect flight are shrouded in the past, and studies of the fossil record and extant insects have provided few answers. Speculations abound with no hard facts to check them, and we are left in a rather unconvincing muddle. ADVANCES IN INSECT PHYSIOLOGY VOL. 23 ISBN 0-12424223-0

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The aerodynamic analysis of modern insect flight has become increasingly sophisticated, and it should certainly be possible to extend this approach to the origin of flight. Such an analysis could prove extremely useful, but only one theoretical treatment has been published (Flower, 1964) and all details of the calculations were omitted! In more recent years two studies have investigated some aspects of the aerodynamics of early flight using an empirical approach with models (Kingsolver and Koehl, 1985; Wootton and Ellington, 1991). These experiments are both helpful and fun, and it seems likely that others will follow the lead. A theoretical framework is again needed for guiding such experiments and for interpreting the results most effectively. The aim of this review is to evaluate the plausibility of different hypotheses about the origin of flight using suitable theories. The major aerodynamic problems in the evolution of flight are identified and analysed as far as possible. Many of the “conclusions” are necessarily tentative because empirical data are lacking and have to be approximated from results on related problems in engineering and aerodynamics. With future modelling studies all of the necessary data can be collected, and we may then be able to review the origin of insect flight once again with greater confidence.

2 Early insects The insect fossil record provides few hints to the origins of flight, thus ensuring that it will remain a topic for endless speculation. Nevertheless, the record must first be consulted for valuable clues about the “protopterygotes”, the immediate ancestors of winged insects. What did they look like? When did they appear? How did flight begin?

2.1

FOSSIL RECORD

In terms of numbers and diversity, the insect fossil record begins in earnest around the Middle to Upper Carboniferous period; see Wootton (198 1) for a comprehensive review of Palaeozoic insects. There were primitive wingless forms (Apterygota, such as the silverfish Thysanura), but winged insects (Pterygota) were far more common. Indeed, the winged insects had already split into two groups with substantial radiations: the Palaeoptera, which had outstretched wings like modern dragonflies, and the Neoptera, which could fold their wings back over the body. Although somewhat disputed, it seems probable that the first winged insects were palaeopterous, so this group is more central to our interests; wing folding was a later development and must

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have greatly improved mobility on the ground. The Pterygota and Neoptera are likely to be monophyletic groups, but there is some uncertainty about the Palaeoptera (Hennig, 198 1; Kristensen, 198 1; Wootton, 1981; Carpenter and Burnham, 1985). Following Wootton (1990), Fig. 1 shows the number of families of the principal definable groupings which resulted from the early radiations of the Palaeoptera and Neoptera in the Upper Carboniferous and Early Permian. The Palaeoptera were already divided into at least three clades, the Palaeodictyopteroidea, the Odonatoidea and the Ephemeroptera. The Palaeodictyopteroidea were a group of three or four fluid-€ceding orders, characterized by piercing and sucking mouthparts, which were among the most numerous Carboniferous insects but which disappeared by the end of the Permian. They covered an enormous size range, with wingspans between 10 and 560mm, and the form of their wings suggests a variety of adaptations in flight style (Wootton, 1981). Prothoracic lateral lobes were well developed and are widely assumed to be serially homologous with the wings. The Odonatoidea were the predacious dragonfly group, including the primitive Protodonata and later, in the Permian, the first true Odonata. Again, there was a remarkable size range with wingspans from 30 to 710 mm. Their wings are similar enough to extant dragonflies to suggest comparable flight capabilities, although perhaps with less refinement (Wootton, 1981). The Ephemeroptera (mayflies) form the third group of the Palaeoptera, and they appeared rather later than the others. Onefamily

Lower Permian

Wer Carboniferous Palaeodictyopteroidea Ephemeroptera Caloneurodea Dictyoptera Odonatoidea Protorthoptera Onhoptera

Palaeoptera

B Neoptera

FIG. 1 Number of insect families in the major groups during the Upper Carboniferous and Lower Permian. (From Wootton, 1990.)

The Neoptera comprised four distinct orders: the Orthoptera (grasshoppers and crickets with jumping hind limbs), the Dictyoptera (cockroaches, remarkably like modern forms), and the extinct Caloneurodea and Miomoptera. A rag-bag collection, the ‘Protorthoptera’, await a consensus on their

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classification; they are rather generalized neopterans with chewing mouthparts, and some have well-developed prothoracic lateral lobes. This rich variety of early winged insects was equally matched by diverse ecological adaptations (Carpenter, I97 1; Wootton, 1972), plant-arthropod interactions (Smart and Hughes, 1972; Sharov, 1973; Scott and Taylor, 1983; Taylor and Scott, 1983; Scott et al., 1985), and predator-prey interactions with each other, with other arthropods, and with the early tetrapods (Bolt, 1969; Carroll, 1977; Wootton, 1981). Perhaps it is not surprising that, in such a well-established community, we can find no hints to the origins of insect flight. What did the “protopterygotes”, the immediate ancestors of winged insects, look like? Primitively wingless insects are found as early as the Lower Devonian (Collembola, springtails: Whalley and Jarzembowski, 1981) and Middle Devonian (Archeognatha, bristletails: Shear et nl., 1984; Labandeira et al., 1988). Diplura (bristletails with mouthparts hidden by folds of the head capsule), Thysanura and the extinct semi-aquatic Monura occur later in deposits from the Upper Carboniferous (Kukalova-Peck, 1987). Thus the fossil record is poor until the appearance of winged insects in the Middle and Upper Carboniferous, and no intermediate protopterygote forms have been found. Given the diversity of insects by the Middle Carboniferous, it is likely that considerable radiation had already taken place by the Middle to Upper Devonian (Hennig, 1981; Wootton, 1976, 1986, 1990). The Upper Devonian would have been especially well suited to the evolution of insect flight with tree-like mixed forests of lycopods, sphenopsids, pteridosperms and progymnosperms (Wootton and Ellington, 1991). Kukalova-Peck (1987) argues that the first major phyletic splitting of the insects occurred much earlier, following the colonization of shores by plants in the Ordovician. However, there are three other major splits in her scheme before the appearance of pterygotes, and there is no compelling reason to look earlier than the Upper Devonian, perhaps 345 million years ago, for the origin of flying insects (Wootton, 1986). It also seems probable that the origin of flight was a single event in the evolution of insects (Forbes, 1943; Wigglesworth, 1963; Kukalova-Peck, 1978, 1983; Wootton, 1986), although some believe that flight arose independently more than once (Matsuda, 1981; Le Greca, 1980).

2.2

PROTOPTERYGOTES

There is now common ground in believing that the protopterygotes possessed a segmental series of paired lateral projections on the thorax and abdomen. However, the anatomical origin of these “winglets” or “protowings” has

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been argued about for over a century: for recent reviews see Quartau (1985, 1986), Wootton (1986) and Kukalova-Peck (1 987). The paranotal lobe hypothesis, initiated by Miiller (1875) and restated and developed by others (e.g. Forbes, 1943; Hinton, 1963; Rasnitsyn, 1981), proposes that the winglets were immobile extensions of the terga. The other hypothesis, due originally to Oken (181 1) and extended by Wigglesworth (1973, 1976) and Kukalova-Peck (1978, 1983, 1987), claims a pleural origin for winglets that were already articulated and movable. These winglets would be homologous with the exites of archaic basal leg segments and more recently with the gillplates of many aquatic nymphs, such as mayflies. Indeed, Kukalova-Peck suggests that the winglets originally functioned in juvenile forms for aquatic respiration, ventilation and possibly locomotion; Hennig (1981) and Rasnitsyn (1981), however, consider that the aquatic habitat is a secondary adaptation. In both hypotheses, the primitive insect with two pairs of wings would result from enlargement of the meso- and metathoracic pairs of winglets and loss of the other pairs. The paranotal lobe hypothesis would necessarily require the development of a mobile articulation de novo a t some stage in this process, but the weight of modern evidence seems to favour a pleural origin and movable winglets instead. Most insect palaeontologists are naturally reluctant to invite criticism by publishing drawings of protopterygotes, but Wootton (1986) has done so in a delightfully informal review (Fig. 2). The size and shape of the winglets are conjectural, and “absolutely no reliance is to be placed on any detail of that figure!” (R. J. Wootton, personal communication), but it gives an idea of the basic groundplan.

2.3

THE FIRST FLIGHTS

Initially the winglets must have been small relative to the body size. Were they too small to be of any aerodynamic benefit before enlargement? Many ideas have been put forward over the years about the early function of winglets, ranging from the practical (they protected the legs) to the fanciful (they prevented insects from sinking in water, or acted as stabilizers); for a fascinating discussion see Proc. R.ent. SOC.Lond. C 28, 21-32 (1963). More reasoned arguments (Alexander and Brown, 1963) suggest that they were involved in courtship display and only assumed an aerodynamic function after enlargement. Others (Whalley, 1979; Douglas, 1981; Kingsolver and Koehl, 1985, 1989) support an early thermoregulatory role for the winglets, and Kingsolver and Koehl(l985) demonstrated with model experiments that selection for increased thermoregulatory capacity would indeed favour

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FIG. 2 Drawing of a possible protopterygote. (From Wootton, 1986.) See disclaimer in the text.

enlargement of small winglets. However, if Kukalova-Peck is correct in that winglets were initially derived for aquatic purposes, they may have been of substantial dimensions before the transition to land; perhaps they were large enough for an immediate aerodynamic benefit after all. At some stage, perhaps from the beginning, the winglets would assume an aerodynamic role. The route by which flight then evolved is another longstanding contentious issue, with advocates falling into three broad groups. 1. The paraglider rheories (Forbes, 1943; Hinton, 1963; Flower, 1964; Rasnitsyn, 1981). The most popular hypothesis is that the winglets facilitated stable parachuting and gliding by arboreal protopterygotes falling or jumping from tall vegetation. A steady improvement in glide performance is envisaged with increasing winglet size. This hypothesis will be elaborated upon in later sections. 2. Thejuating theories (Wigglesworth, 1963, 1976; Norberg, 1972). Many small insects, winged and wingless, rely on passive dispersal by convective air currents. This hypothesis states that flat or fringed winglets acted as viscous high-drag devices to improve dispersal for tiny insects. There are two difficulties with this proposal, both arising from the small size of the protopterygote. First, as seen in many plant seeds, plumed structures would be better for high drag than flat winglets; orientation has little effect on drag for such small sizes, so there are no obvious selection pressures for the plumes to become flat. Second, there would be an awkward transition between the drag-producing winglets and the

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lift-producing winglets required for true flight. The body length would have to increase by at least an order of magnitude before lift generation becomes effective, but any increase in size would worsen the floating ability. Tiny protopterygotes could conceivably improve their lift production using unsteady aerodynamic mechanisms (see Ellington, 1984), but this would require the evolution of flapping flight with a complex wing motion at an improbably early stage. 3. The running jump]flying leap theory (Kukalovri-Peck, 1987). It has been suggested that winglets improved the glide performance of cursorial insects during aerial excursions initiated by a jumping or running start. With articulated winglets, active flapping might even have been employed from the beginning. It should be noted that the running jump hypothesis is most unlikely, because running protopterygotes almost certainly could not have attained take-off speed (Wootton, 1986). However, a powerful jump to gain height and speed, followed by a glide, is still worthy of consideration. Apart from the single speculation about active flapping, all aerodynamic theories postulate some form of gliding as the first step; parachuting and floating are simply examples of a poor gliding ability. Would tiny winglets significantly improve the gliding performance of protopterygotes? If not, a non-aerodynamic role which favoured enlargement must be accepted before the evolution of flight got off the ground. What are the reasons behind the enlargement of some winglet pairs and the loss of others? Would movable articulations be beneficial for gliding, or were they not required before active flapping developed? These are some of the questions that must be answered about the evolution of insect flight. Speculation on such topics has been widespread but is no substitute for a detailed aerodynamic analysis of gliding protopterygotes. Ideally, the analysis should be a blend of theory and modelling experiments, but the experimental approach has been sorely neglected. Kingsolver and Koehl (1985) first opened the door to modelling experiments, measuring lift and drag characteristics of protopterygote models in a wind tunnel. This was followed by Wootton and Ellington (1 991), who studied the glide performance of models. Both studies are of limited scope, but they mark the beginning of a fascinating new approach to the problem. The theoretical aerodynamic approach has been neglected even more; Flower’s (1964) study is the only serious theoretical treatment. There is much in the engineering and aerodynamic literature that may prove relevant to the origins of insect flight, probably to the surprise of those authors, and the purpose of this review is to assemble such material. The resulting theoretical framework may help to decide on the plausibility of existing hypotheses, and to guide future experimental investigations on protopterygotes.

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3

Basic aerodynamics

Before we can begin to understand protopterygote flight, a basic knowledge of aerodynamics is necessary. There are some excellent general reference books; my personal favourites are still Prandtl and Tietjens (1957a,b) for their appeal to physical intuition, Batchelor (1970) for his combination of mathematics and physics, and Milne-Thomson (1973) for his concise mathematical treatment. Other helpful texts are von Mises (1959), Duncan et al. (1970) and Bramwell (1976). Hoerner (1958) and Hoerner and Borst (1975) provide a marvellous compendium of fluid forces in conventional and unconventional situations, but beware of numerous errors in the equations. The splendid book by Vogel(l981) deserves special mention; this is an expert introduction to biological fluid dynamics, written by a biologist in a style that is absorbing, engaging and painless (except for the puns!). Many biologists are put off the subject of aerodynamics by an apparent abundance of equations, but this is an unfortunate reaction. Most of the basic equations are of an identical form which can readily be understood and, once this is done, the rest follows fairly easily.

3.1

FLUID FORCES

Any object moving through a fluid, such as air or water, produces a drag force resisting the motion. As drawn in Fig. 3 from the conventional viewpoint of an observer stationary with respect to the object, the fluid flows around the object and causes a drag force directed towards the right. Particles, or elements, of the fluid must experience a reaction force directed towards the left as they pass the object, and this slows them down. As new fluid elements pass by they are also decelerated, and so there is a continual loss of momentum in the fluid. The drag force can therefore be interpreted as this rate of change of momentum in the fluid. There are two, fundamentally different types of forces which can cause deceleration (or acceleration) of a fluid element: viscous forces which act tangentially on its surface, and inertial forces which are manifest as pressures normal to the surface. The viscous force arises from a shearing motion, and is best illustrated by a flat plate parallel to the airflow (Fig. 3a). Air immediately in contact with the surface of an object cannot move relative to it: a phenomenon known as the no-slip condition. Some distance away from the surface, however, the air moves along at the velocity Vof the undisturbed air. The air velocity necessarily changes between these regions, with thin layers of air sliding past each other. This gives rise to a velocity gradient confined to a boundary layer of thickness 6, usually defined rather arbitrarily as the

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a

b

FIG. 3 (a) Laminar flow around a smooth flat plate oriented parallel to the airstream. The flow is retarded in the boundary layer, which grows in thickness 6 along the plate. The momentum loss is evident in the wake downstream of the plate. The vertical scale is greatly exaggerated. (b) A flat plate perpendicular to the a i d o w produces a large, slowly moving wake. Over the range of Reynolds numbers appropriate to insect flight, it is quite common for vortices to be shed alternately from either side of the plate.

distance where the air velocity reaches 99% of the undisturbed, free-stream value. Air within the boundary layer was initially moving at the free-stream velocity before the object acted upon it, so the viscous force has decelerated the air in the boundary layer. As this retarded air moves beyond the plate, it becomes the wake: a slowly moving region where the momentum loss is clearly evident. The viscous force reflects the continual stretching and breaking of weak bonds (e.g. van der Waals forces) between fluid molecules due to the shearing motion in the boundary layer. The work done in stretching these bonds is converted into heat when they break, and it is equal to the kinetic energy loss of the retarded flow. The object experiences a viscous drag force parallel to its surface, appropriately known as the skin friction. As might be expected this force is

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C. P. ELLINGTON

determined by the surface area, or wetted area, A , exposed to the fluid, and by the velocity gradient, which is proportional to V / 6 .The viscous force F, is therefore proportional to

where p is the viscosity of the fluid. The other type of drag is an inertial force acting normal to the surface of the object. Imagine a flat plate perpendicular to the airflow (Fig. 3b), impeding the air just as if you held up a hand to stop it. The fluid arrives with velocity V , and the area of fluid affected will be roughly proportional to the cross-sectional area, orfrontal area, A , of the plate. The volume of affected air arriving per unit time is therefore A,V, and the corresponding mass per unit time is pA,V, where p is the density of air. The momentum per unit time carried along with this air is the mass per unit time multiplied by its velocity, and equal to pA,V2. If the plate took away all of this momentum per unit time, i.e. if the air was completely stopped, the rate of change of momentum would simply be pA,V2. The force experienced by the plate must be equal to this same rate of change of momentum. In reality, the plate cannot extract all of this momentum because the air flows around it: the air is decelerated but not stopped. Nevertheless, we should still expect the inertial force F, to be proportional to pA,V2:

F,ccpA,V2. Downstream of the plate there is a broad, slowly moving wake, indicating that the inertial drag force is very large.

3.2

REYNOLDS NUMBER

The relative importance of these two types of forces is given by the Reynolds number Re, which is the ratio of inertial forces to viscous forces. The wetted area A , and frontal area A, are both proportional to the square of some “characteristic length” I of the object, and the boundary layer thickness 6 is proportional to 1. We can therefore use equations (1) and (2) to write F,/F,oc p W / p = IVjv, where v (= p/p) is the kinematic viscosity of the fluid. The Reynolds number is thus defined as

Re=lV/v.

(3)

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For air at 15°C and standard atmospheric pressure, p is 1.23 kgm-3, p is 1.80 kgm-I s-’, and v is 1.46 x 10-’m2 s-’. The “characteristic length” for the Reynolds number is defined by convention for different flow situations. For an object like a gliding insect or protopterygote body, the body length is usually the appropriate dimension. We shall see that the Re range of interest to gliding protopterygotes is about 102-104,indicating that inertial fluid forces dominate. This is true for all types of macroscopic animal locomotion as well as conventional aerodynamics and hydrodynamics, and it explains the ubiquitous form of the force equation

F= ‘I2pAV2CF.

(4)

This equation, derived from equation ( 2 ) , is appropriate whenever inertial forces are paramount. The proportionality coefficient C, is the force coeficient which often has to be determined empirically. It is a very useful, dimensionless measure of the force on an object of given area moving at a given speed. The factor of 112 has significance in fluid dynamics, because it relates the equation to the dynamic pressure term in Bernoulli’s equation, but such historical niceties need not concern us here. This useful equation appears under many guises; it can be used for drag forces or lift forces, in which case the force coefficient is known as the drag coeficient C, or lift coejicient C,, respectively. The appropriate area A in such equations is fixed by convention; it is often the frontal area for drag-producing objects like an insect body, but the planform area is used for the lift and drag forces on wings. The Reynolds number is a convenient guide to the importance of inertial and viscous forces, but it should not be taken literally as their ratio: too many complicating factors and proportionality constants have been swept under the carpet in the quest for convenience. For example, the skin friction on objects is still a significant component of their total drag for Re up to several thousand; it is not less than 0.1 YOof the total drag, as a strict interpretation of the ratio might suggest. The flight of protopterygotes and insects is therefore in an awkward fluid regime between viscous forces which are proportional to V , and inertial forces which are proportional to V 2 . The resulting quadratic dependence of fluid forces on velocity causes a systematic change in force coefficients for the standard, inertial equation (4); drag coefficients will tend to decrease with Re because the viscous force component increases only as the first power of velocity and not the second power, which is the form assumed in the equation. Larger and faster objects thus have relatively lower drag over the Re range of interest to us. We shall see that the performance of gliding protopterygotes is a critical function of their size because of this Re dependence of forces.

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Although this Re range complicates the force coefficients, it greatly simplifies the nature of the fluid flow: there is no turbulence. Turbulent flows, in which chaotic and energetic random motions are superimposed on the mean airflow pattern, occur at Re more relevant to bird flight (around lo5). For insects and protopterygotes the flow remains laminar, moving in an orderly and more or less parallel fashion, as in Fig. 3a. Even sharp, protruding edges cannot provoke the well-behaved flow beyond a graceful reply. In Fig. 3b the flow separates from the plate at the edges, but curls up on itself and attaches to the plate as a vortex. The attached vortices are unstable at these Re, alternately detaching from either edge and forming a picturesque wake of vortices spinning in opposite directions. Similar flow patterns are seen around circular cylinders, forming what is known as a von Karman wake. 3.3

A MISCELLANY OF FORCE COEFFICIENTS

To the uninitiated, force coefficients for bodies and wings may seem meaningless; some background knowledge is needed to appreciate their values. The force coefficients for geometrical shapes commonly provide such a framework, and several examples are presented here. They have been chosen for their utility in sections that follow. 3.3.1 Flat plate in parallelflow The simplest case has already been mentioned: the laminar flow over a smooth flat plate parallel to the airflow (Fig. 3a). Laminar skin friction is the only source of drag, and this drag is indeed the theoretical minimum for a flat plate: surface irregularities and other orientations of the plate will increase the drag. The classical solution of the laminar boundary layer equations by Blasius gives the skin-friction drag coefficient C, for use in equation (4) as

C,= 1.33 Re,-'/*,

(5)

where the appropriate area to use is the wetted area A,,,, and the Reynolds number is based on the length I in the direction of flow (Re,=IV/v). For comparison with the drag of wings, a skin-friction drag coefficient C,,f based on the wing planform area S (maximum projected area) is more convenient. S is half the wetted area, and so C,,, is simply twice C,: C,,,

= 2.66 Re-'''.

(6)

The marked decrease of the skin friction coefficients with Reynolds number (Fig. 4) is because viscous forces are proportional to V and not V 2 .

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AERODYNAMICS AND THE ORIGIN OF INSECT FLIGHT

n 2

3

4

5

6

log(R4

FIG. 4 Drag coefficients for plates and cylinders plotted against log (Re).The curve for the normal flat plate is from equation (7), and the dashed region indicates the extrapolation of Eisner (1931). The dashed line for the normal cylinder is given by equation (9). The curve for the parallel flat plate is from equation (6).

3.3.2 Flat plate in normalflow At the other extreme we want to know the drag coefficients in fully separated flow, such as a flat plate normal to the flow (Fig. 3b). The drag varies with the ratio of the width d to length I (both dimensions are normal to the direction of flow), so results for a very long (i.e. infinitely long) plate are usually considered first. The flow is then virtually two-dimensional, or identical everywhere along the length; the drag coefficient for this two-dimensional case will be denoted by CD'.The corresponding area to use in equation (4) is the frontal area dl, and Reynolds number is based on the width (Re,= dV/v). There are many experimental measurements of CD' at Re above several thousand, but I have not found published values in the range of lo2-lo3. Eisner (1931) presents an extrapolation over this range, joining up experiments at higher and lower Re. A useful approximation for his results is given by

+

CDp = 1-95 50/Re,,

(7)

which is shown in Fig. 4 with the extrapolated region dashed; this equation fits his data to an accuracy of better than 5%. The inertial pressure drag is

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more or less constant and dominates the viscous skin friction except at low Re, where C,‘ then rises. There is some evidence for a localized hump in the drag coefficient at Re around 300 for square and circular plates, associated with a change in the vortex wake structure (Hoerner, 1958). The extent to which this might occur for infinite plates needs investigation. The drag coefficient for plates of finite length C, drops very quickly, because the air “sneaks” around the ends instead of flowing around the width. Hoerner (1958) summarizes results for laminar experiments at Re, greater than lo3. For a width:length ratio (d/l)of 0.1, C, is only 68% of that for an infinite length, and it rapidly stabilizes at a value of 61% for ratios greater than 0.2. The curve in Fig. 5 is an approximation to his results, accurate to better than 2%, and is given by

C,/C,‘

= 0.61

+ 0.39exp( - 17d/l),

(8)

where CD‘is the value for an infinite plate, given by equation (7). 3.3.3

Cylinder in normal flow

Elongate insect bodies are rather similar to circular cylinders, which will prove to be a worthwhile addition to this miscellany. The drag coefficient for

diameter:length ratio (dil)

FIG. 5 Drag coefficients for plates and cylinders of finite length divided by the corresponding value for an infinite length, plotted against the diameter : length ratio. The solid line for the plate is from equation (8), and the dashed line for the cylinder is from equation (10).

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circular cylinders normal, or broadside, to their motion is one of the best studied subjects of experimental fluid mechanics. A summary graph, derived from Goldstein (1965), is shown in Fig. 4 for the normal drag coefficient CN‘ of very (infinitely) long cylinders plotted against Reynolds number based on the width, or diameter (Re, = dV/v).The appropriate area for use in equation (4) is the planform area dl, where I is the length of the cylinder. C,’ can be divided into two components: an inertial pressure drag which is more or less constant from Re around 30 to lo’, and a viscous skin friction which increases with lower Re. The rise in C,‘ at the left-hand end of Fig. 4 largely reflects this higher skin friction. As Re increases the pressure drag gradually dominates, and C,’ settles down to a more or less constant value of 1.1 (*0.1). The sharp drop above Re of 10’ is where the boundary layer becomes turbulent, and flow separation is reduced; there is an increase in skin friction because of the turbulent boundary layer, but this is more than offset by a strong reduction in pressure drag. Even for the laminar flow regime, the “wobbles” in the graph preclude a detailed analytical approximation for C,‘. However, I have found the following approximation for laminar conditions to be useful in many situations:

C,’= 1.1 +22/Re,.

(9)

This approximation is shown by the dashed line in Fig. 4 and is accurate to better than & lo%, which is adequate for our purposes. As for flat plates normal to the flow, the drag coefficient C, for cylinders of finite length falls off very quickly. Hoerner (1958) again summarizes results for laminar experiments at Re, = 10’. When the diameter : length ratio ( d / l )is 0.1, C, is only 73% of that for an infinite length, and it is 57% for ratios greater than 0.4. The results cannot be described very well by a single, simple function for all values of d/l. Ignoring the initial rapid decline, however, an approximation accurate to better than 2% can be found for d/l> 0.04: C,/C,‘

= 0.57

+ 0.34exp(

-

7.6d/l),

where C,’ is the value for an infinite cylinder, given by equation (9). The approximation is shown in Fig. 5, but it should be noted that confirmation at Re, lower than lo5 is needed. 3.3.4 Cylinder in axialflow The drag of a cylinder with its axis parallel to the airflow is very relevant to our interests. because it is closer to the usual orientation of insect bodies in

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flight. However,-results for axial flow reveal that the pressure drag component can be markedly influenced by the exact shape of the ends of the cylinder, making generalizations difficult. This problem can be ignored for cylinders with low d : I ratios, where the skin friction over the large surface area is much greater than the pressure drag over the relatively small frontal area. For cylinders with ratios less than about 0.15, therefore, a reasonable approximation to the drag is given by skin friction alone (Hoerner, 1958). We shall define a tangential drag coefficient C, for circular cylinders in axial flow, as a natural partner to the normal coefficient described above; the skin friction force is tangential to the axis of the cylinder. The tangential drag can readily be estimated from the laminar skin friction coefficient C, in equation (3, using a Reynolds number based on the cylinder length (Re,= IVjv) and the wetted area A , ( = ndl) of the cylinder. However, this will not quite be correct; C, is derived for parallel, two-dimensional flow, but the boundary layer around the cylinder is circular. A correction for the cylindrical geometry, which becomes large at low R e because of relatively thicker boundary layers, is usually added to the C, estimate. The tangential drag coefficient based on planform area ( d o , for compatibility with C,, is given by

where the second term represents the correction (Hoerner, 1958). To convert this to a tangential drag coefficient based on wetted area simply divide by x , and for one based on frontal area multiply by 41/nd.

3.4

MECHANICS OF GLIDING

The final piece of aerodynamic background needed for our jigsaw puzzle is the mechanics of gliding. A steady glide in still air is characterized by the flight speed V and the glide angle 8 of the flight path relative to horizontal. Conventionally, this is drawn from the viewpoint of an observer moving with the glider, so that the glider is stationary with air flowing around it (Fig. 6a). By definition, the lift force L is perpendicular to the air motion and the drag force D is parallel. When added together, these forces constitute the resultant aerodynamic force R acting on the insect. The only other force involved is the weight mg acting downwards, where m is the mass of the insect and g is gravitational acceleration. For steady, unaccelerated motion the forces must balance, so the resultant force must be vertical and equal in magnitude to mg. The lift and drag forces are related to the weight and glide angle by simple trigonometric expressions:

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AERODYNAMICS A N D THE ORIGIN OF INSECT FLIGHT

L = mg cos0, D = mg sin@. The lift and drag are produced by both the body and the wings, and they can also be written in the usual form for inertial forces:

L = Il2pAV2C,, D = ' / , P A V2CD. where C, and C, are the lift and drag coefficients. The definition of the appropriate area A will be given below as needed. Combining these four equations, the glide angle 0 is expressed by

0 = arctan(D/L) = arctan(C,/C,),

(16)

and the magnitude of the resultant force is given by mg= [L2+ D2]112 = ' / 2 p Ap[C: 4- CD2]1'2.

(17)

These two equations ensure that the resultant aerodynamic force is vertical, and that its magnitude supports the body weight.

a

b R

R

FIG. 6 (a) In gliding the lift L and drag D are respectively perpendicular and parallel to the air velocity V. The resultant aerodynamic force R balances the weight of the insect. (b) In a resolved-flow analysis, the air velocity is broken down into normal VN and tangential V, components relative to the longitudinal axis of the body. The angle between this axis and the air velocity is the angle of attack of the body E ~ . Normal and tangential drag forces D, and D, are calculated from the velocity components, giving the resultant force R.

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The glide angle 8 is shallow for good gliders such as birds and man-made machines, allowing small angle approximations to be employed: cos0 z 1, and sin0z0, for 0 measured in radians. Some of the equations are then reduced to simple forms: L z mg, D z mg0, and mg z ' / 2 p AVC,. Because insects and presumably protopterygotes generally have steeper glide angles, these approximations are best avoided. By observing the glide angle and speed for a particular insect, its lift and drag coefficients can readily be calculated from the relations above. Of more general interest is the prediction of glide performance for different protopterygotes and insects, assuming values for the force coefficients. This is straightforward in principle, but may involve iterative calculations. The lift and drag coefficients are primarily functions of the orientation of the body, so the glide angle can be estimated from equation (1 6) for a given orientation. The glide speed V required for weight support can also be calculated from equation (17). In the range of Reynolds numbers of interest, however, the lift and drag coefficients can change significantly with Re in addition to the orientation. The estimated glide speed is therefore used to calculate Re, and new values for the coefficients appropriate to that orientation and Re are chosen. The glide angle and speed are then re-calculated, and this iterative process is continued until convergence on values of 0 and V is satisfactory.

4

Gliding cylindrical bodies

We now turn to the problem of estimating glide performance for the protopterygotes. J. W. Flower, an aeronautical engineer, became interested in the origin of insect flight through Hinton, and in 1964 he published what is still the most detailed aerodynamic study of the problem. He likened insect bodies to circular cylinders, and noted that glide angles of 45" might be possible if the cylinders could be held in a suitable attitude. He reasoned that the winglets were initially too small to improve this performance significantly, and thus their original function would have been for attitude control, enabling the insect to realize the best glide from its much larger body area. From this apparently sound aerodynamic reasoning, Flower went on to consider the important effects of body size on glide performance. He concluded that insects about 1 cm long were the prime candidates for evolving wings to improve their glide performance; selection pressures to improve wind-dispersal by small insects would favour even smaller bodies much more than the development of wings, and for larger insects their high glide speeds could be injurious. Flower's conclusions are based on predictions of glide performance, but he gives no details whatsoever of his calculations. Because the work is so seminal and important, I have therefore

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attempted to reconstruct his analysis. Almost certainly it is a resolved-flow, or cross-flow, analysis of a type commonly used in aerodynamics, and which has also found application in resistive models of animal swimming (e.g. Taylor, 1952; Hancock, 1953; Gray and Hancock, 1955).

4.1,

RESOLVED-FLOW ANALYSIS

The resolved-flow analysis is illustrated in Fig. 6b for a gliding body. The orientation is specified by the angle of attack clb between the longitudinal body axis and the glide path. The velocity Vis resolved into a component V, ( = Vsina,) normal to the body axis and another component V, ( = Vcosa,) tangential, or parallel, to the body. The drag forces for these simple cases of normal and tangential flow can readily be estimated and combined to find a resultant aerodynamic force R on the body. It is assumed that this resultant force is identical to that actually experienced by the body; i.e. the normal and tangential flow patterns do not interact and can be analysed independently. This is almost certainly not true. The normal flow pattern should produce alternate shedding of vortices, creating a periodic von Karman wake behind the cylinder. A steady wake is found behind inclined cylinders, however, suggesting that the tangential flow stabilizes the vortex shedding from the normal flow. None the less, there is good agreement between measured forces and those predicted by the resolved flow analysis at laminar Re (Hoerner, 1958), so the assumption proves to be a reasonable approximation for estimating forces. The normal D, and tangential D, drag forces are written in the usual form for inertial forces:

where C, and C, are the respective drag coefficients. A is the projected plan area of the body, equal to the product of body diameter and length (do;plan area is chosen in preference to frontal area so that cylinders of the same diameter but different lengths can be compared meaningfully. The resultant force R given by D, and D, can then be resolved into the conventional lift and drag components. Expressing these forces in the usual inertial form of equations (14) and ( I 5), and using A for the area, the lift and drag coefficients for the gliding body can be found: C, = sin2a, comb C, - cos2absina, C,,

(20)

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C,

= sin3abC,

+ cos3abC,.

(21)

The “lift” on a long gliding cylinder actually arises from the normal drag force, as shown by Fig. 6 and equation (20); it is not a circulatory lift such as wings produce. The lift will increase with higher values of C,, but it is reduced by the tangential drag coefficient C,. This is evident from Fig. 6b, where the tangential drag necessarily points below the velocity vector I/ and thus makes a negative contribution to the lift force L. The net lift can even become negative at small angles of attack ab:the normal velocity and thus the normal drag will be reduced, allowing the tangential components to dominate the force balance. The angle at which the lift changes from positive to negative is given by equation (20) when C, is equal to zero, and is simply t a m b = C,jC,. Predictions of the glide performance at any angle of attack are now possible given only a knowledge of the normal and tangential coefficients, which are functions of the Reynolds numbers based on diameter and length, respectively. In a strict application of the resolved-flow analysis, these Reynolds numbers should be calculated using the corresponding normal and tangential velocities. This carries the assumption to its extreme conclusion that the two flow components are completely independent but, as already mentioned, the assumption is only approximately true. The analysis will therefore be simplified by using the glide speed, instead of the respective velocity components, to estimate both Reynolds numbers. Flower’s calculations for gliding cylinders were for a 1:dratio of 10, which is at the upper end for relevant modern forms ( 6 1 0 ; Wootton and Ellington, 1991). For comparability with his results, I will also assume a ratio of 10. Expressions for the normal and tangential drag coefficients as functions of Re were given in Sections 3.3.3 and 3.3.4: equation (1 1) gives C,, and equation (10) shows that C, for a 1O:l cylinder is 73% of the value from equation (9) for an infinitely long cylinder. Calculated values of C, and C, are then used in equations (20) and (21) to determine the corresponding lift and drag coefficients.

4.2

LIFT AND DRAG AT CONSTANT REYNOLDS NUMBERS

The lift and drag coefficients are functions of the angle of attack of the body the normal and tangential drag coefficients, C, and C,, which are in turn functions of the Reynolds number Re. We therefore begin by investigating how C, and C, change with abover a range ofRe, in order to separate the effects of these variables on the forces. At an intermediate Re of 1000, C, is 0.962 and C, is 0,195 for a 10:l cylinder. The lift coefficient calculated from equation (20) is plotted against clb and

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body angle in Fig. 7a, together with the contributions from the normal and tangential drag coefficients which constitute C,. The normal contribution is necessarily zero at abequal to zero: the cylinder is parallel to the flow, so there is no normal velocity component to produce a normal drag force. The normal contribution is also zero at ab= 90" where the cylinder is broadside to the flow: the normal drag is then a maximum, but it is in the direction of flow and does not contribute to the perpendicular lift force. Between these extremes, the normal component is positive and peaks at 54". The contribution of the tangential drag force to lift is also zero at either extreme. When the cylinder is parallel to the flow there is no component of this force in the direction of lift; at ab= 90" there is no tangential flow along the cylinder, and thus no tangential drag to contribute to the lift. At other body angles the tangential component makes a small, negative contribution to the lift. The net lift, given by the sum of these contributions, is slightly negative at ab less than 1 lo, and positive elsewhere with a peak at 57". Fig. 7b shows the drag coefficient C, and its components from the normal and tangential drag on the cylinder. The tangential contribution is largest at ccb=O, and then falls off with ab because of the geometry. The normal contribution to drag is zero when the cylinder is parallel to the flow, and increases to a maximum at ab= 90" when the normal drag is greatest and in the direction of flow. The drag components are always positive, and the total drag increases with abuntil the cylinder is broadside to the flow at ab= 90". The effect of Reynolds number on the net lift and drag coefficients is shown in Fig. 8 for Re ranging from lo2 to lo4. The steep increase in normal and tangential drag coefficients at lower Re, discussed in Section 3.3, is reflected in the greater divergence of the curves for Re below about 1000. It may seem surprising that maximum lift coefficients increase with lower Re, but this is due to the larger normal drag coefficients that primarily constitute C,. The drag coefficient C, increases even more than C, with lower Re, so the lift-to-drag ratio actually declines. The glide angle of a cylinder is fixed by the lift-to-drag ratio, 0 = arctan(C,/C,), and it is of more interest to us than the ratio per se. Fig. 9 shows that 0=90" for body angles of 0" and 90", which simply indicates vertical parachute drops with the body in a vertical and horizontal orientation, respectively. At moderate angles of attack the glides can be quite impressive, reaching a glide angle of 40" at Re= lo4 and ab=30":i.e. the glide path is inclined by 40" to the horizontal and the body axis is 30" to this path, or 70" relative to the horizontal. Glide angles progressively worsen at lower Re, and the range of body angles where lift is negative (€I > 90") increases. The resolved-flow analysis is quite simple, but it would be reassuring to have some experimental verification of the results. However, I have not found lift and drag, or normal and tangential, coefficients for finite cylinders

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0.5

I

I

I

0

I

I

I

I

I

I

I

I

I

I

I

CT wnrnbut

1

0 0

20

40

60

80

Body angle of attack a b (degrees)

FIG. 7 Results from the resolved-flow analysis for a cylinder with 10:l 1ength:diameter ratio at a Reynolds number of 1000. (a) Lift coefficient C, and (b) drag coefficient C, plotted against the angle of attack of the body ab.The contributions from the normal and tangential drag coefficients are shown in both cases.

over a wide range of ct,, for these Reynolds numbers. The geometry of the analysis is straightforward, so any doubts should be focused on the assumed values of C, and C,. The estimates of C, should prove quite robust because it is a problem of separated flow; surface irregularities and roughness are unlikely to affect the pressure drag very much. The estimates of C, are more problematical. Although skin friction should dominate the drag in tangential flow for cylinders with a high I : dratio, some pressure drag must exist but has not been taken into account. The assumed skin friction is from the theoretical solution for laminar flow over a perfectly smooth surface, and the actual value must be substantially higher for real bodies that are not perfect cylinders and that have surface roughness. For insect wings the drag in

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1 ,

I

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I

I

I

I

I

I

2

n

0,

-8 5 "2

8

c T 1

n 0

20

40

60

80

Body angle of attack ab (degrees)

FIG. 8 (a) Lift coefficient C, and (b) drag coefficient C, for a cylinder with 1O:l 1ength:diameter ratio operating over a range of Reynolds numbers which are indicated on the curves. The line for Re= 1000 has not been marked in (a) for clarity.

parallel flow is about twice the theoretical minimum for a perfect flat plate (Ellington, 1984), and I would not be surprised to find a similar difference between C, for real cylinders and the theoretical value. Without sorely needed experimental work we cannot estimate the difference, so these calculations have to be viewed in the knowledge that C, is almost certainly a serious underestimate of the real value. Nevertheless, the results for insect wings demonstrate that the drag still scales with Re as expected on theoretical grounds; i.e. the exponents of Re in the equation for the drag coefficient are the same for real and perfect wings, even though the magnitude of the drag on real wings is twice as high. The trends reported here for the effects of Re on the force coefficients and glide angles should therefore be found in actual measurements.

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C. P. ELLINGTON 100

80 h

f

s

60

40

0

20

40

60

80

Body angle of attack ab (degrees)

FIG. 9 Glide angle 0 is plotted against the angle of attack of the body cq,for the data of Fig. 8. Values of the Reynolds number are indicated for each curve.

4.3

GLIDE CHARACTERISTICS OF CYLINDERS

The results of the preceding section are directly relevant to experimental measurements of the force coefficients on cylindrical bodies. Such measurements would normally be done over a range of body angles in a windtunnel operating at a given speed, and then repeated for different speeds. The Reynolds number is constant at any given speed, so the results would form a family of curves, one for each Re value. For a cylinder in gliding flight, however, the Re must change in sympathy with the glide speed at different body angles; the force coefficients necessarily alter with ab,and thus the glide angle and glide speed for the force equilibrium must vary. The problem is further complicated because the change in Re then feeds back on the force coefficients which, as we have seen, are strongly influenced by the Reynolds number. The calculation of glide characteristics for any given cylinder therefore involves an iterative process. For each angle of attack ab of the body, C, and C, are estimated from an initial guess at the Re. The corresponding lift and drag coefficients are calculated, leading to estimates of the glide angle 0 and the glide speed V. A new Re is then determined from the glide speed, and this is used for another round of calculations. The iteration converges rapidly on final values of 0 and V . The glide characteristics for cylinders of various sizes can thus be estimated and compared with Flower’s (1964) results. The I : d ratio is taken as 10 : I , and cylinder size will be

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AERODYNAMICS AND THE ORIGIN OF INSECT FLIGHT

specified by the length. The cylinder weight for the force balance is calculated assuming a density equal to water (1000 kg m-’). The results for cylinders of different lengths are presented in Fig. 10 as conventional glide polars. The sink speed V, is the rate of vertical descent ( = Vsine), and it is plotted against the horizontal speed V , ( = Vcos8); glide polars for vertebrate and man-made gliders simply have the speed V along the horizontal axis, but this is appropriate only when small angle approximations are valid for 8. The glide polar provides a marvellously concise summary of a glider’s performance. The sink and horizontal speeds can be read off directly for each angle of attack a,,, which is indicated parametrically on the curve. The length of a line from the origin to any point on the curve equals the glide speed V = [V:+ V2]1!2, and the angle of this line to the horizontal axis gives the glide angle 8. Two sides of the boxed figure are marked off as a protractor to aid measurements of 8. Horizontal speed (m s 20 0

10

10

-.a

E

I v

; Bi;

40

20

90

80

70

60

50

FIG. 10 Glide polars for cylinders of different lengths but with the same 10:1 length : diameter ratio. See text for explanation.

Standard indicators of glide performance can easily be read off the glide polar. The shallowest glide angle is found from the tangent to the curve, and this permits the maximum horizontal movement for a given loss of height. In many situations this provides the “best glide” in still air; for the 1 cm cylinder 8,, is 44” at Vbg equal to 8.1 m s-I. If the ambient wind contains updraughts, the glider can soar if its sink speed is less than or equal to the updraught. “Minimum sink” is therefore advantageous for soaring, and it corresponds

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to the highest point on the polar. The minimum glide speed might also be important to reduce the risk of damage upon landing. For cylinders this obviously occurs when they parachute vertically (0 = 90') with the body horizontal (ab= 90"), and the corresponding speed can be read off the vertical axis. The conditions for minimum sink and minimum glide speed correspond for cylinders, but they differ for conventional gliders with wings. For the 1 cm cylinder €I,,is 90" at V,,, equal to 3.8 m s-I, so this cylinder could only soar in very strong updraughts with speeds greater than 3-8 rn s-'. Before discussing the implications of the results in Fig. 10, they will first be compared with those of Flower (1964). Our sink speeds for horizontal and vertical body orientations match closely, indicating that our values of C, and C, agree to within about 10%. Glide angles are also very similar, to within about 5", for body lengths of 5 mm or more. For shorter lengths, his glide angles are almost 20" shallower than mine for angles of attack clb around 30", and a small but systematic 6" higher than mine for clb around 60". The probable source of the discrepancy is that he used an approximate form of the resolved-flow equations (20) and (21) which is more appropriate for moderate to high Re where C, is typically much greater than C,. In such cases the term involving C, always dominates the lift coefficient equation, so the C, term is commonly neglected. Similarly, the C, term contributes significantly to the drag coefficient equation only when the angle clb is small, and so c0s3ab is usually approximated by unity. The effects of these approximations are two-fold at lower Re, where C, is too large for such approximations: (i) for moderate body angles C, is seriously overestimated and hence glide angles are falsely optimistic, and (ii) at high body angles C, is overestimated and thus glide angles are worse. I am reasonably confident of this explanation for our differences because, if my equations are modified to the usual approximate form, our results then agree to within 5" even for small body lengths. There is no other significant discrepancy between our results. The implications of Fig. 10 for gliding pterygotes will now be considered. The most obvious feature is that glide speeds increase rapidly with size, indicating that larger protopterygotes would certainly be in risk of damage. Flower suggested that speeds of around 9-13 m s-' might represent the maximum safe limit, but this was clearly just an educated guess. A more practical approach to the problem demonstrated that cockroaches of all nymphal instars suffer damage to antennae and legs when dropped from 5 m on to sheet paper overlying soft plastic foam (Wootton and Ellington, 1991). The equilibrium position in falling is with the body horizontal (ab=90"), and all of these instars would have reached terminal velocity within 5 m. Thus all sizes that were tested are liable to sustain damage during a fall, and any "safe" speed limit must be very slow, indeed. It seems likely that the damage will become even more severe above some speed, but we cannot assign a

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reliable value to such a speed limit for “acceptable” damage. A speed of about 10 m s-‘, which is within the range suggested by Flower and which does not exceed observed flight speeds for extant insects, seems a plausible guess. A gliding protopterygote 5 cm long just scrapes under this limit when parachuting at very large body angles, but it could not exploit its rather impressive glide performance at moderate body angles without sustaining damage on landing. A 2 cm protopterygote could parachute more safely, but better glide angles are again achieved only at dangerous speeds. It thus seems that a 1 cm body is about the largest that could glide without excessive risk over a range of angles, which is the same conclusion as Flower’s. Glide angles improve with larger bodies because C, increasingly dominates over C, at higher Re. The best glide angles for 5 and 2 cm bodies are 28” and 36”,respectively, which are very impressive indeed for gliding bodies sans wings. It is a pity that such remarkable bodies would probably splat on landing! The best glide for a 1 cm body has already been given as 44“, which is still quite respectable; the protopterygote would cover the same distance horizontally as it lost vertically. For even smaller bodies the glides are increasingly disappointing: 54” for a 5 mm length, 64” for 2 mm, and 68” for 1 mm. If the horizontal distance o f a glide was most important to protopterygotes, selection should clearly have favoured the largest bodies consistent with acceptable damage: perhaps about 1 cm long. What if a low sinking speed was more important, say for wind-assisted dispersal? Small protopterygotes with horizontal bodies would obviously be favoured, taking them into the viscous realm where skin friction dominates. The surface area of the body becomes the primary determinant o f drag forces, and it is most unlikely that the first tiny winglets would have reduced the sinking speed significantly. Flower’s conclusion that “the selective pressure in small insects is towards smaller insects, which would have no reason to evolve wings” is again supported, and the “floating” theories of the origin of insect flight seem implausible. Apart from some quantitative differences in glide angles for small bodies, this analysis therefore agrees with Flower’s important conclusions about the gliding potential of protopterygotes. It is rare indeed when mechanical arguments can shed such an illuminating light on an evolutionary problem, in this case specifying the size of the potential ancestor of winged insects. But if the hypotheses on the origin of insect flight are correct in postulating some form of gliding as the first step, then the mechanical arguments in favour of decent glides at angles of about 45” by a protopterygote some 1 cm long are quite persuasive. However, two caveats must be borne in mind. The tangential drag coefficient C, for protopterygotes is likely to be much greater, perhaps by a factor of two, than that assumed for perfect cylinders. Glide angles will therefore be increased, and the “decent” angle of 4.5” for a 1 cm

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protopterygote is more probably around 60". Secondly, the glide speeds are still rather high, and the level of damage on landing may not be acceptably low except for even smaller sizes.

4.4

STABILITY AND CONTROL

Although details of Flower's analysis can be criticized, the thrust of it was certainly on target. The total area of the winglets would initially have been small relative to the body planform area, and so it is unlikely that they could have significantly improved the lift-to-drag ratio. The body forces would therefore determine the best glide angle. A possible role for the winglets, suggested by Hinton (1963) and Flower (1964), was to maintain the correct attitude for the gliding body. Hinton was concerned about the ability of the insect to land right-side-up, favouring a quick escape from terrestrial predators, whereas Flower was more interested in attitude control during the glide. Circular cylinders cannot glide stably at small or moderate angles without some help. The centre of aerodynamic pressure at small angles is located near the front of the cylinder, producing a head-up pitching moment. As the body angle increases the centre of pressure travels towards the middle of the cylinder, reducing the pitching moment (Hoerner and Borst, 1975). A stable equilibrium is only reached at an angle of 90", with the centre of pressure coinciding with the centre of gravity and the cylinder falling broadside to the flow. The best glide performance for cylinders is obtained at moderate angles of attack, around 3&50", but these would obviously be unstable attitudes for simple cylinders. Stable glides at such angles of attack could be achieved by moving the centre of gravity forward; shifts in the centre of pressure with angle of attack would then provide restoring moments about the centre of gravity. This type of pitch stability can be illustrated by an aeromodelling parlour trick, in which flat plates are made to glide stably by a very carefully positioned centre of gravity. The moment arm for the restoring forces is necessarily small, however, and the stability is marginal. Fossil forms suggest that the centre of gravity in protopterygotes was anterior to the middle of the body, as in modern insects, and they may have gained a little pitch stability in this manner. Even so, the larger restoring moments that could have been generated by posterior winglets and caudal projections would probably have been essential. Paired cerci and/or median caudal filaments are found on most insects from the Carboniferous, and it is widely believed that they were present on

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the protopterygotes. In principle, these could provide a weathercock type of stability (Leston, 1963) for both pitch and yaw, the side-to-side rotations of the body. That such projections impart yaw stability in practice has been demonstrated by the modelling experiments of Wootton and Ellington (1991). They constructed lightweight balsa models that would glide at the same Reynolds number as protopterygotes. The “bodies” were cylindrical tubes with a 1:d ratio of 8:1, an average value for the protopterygotes, and bore winglets. The models were launched from a height of 6 5 m , and the glide paths recorded for analysis. Results will be presented in the next section, but it is worth noting here that the yaw stability was poor until a caudal filament was added to the models. Paired cerci would function in the same manner, so there seems little doubt that caudal projections would indeed have provided yaw stability for protopterygotes. Wootton and Ellington also found that caudal projections were necessary for pitch stability in models where the posterior winglets had been removed. It is therefore possible that the problem of maintaining a stable attitude was solved without the winglets; caudal projections could have done the job if they were long enough in the protopterygotes. The results of Wootton and Ellington nevertheless show that the winglets have a profound influence on pitch. Their models had nine identical pairs of rather large winglets; the total planform area of the winglets was 36% of that for the body. The inclination of individual winglet pairs could be adjusted, and pitch stability could always be achieved when they were appropriately set. Furthermore, the most posterior pair proved remarkably effective in controlling pitch; changing their inclination by a few degrees would alter the flight path from a shallow glide to a steep nose-dive. The models were overly sensitive to adjustments of these winglets, in fact, suggesting that a practical role in pitch control would require smaller posterior winglets. The available evidence suggests that these winglets were indeed smaller for the protopterygotes. Control over the glide path could also have been achieved by movements of caudal projections, legs and possibly antennae, so winglets might be redundant for control as well as for stability. The excellent control which they offer seems likely to have been exploited at some stage during the evolution of flight, but it may not have been essential from the outset if the caudal filaments and cerci were long enough. In addition to pitch and yaw, gliders also require stability in roll to prevent rotation about the longitudinal body axis. Kingsolver and Koehl (1985) measured very small rolling moments on their model protopterygotes in a windtunnel, and concluded that tiny wings could not have provided enough roll stability. However, the moment of inertia about the roll axis is small for cylinders, and these rolling moments might still be adequate for stability. During the course of their experiments, Wootton and Ellington never

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detected any roll instability in their gliding models, although they encountered almost every other kind of instability. I therefore think that stability in roll was a very minor problem at most for the protopterygotes. 5 Aerodynamic function of winglets

The preceding section leaves in doubt the hypothesis that tiny winglets initially functioned in attitude stability and control for the protopterygotes; caudal projections could have fulfilled that role if they were long enough. There are two other aerodynamic benefits which the winglets could possibly have offered, and which could have provided an aerodynamic reason for subsequent enlargement of the winglets. Because there are only two parameters which describe the glide path, it is not difficult to guess these proposed benefits: an improvement in the glide angle, and a reduction of glide speed. 5.1

IMPROVEMENT OF GLIDE ANGLE

Many, if not most, of the supporters for a gliding origin of flight seem to assume that an enhanced lift-to-drag ratio will automatically result from enlargement of the winglets. Wings have higher lift-to-drag ratios than cylindrical bodies, of course, so their addition to a body will increase the total lift more than the total drag. The lift and drag from all winglets can be written in the usual inertial form

where the subscript “w” denotes values for the winglets and A , is the total planform area of winglets. The lift and drag on the cylindrical body can be written in the same format with an identifying subscript “b”. Total lift is the sum of L, and L,, and total drag is similarly given by the sum of D, and D,. The equations can readily be manipulated to give the lift-to-drag ratio of the body plus wings, “b + w”, divided by the ratio for the body alone. This is the relative lift-to-drag in the terminology of Kingsolver and Koehl (1985), which shows the improvement of the lift-to-drag ratio compared with that of the body:

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Thus the improvement is simply given by the ratios of winglet and body areas and force coefficients. The glide angle is determined by (CL/CD),and the relative angles can also be calculated from the analysis. Wings must produce a higher lift and a lower drag than a body of the same planform area, so the ratio in equation (24) must be greater than unity and will increase as winglet area increases. It is very tempting to estimate the relative lift-to-drag ratio for different winglet areas as a fraction of body area, especially for the optimistic case when the maximum lift-to-drag ratio of the winglets is combined with the best ratio for the body. At first glance all the necessary data are available: force coefficients for cylinders are given in Section 4.2, and for insect wings (C,/C,),,, is typically 2 4 , and corresponding C, and C, values are about 0.6 and 0.2, respectively (Vogel, 1981; Ellington, 1984; Dudley and Ellington, 1990). However, such calculations would be most unreliable at present, because they would combine force coefficients for real wings and theoretical bodies; the tangential drag coefficient for cylindrical bodies is unreliable, as already noted, and thus the body force coefficients will not be comparable with measurements on real wings. Nevertheless, equation (24) suggests an improvement in glide performance with increasing winglet area, but experimental verification is sorely needed. This analysis assumes, as others appear to have done, that the lift and drag per unit area of winglet is the same as for a fully-fledged wing. However, tiny winglets would initially have been within the boundary layer of the body and exposed to a retarded airstream. Furthermore, they would probably have been rather short and stubby compared with the long, slender wings of extant insects, and the lift coefficient for such low “aspect ratio” wings is smaller. These two factors indicate that the lift of tiny winglets is lower than would be predicted from their area alone; lift coefficients and maximum lift-to-drag ratios would be smaller than values for proper wings. For very small winglets, therefore, the improvement in glide angle would be much less than otherwise expected. Would it be sufficient to favour enlargement of the winglets? Kingsolver and Koehl (1985) addressed this point by measuring the maximum relative lift-to-drag for protopterygote models with different wing lengths. Their models had body lengths of 2, 6 and 10 cm, and results were presented for tests at Reynolds numbers based on body length of about 3.4 x lo3, 1.0 x lo4 and 1.7 x lo4, respectively. There were two pairs of thoracic winglets, instead of the complete series of thoracic and abdominal winglets thought to be present in the protopterygotes. For small models, 2 cm and 6 cm in body length, they found no substantial improvement in the relative maximum lift-to-drag ratio until the wing lengths exceeded 70% and 10% of the body length, respectively (Fig. 11); the corresponding total

C. P. ELLINGTON

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0

I

I

I

I

I

1

0.2

0.4

0.6

0.8

10

1.2

Relative wing length

FIG. 11 Maximum lift-to-drag ratio of model pterygotes with wings of different lengths, divided by the ratio for the body alone, is plotted against wing length divided by body length. Body lengths are 2, 6 and 10 cm, and the airspeed is 2.5 m s-I. (From Kingsolver and Koehl, 1985.)

winglet areas would be 3.76 and 0.54 times the body planform area. Contrary results were obtained for a larger model with 10 cm body length. The lift-to-drag ratio increased rapidly with larger wing lengths, supporting the aerodynamic explanation for enlargement of the winglets. Their results for the 2cm model are puzzling, and it is difficult to understand why a winglet area almost 4 times the body area is required before the lift-to-drag ratio improves significantly. The winglets were operating at a Reynolds number of about 400 based on the chord, or width, of the wing. The lift-to-drag ratio of wings decreases at lower Re because of the enhanced viscous skin friction, but Vogel (1967) still found a decent maximum lift-to-drag ratio of 1.8 for cambered Drosophila wings at a similar Re of 200. The winglets on the 2cm model might therefore be expected to improve its performance more substantially. The model had flat rather than cambered wings, and Vogel measured a more disappointing lift-to-drag ratio of 1-2for flat wings, so perhaps Kingsolver and Koehl did not offer the most favourable conditions for evaluating the influence of winglets. Their error bars are also too large relative to the shallow gradient in Fig. 11 for an accurate determination of the wing length at which performance is improved. Their results for the 6 cm model can be compared with those for the small model of Wootton and Ellington (1991), which glided at Re of about 8 x lo3. This model had serial square winglets with a total area some 36% of the body

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area; this is smaller than the 54% found by Kingsolver and Koehl(l985) for an improvement in the lift-to-drag ratio, and the winglets were of low aspect ratio and more subject to the boundary layer of the body. During free glides the performance was hardly impressive, and the best glide angle obtained was 54". This is less than the best glide of 40" predicted for a comparable cylindrical body in Section 4.2, but it was noted that the value for a real body would be substantially worse. It seems likely that the winglets did improve the relative maximum lift-to-drag ratio, but the increase would have been slight in agreement with the conclusion of Kingsolver and Koehl. The 10 cm model of Kingsolver and Koehl showed a remarkable increase in performance with even very small winglets, and it is consistent with the classical aerodynamic explanation of winglet enlargement. Wootton and Ellington also tested a large model, scaled up two-fold from their smaller one, which flew at Re around 2-3 x lo4. The gliding ability was radically transformed by this increase in size, and shallow glides at angles as low as 17" were achieved. There seems little doubt that tiny winglets can dramatically improve the performance of gliders in this Reynolds range. If the initial enlargement of winglets was favoured by increases in the liftto-drag ratio, then the protopterygotes must have been operating a t ,Re somewhat above lo4. Judging from the glide performance estimates for cylinders without winglets this would correspond to body lengths of 2 cm or longer, which is consistent with most views on the size of protopterygotes. Kingsolver and Koehl interpreted their model results directly in terms of the body sizes of protopterygotes, however, and concluded that improbably large sizes would be involved; their 2 cm model showed little evidence for an enhancement of the lift-to-drag. This interpretation is quite incorrect. Their force measurements were taken at a constant 2.5 m s-', a value fairly typical of insect flight speeds from the literature (Kingsolver and Koehl, 1989), but this is much slower than the equilibrium gliding speed of bodies without wings. A model length of over 6cm is required at 2.5ms to mimic the equilibrium glide performance of a 2 cm long protopterygote. Their interpretation led to a rejection of the classical aerodynamic arguments for winglet enlargement in favour of a thermoregulatory explanation. This received a great deal of attention, and it is ironic that, if properly interpreted according to the Reynolds numbers of the experiments, their results actually support this aerodynamic hypothesis.

5.2

REDUCTION OF GLIDE SPEED

Having shown that an improvement in the glide angle could account for winglet enlargement in protopterygotes larger than about 2 cm, we return to

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the awkward problem of high glide speeds. It was suggested in Section 4.3 that a 1 cm body is the largest that could glide without excessive risk on landing, and that a 2cm body could achieve decent glide angles only at dangerous speeds. This small discrepancy in body length is certainly within the possible errors of the analysis, but it illustrates that the glides of early protopterygotes were marginal in safety. Could an early function of the winglets have been a reduction of glide speeds? Small winglets would have little effect on the tangential drag of the cylindrical body, given its much larger surface area. However, even the tiniest winglets would be likely to increase the normal drag coefficient substantially. The flow around a circular cylinder separates on its downstream side, producing a wake that is smaller than the cylinder diameter. The introduction of sharp edges laterally on the cylinder will force separation to occur at those edges, and this can increase the size of the wake and therefore the pressure drag coefficient for the normal flow. Hoerner (1958) gives similar examples for other objects with sharp lateral edges; squares with the flow directed along a diagonal, and triangles pointing into the flow. The drag coefficient for these objects is half-way between a cylinder and a flat plate normal to the flow. For Reynolds numbers based on diameter greater than 1000, for example, the drag coefficients plateau at about 1.1 and 1-95for the cylinder and plate, respectively, and we might expect a cylinder with sharp edges to have a value close to 1.5. By forcing separation, a series of winglets along the body might therefore increase the normal drag considerably, and this would be true even for the smallest winglets. By increasing the normal drag coefficient C,, the early stages of winglet development might usefully reduce the terminal velocity of glides at large angles of attack, i.e. parachuting with the body almost horizontal. Fortunately the drag coefficients for separated flows are relatively constant except at quite low Re, so for a first approximation we can ignore changes in C, as the terminal velocity and hence Re decrease. The force balance for parachuting is easily derived by equating the weight to the normal drag, and the square of the terminal velocity is given by

V 2= 2rng/(pAC,).

(25)

In landing at this terminal velocity, the corresponding kinetic energy '/2nzV2 of the body must be absorbed upon impact. Parts of the body will deform and, if the protopterygote is lucky, the energy will be absorbed harmlessly as elastic strain energy and then be dissipated on the rebound. If the energy involved is too great, damage will ensue as parts are strained beyond their elastic limit. This limit is set by the strain energy per unit mass of the deforming material, which is simply proportional to V 2given the expression

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for the kinetic energy. Thus the risk of damage in landing should be proportional to the square of the terminal velocity. This non-linear relation is probably why we tend to think of a speed limit for acceptable damage, beyond which the costs increase rapidly. Equation (25) shows that the risk will be inversely proportional to both the area and the normal drag coefficient. If the first winglets forced separation of the flow and increased C, to 1.5, this would immediately cut the risk by 27% -a very promising start. Subsequent enlargement of the winglets increases the planform area, and thus further decreases the risk. We can define a relative risk during this process as the risk for the body with enlarged winglets divided by that for the very promising start, i.e. a negligible winglet area but an enhanced C, because of the more separated flow. From equation (25) this ratio is simply given by the ratio of planform areas, Relative risk =

A All + A ,

N

A, 1 --, A,

where the subscripts “b” and “w” identify the body and winglet areas, and the approximation is valid when the winglet area is small compared with the body area. During enlargement, the relative risk therefore decreases linearly as winglet area increases: e.g. when the winglets are 10% of the body area, the risk is 90% of the initial value. This simple analysis provides a very plausible aerodynamic reason both for the appearance of winglets and their subsequent enlargement. The winglets might already have existed for other functions before the first parachute drops, but if not then there would be strong selection pressures for the development of a pair of ridges along the body. A tiny ridge is all that is required to force separation and, given that the insect body is not a smooth cylinder, it should not have been difficult to achieve. Enlargement of the ridge into winglets, and further increases in winglet size, should have been favoured strongly; by reducing the risk of damage, the relative fitness of the protopterygotes increases in direct proportion to the winglet area. This mechanistic explanation could also be advanced in support of the floating theories for the origin of flight (Section 2.3), where a reduction in terminal velocity improves dispersal by the wind. The protopterygotes would initially have to be small in order to have a low terminal velocity, and this could be reduced further by the development of winglets. However, at such low Reynolds numbers viscous drag forces become more important than the pressure drag of this mechanism, so it is better suited for exploitation at higher Re. The results of this section are at odds with the results of Kingsolver and Koehl (1985). To assess the effects of winglet size on the floating theory, they

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measured the maximum drag of their models broadside to the airflow. The maximum drag did not increase significantly above that for the body alone until the wings were at least 20% of the body length for all three models. At this winglength the area of the two wing pairs was approximately equal to the planform area of the body, and thus the total area exposed to the flow was doubled without a significant increase in drag. This would be a remarkable achievement, but it seems most unlikely. Other aspects of their study are disturbing, such as the enormous wings required on the 2 cm model for an improvement of the lift-to-drag ratio. Their 10 cm model also gave anomalous results; they presented data as a relative lift-to-drag ratio, but commented that the absolute value of the lift-to-drag ratio for the large model was some four or five times worse than that for the two smaller models. This radical deterioration at higher Reynolds numbers is hardly to be expected, unless their models ventured into some of the separated flow phenomena which are not uncommon in this Re range; this possibility is supported by their observations of the airflow patterns. Because some of their results are so contrary to our understanding of aerodynamics, an independent verification is badly needed. If an unusual separated flow pattern is indeed responsible for the discrepancies, we will then have to determine whether protopterygotes glided through a highly unusual realm of aerodynamics, or whether the results were simply an artefact of the models.

6

Concluding remarks

Because we cannot observe the protopterygotes, we shall never fully understand the evolution of insect flight. It has long been a topic of speculation, punctuated now and then by periods of increased interest and awareness. The study by Kingsolver and Koehl is already a modern classic, in that it first brought experimental methods to bear on the problem. Future investigations using models are certain to follow, and for that reason I have attempted to provide a theoretical framework for such studies. There are many strange fluid phenomena in this range of Reynolds numbers, and the modelling can be tricky. We need to know when experimental data seem odd, so that the cause can be traced. Equally well, the theoretical approach is quite limited in that the force coefficients ultimately have to be determined empirically. The coefficients used here are little more than a guide, drawn from related problems in aerodynamics. As it stands, this analysis presents a very favourable aerodynamic explanation for the origin of flight. I think it is robust enough for the tentative conclusions to be received with some confidence, but much more experimental work is obviously needed for verification.

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There seems to be common ground in believing that the protopterygotes possessed a segmental series of winglets along the thorax and abdomen. Whether or not they would be advantageous for gliding flight before reaching some critical size is one of the major questions. Kingsolver and Koehl concluded that they would not, and that an initial thermoregulatory role for the winglets seemed more likely. My analysis suggests that the winglets might have a strong aerodynamic advantage from the outset, by reducing the terminal velocity of vertical descents and therefore lowering the risk of injury. The terminal velocity of insects even a few centimetres long is dangerously high. The winglets could also provide pitch stability and control if articulated and mobile, as shown by the model experiments of Wootton and Ellington. Caudal filaments and/or paired cerci were also likely to be used for stability and control of pitch and yaw. If injurious landings were a major problem, the pitch control should have been used for vertical descents with a horizontal body; the speed is least in that attitude. Enlargement of the winglets would further reduce the speeds, offering more insurance against damage, and it would also permit the protopterygotes to expand their flight repertoire beyond simple parachuting. Their cylindrical bodies are capable of gliding at different angles, if the attitude can be controlled by the winglets or caudal projections, but the glide speeds are necessarily higher than the terminal velocity of parachuting. Moderate-sized protopterygotes that were initially liable to damage in a descent would be able to exploit their gliding ability only after speeds had been reduced. Glide angles and thus the horizontal distance travelled during a descent improve remarkably for bodies longer than about 1 cm, but this is the range where the risk of damage on landing increases rapidly with size. The best potential gliders were therefore at most risk, and enlargement of their winglets should have been strongly favoured. An increase in winglet area to reduce dangerously high speeds provides an aerodynamic route by which the winglets arrive at a size where they can begin to influence the lift-to-drag ratio of protopterygotes a few centimetres long. The effect would be even more pronounced for larger ones, but their terminal velocities would be so high that even winglet enlargement might not prevent severe damage. For protopterygotes of moderate size, however, further enlargement of the winglets substantially improves their already impressive gliding ability, and they are set on the road to the full development of flight capabilities. In the course of time the meso- and metathoracic winglets became disproportionately enlarged, and the others lost. The reasons for this may lie in the modelling experiments of Wootton and Ellington, who found that the pitch was unduly sensitive to the most posterior pair of winglets. The large moment arm of posterior winglets is not compatible with the production of large forces, whereas the meso- and metathoracic winglets are close to

208

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the centre of mass and exert small moments. A differential enlargement of the winglets may therefore have proceeded from early stages to satisfy the pitching moment balance for the gliding protopterygotes. The stability and control functions of the posterior abdominal winglets could have been taken over by the caudal filaments at any stage, although control would probably not have been as effective. Whether or not the loss of abdominal winglets occurred before active flapping flight took over the role of stability and control is uncertain: it could have happened either way. Acknowledgements

I am most grateful to Dr R. J. Wootton for his helpful comments on the manuscript and for his reluctant permission to reproduce Fig. 2.

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Sharov, A. G. (1973). [Morphological features and way of life of Palaeodictyoptera]. Dokl. 24 Chtenii Pumiuti N.A. Cholodkovkogo, pp. 49-63. Akad. Nauk SSSR, Moscow. Shear, W. A,, Bonamo, P. M., Grierson, J . D., Rolfe, W. D. I., Smith, E. L. and Norton, R. A. (1984). Early land animals in North America: evidence from Devonian age arthropods from Gilboa, New York. Science, N Y 224, 492494. Smart, J. and Hughes, N. F. (1972). The insect and the plant: progressive palaeoecological integration. Symp. R. ent. Soc. Lond. 6, 143-155. Taylor, G. I. (1952). The action of waving cylindrical tails in propelling microscopic organisms. Proc. R. SOC.Lond. A 211, 225-239. Taylor, T. N. and Scott, A. C. (1983). Interaction of plants and animals during the Carboniferous. Bioscience 33, 488493. Vogel, S. (1967). Flight in Drosophila. 111. Aerodynamic characteristics of fly wings and wing models. J . exp. Biol. 46, 431443. Vogel, S. (1981). “Life in Moving Fluids.” Willard Grant, London. von Mises, R. (1959). “Theory of Flight.” Dover, New York. Whalley, P. E. S. (1979). New species of Protorthoptera and Protodonata (Insecta) from the Upper Carboniferous of Britain, with a comment on the origin of wings. Bull. Br. Mus. nat. Hist. Geol. 32, 85-90. Whalley, P. E. S. and Jarzembowski, E. A. (1981). A new assessment of Rhyniellu, the earliest known insect, from the Devonian of Rhynie, Scotland. Nature 291, 317. Wigglesworth, V. B. (1963). The origin of flight in insects. Proc. R. ent. Sac. Lond. C 28. 23-24. Wigglesworth, V. B. (1973). Evolution of insect wings and flight. Nuture 246, 127129. Wigglesworth, V. B. (1976). The evolution of insect flight. Symp. R. ent. Soc. Lond. 7, 255-269. Wootton, R. J. (1972). Nymphs of Palaeodictyoptera (Insecta) from the Westphalian of England. Palueontology 15, 662-675. Wootton, R. J. (1976). The fossil record and insect flight. Symp. R. ent. Soc. Lond. 7, 235-254. Wootton, R. J. (1981). Palaeozoic insects. Ann. Rev. Entomol. 26, 319-344. Wootton, R. J. (1986). The origin of insect flight: where are we now? Antenna 10, 8286. Wootton, R. J. (1990). Major insect radiations. In “Major Evolutionary Radiations” (Eds P. D. Taylor and G. P. Larwood), Systematics Association Special Vol. 42, 187-208. Wootton, R. J. and Ellington, C. P. (1991). Biomechanics and the origin of insect flight. In “Biomechanics in Evolution” (Eds J. M. V. Rayner and R. J. Wootton), pp. 99-1 12. Cambridge University Press, Cambridge.

Index a-adrenergic agonists, 90 Acanthacris rujicornis, 17 Acheta domesticus, 97 activation continuum, 102-3 Acridu turrira (grasshopper), 3, 17 Acrididae, 6 acridids, 10 colouration, 12, 13, 1.5, 17, 18, 21 hopper development, 27 male sexual behaviour, 22 reproductive parameters, 24 VG, 45, 46 Acridinae, 17 activity and behaviour, 31-7 activity, voluntary cessation of, 103-4 adaptation and genotypic variability, 160-2 additive genetic variance, 126 adipokinetic hormone see AKH aerodynamic function of winglets, 200-6 glide angle improvement, 200-3 glide speed reduction, 203-6 aerodynamics, basic, 178-88 fluid forces, 178-80 force coefficients, 182-6 cylinder in axial flow, 185-6 cylinder in normal flow, 184-5 flat plate in normal flow, 1 8 3 4 flat plate in parallel flow, 182 gliding, mechanics of, 1 8 6 8 Reynolds number, 180-2 aerodynamics and origin of flight, 171--208 age castes, 129, 152, 155, 1.59 age demography, 136, 140 age polyethism, 118, 129, 133 division of labour plasticity, 130, 131, 132, 135 genotypic variability, 140 AKH, 35, 36,49, 54 arousal syndrome, extended, 90, 91 endocrine control, 84, 85 insecticide design, 105

metabolic substrates, 92 paralysis/insecticide poisoning, 101 alarm pheromone, 105, 133 alarm stimuli, 82 albinos, 3 1 allatectomy behaviour/activity, 34 colouration, 20 male sexual behaviour, 22 physiology/biochemistry/molecular biology, 28, 29 reproductive parameters, 24, 25 unilateral, 26 allatostatins, 105 allatotropin, 48 American cockroach see Peripianeta americana amines, biogenic, 49 anatomy, 8-12 Anopheles stephensi (mosquito), 90 anti-juvenile agents, 54 ants, 4 aphids, wing polymorphism of, 3, 4 Aphisfabae, 94, 103 Apis mellifera see honey bee Apterygota, 172 arousal response, postulated and octopamine/neuropeptides, 86-7 arousal response, postulated and stress/flight/insecticide/feeding, 88-9 arousal syndrome, 81-106 extended, 85-99 ions, 98-9 metabolic substrates, 92-3 nerve and muscle effects, 90-2 water, 94-8 arrhenotoky, 119 arthropods, 174 Australian plague locust (Chortoicetes terminifera), 6, 9 autoneurotoxin, 10I

21 2

B chromosomes, 30 p-carotene, 20 baculovirus, 54, 55 ‘bands’, 31 basic aerodymics see aerodynamics, basic bees, 4, 130 see also bumble bees; honey bees behaviour and activity, 3 1-7 adults, 33-7 hoppers, 3 1-3 behavioural canalization and heritability, 160 dominance, 149, 152 colony-level integration of individual behaviour, 146-8 feedback loops, negative, 147, 148 interactions, 148 modification, 156 modularity, 157-9, 159, 160 response thresholds, 148 variability, 143, 144, 145 co- and cross-fostered cohorts, 13940 members of different subfamilies, 137-9 within subfamily, 1 4 3 4 ‘being touched’, 31, 32 Bernoulli’s equation, 181 bile pigments, 15 binomial probability density function, 122 biochemistry, 28-30 biochromes, 15 biology, molecular, 28-30 Blatella germanica, 92 Bombyx pupae, 32, 33 Boolean switching functions, 149 boundary layer, 180, 186 fluid forces, 178, 179 laminar, 182 turbulent, 185 bristletails (Diplura), 174 brood viability, 123, 124 ‘brother’ drones, 124-5 brown locust (Locustana pardaha), 4, 7 Brown strain, 140 bumble bee, 94, 103 Bursicon, 18

CA (corpora allata), 3, 21, 53, 54 behaviour/activity, 34, 35 colouration, 19, 21

INDEX

endocrine organs, 37, 38, 39,40, 44, 45 hopper development, 27 and JH, 3 7 4 5 locust phase characteristics, 41-3 male sexual behaviour, 22 metabolic substrates, 97 morphology/morphometrics/anatomy,9, 10, 11, 12 pheromones, 50, 52 physiology/biochemistry/molecular biology, 28, 29 reproductive parameters, 24, 25, 26 VG, 46,48 Calliphora, 46 Caloneurodea, 173 canalization, behavioural, 160 carbon dioxide see CO, Carboniferous, I98 Middle, 174 Middle to Upper, 172 Upper, 173, 174 carotenoids, 15 Carpenter bee (Xylocopa capitata), 94 caudal filaments, 207, 208 median, 198, 199 CC (corpora cardiaca), 17, 18 behaviour/activity, 35, 36 endocrine control, 85 locust, 90 metabolic substrates, 97 see also neurosecretory cells VG, 48, 49 central nervous system (CNS), 132, 133 cerci, paired, 198, 199, 207 cessation of activity, voluntary, 104 chemosensilla, 90 chiasma frequency, 30, 31 Chortoicetes terminifera (Australian plague locust), 6, 9 CNS see central nervous system CO, (carbon dioxide), 30, 36, 39, 49 cockroach, 82 activation continuum, 103 arousal syndrome, extended, 90 cessation of activity, voluntary, 104 Dictyoptera, 173 Diploptera punctata, 105 endocrine control, 83 glide characteristics, 196 hypertrehalosaemia, 84

INDEX

insecticide design, 104 insecticide-poisoned, 84 ions, 98 see also Periplaneta americana cohorts, co- and cross-fostered, 13940 colony defence, 138 fissioning see swarm genotypic composition, 124-5 single-cohort see single-cohort colonies colony-level integration of individual behaviour, 143-9 behavioural dominance, 146-7 behavioural variability within subfamily, 144 idiosyncratic/elite/reserve workers, 148 plasticity in division of labour, 144-6 colony-level selection, 142, 162 colouration, 12-21 acridids, 12, 13, 15, 17, 18, 21 adult locusts, 18-20 hoppers and associated hopper-adult features, 12-18 components and division of labour, 13742 concave functions, 123 congregans, 4 convex functions, 123 corpora allata see CA corpora cardiaca see CC corpse-removal, 145 cricket, 97, 173 Gryllus campestris, 3 cross-flow analysis, 189 crowded, 28 see also gregaria; gregarious cyclic AMP, 29, 51, 101 cylinder, 188-200 axial flow, I 8 5 6 glide characteristics, 194-8 normal flow, 184-5 Cyrtacanthacridinae, 6, 7, 17 cytology, 30-1

daughter colonies, 158 DDT, 82, 100, 101 insecticide design, 104, 105 ‘death feigning’, 103 defenders, 138 desert cicada (Diceroprocta apache), 98

21 3

desert locust (Schistocerca gregaria), 6, 7, 91 Devonian, 174 diacylglycerol, 36 Diceroprocta apache (desert cicada), 98 Diciostaurus maroccanus (Moroccan locust), 6 Dictyoptera (cockroach), 173 die1 changes, 91 Diploptera punctata (cockroach), 105 Dipiura (bristletaiis), 174 dissocians, 4 diuretic hormone, 99, 100 division of labour, 128-36 evolution, 149-62 genotypic variability and adaptation, 1662 natural selection, 154-6 on F‘, 156 on K, 155 on N, 155 organizational structure, 1 5 M O behavioural canalization and heritability, 15940 behavioural modularity, 157-9 self-organization, 149-53 extreme, 152 genetic basis, 136-43 components, 13742 genotypic variability for performing tasks, 13740 genotypic variability for rate of behavioural development, 140 genotypic variability for response to changing colony conditions, 141-2 queen and brood, 142 genetics, 117-62 hormonal regulation, 131-3 model, 129 patterns, 128-30 plasticity, 1361, 135 colony-level integration of individual behaviour, 144-6 division of labour, plasticity, age polyethism, 130, 131, 132, 135 Dociostaurus maroccanus, 9 dopamine, 37 dorsal unpaired median (DUM) neuron, 83, 84, 91 drag coefficient, 183, 184, 185, 187, 188, 193 glide characteristics, 194, 195

214

drag coefficient-contd giide speed reduction, 204, 205 resolved-flow analysis, 189 Reynold's number, 181 skin friction, 182 tangential, 186, 197, 201 drag force, 178 glide characteristics, 197 resolved-flow analysis, 189 tangential, 190, 191 viscous, 205 dragonfly group, 173 Drosophila, 202 DUM see dorsal unpaired median neuron dynamic systems parameters, 154-6

E/F ratio, 10, 40, 47 early insects, 172-7 first flights, 175-7 fossil records, 172-4 protopterygotes, 174-5 ecdysone, 18 bisynthetic activity, 46 ecdysteroids, 32, 33, 45-9 elite workers, 148 endocrine, 1-55 control, 83-5 effects and phase characteristics, 8-37 organs and hormones, 3 7 4 9 CA and JH, 3 7 4 5 ventral glands and ecdysteroids, 45-9 environment, 133, 136 Ephermeroptera (mayflies), 173, 175 Ephestia kuehniella (flour moth), 90, 105 evolution and division of labour, 149-62 excitatory hypertrehalosaemic see EXIT response EXIT response, 81, 92 exocrine gland development, 129 extended arousal syndrome see arousal syndrome, extended

5-HT, 83, 85, 90

F' selections, 156 F/C ratios 10, 11, 47, 51 feedback loops, negative, 147, 148, 151, 152 feeding, 88-9

INDEX

fight or flight syndrome/response, 81, 82, 101-2 flat plate in normal flow, 1 8 3 4 flat plate in parallel flow, 182 flight, 88-9 first, 175-7 insect, 171-208 locust, 34, 99 origin and aerodynamics, 171-208 physiological correlates, 99-1 00 response see fight or flight syndrome/response floating theories, 17&7, 205 flour moth (Ephestia kuehnieffa),90, 105 flow, laminar, 192 flow, tangential, 192 fluid forces, 178-80 fluid viscosity, 180 foraging, 135, 138 see afso nectar foraging; pollen foraging; precocious foraging force coefficients, 182, 183, 185, 186 aerodynamics, basic, 1 8 2 4 cylinder in axial flow, 1 8 5 4 cylinder in normal flow, 18&5 flat plate in normal flow, 1 8 3 4 flat plate in parallel flow, 182 formamidine insecticide, 104 fossil records of early insects, 1 7 2 4 frontal area, 180 Galleria, 39 Gastrimargus africanus, 17 general arousal syndrome see arousal syndrome genetic covariance, 158 genetic polymorphism, 161 genetic specialist, 161 genetic variance, additive, 126 genetic variation, intracolonial, 143, 144 genetics of honey bee, 119-27 genomes, 125, 127, 158 genotypic composition of colonies, 1 2 4 5 differences, 140, 148 distribution, 127 variability, 141 adaptation, 16&2 behavioural, 144, 145 performing tasks, 1 3 7 4 0

INDEX

polyandry, 126, 160, 161, 162 rate of behavioural development, 140 response to changing colony conditions, 141-2 glide angle, 194, 197, 198, 207 improvement, 2 0 6 3 characteristics, 194-8 path, 199 polars, 195 speed, 196, 198 reduction, 2 0 3 4 gliding, 186-8, 196 cylindrical bodies, 188-200 glide characteristics, 1 9 4 8 lift and drag at constant Reynolds numbers, 190-3 resolved-flow analysis, 189-90 stability and control, 198-200 glycolysis, 30 Gomphocerinae, 6 grasshopper Acrida turrita, 3, 17 migratory, 6 Orthoptera, 6, 20, 173 short-homed, 6 gregaria, 4, 5 gregarization pheromone, 51, 52 Gromphadorhinaporrentosa, 98 Gryllidae, 8 Gryllus campestris (cricket), 3 guaiacol, 52 Hubrobracon juglandis (parasitic wasp), 119,

120 haemolymph, 82 behaviour/activity, 32, 33, 36 cytology, 31 physiology/biochemistry/molecular biology, 29 Schistocercu, 1I haplodiploidy, 119, 126 Hardy-Weinberg population assumptions, 121 hatchlings, maternal effects on, 22-6 2-heptanone, 132 heritability, 159-60 homochromy, 13, 15, 16, 21 black, 18 brown, 16

21 5

homozygosity, lethal, 119 homozygous expression, 120 homozygous queens, double, 120 honey bee, 3 , 118 age polyethism, 133 arousal syndrome, extended, 90 colony fission demography change, 135 demography changes, 134 division of labour genetics, 117-62 genetics, 119-27 genotypic composition of colonies, 1265 haplodiploidy, 119 mating behaviour, 120-1 polyandry and genotypic variability, 126 polyandry and sex determination, 1 2 1 4 sex determination, 119-20 insecticide design, 105 societies and organizational structure, 15640 behavioural canalization and heritability, 159-60 behavioural modularity, 157-9 division of labour evolution, 15660 subfamilies and given tasks, 138 hopper, 12-18 behaviour and activity, 31-3 development, 26-8 hormonal regulation and division of labour, 131-3 hormonal regulation of plasticity, 132-6 hormones see endocrine organs Humbe tenuicornis, 1I hydrocarbons, cuticular, 30 Hymenoptera, 118, 123 division of labour, 128 families and different relationships, 125 haplodiploidy, 119 polymorphism, 3 hypergregarious, 10 hyperlipaemia, 84 hypertrehalosaemia, 84 hypopharyngeal gland, 132

idiosyncratic workers, 148 IGRs see insect growth regulators individuals, relationships of, 125 inertial force, 178, 179, 187 resolved-flow analysis, 189

216

inertial pressure drag, 183 insect early see early insects families, 173 flight, 171-208 growth regulators (IGRs), 53, 54 peptidergic, 54 phase polymorphism, 7-8 pigments, 15 polymorphism and endocrine relations, 1 4 insecticide design, 10&6 poisoning, 84, 100-1 treatment, 88-9 instars, 27, 28, 29 ions, 98-9 isoenzynmes, 30 isolated, 28, 29 see also solitaria isopentyl acetate, 132

JH (juvenile hormone), 53, 54 behaviour/activity, 34, 35 and CA, 37745 colouration, 19, 20, 21 division of labour, 131, 132 endocrine organs, 38, 39, 40, 44, 45 environment influence, 136 honey bee age polyethism, 133 honey bee and colony fission demography change, 135 honey bee and demography changes, 134 hopper development, 27, 28 insecticide design, 105 locust phase characteristics, 41-3 male sexual behaviour, 22 phase polymorphism, 8 pheromones, 50, 52 physiology/biochemistry/molecular biology, 29, 30 reproductive parameters, 24, 25, 26 see also CAiJH

K selection, 155

laminar flow, 179, 182, 185

INDEX

Lepidoptera larvae, 17 Leucophaea maderae, 85, 95, 96 lift coefficient, 187, 188, 193 glide characteristics, 194 Reynolds number, 181 lift and drag at constant Reynolds numbers, 190-3 lift-to-drag ratio, 200, 201, 202, 203, 206, 207 Limulus, 90 lindane, 101 lipid, 83 lobster, 84, 90 locomotion, 105 locomotor activity, spontaneous, 34 lodust, 20, 83 arousal syndrome, extended, 91 behaviour/activity, 33 CC, 90 colouration, 18-20 endocrine control, 85 flight, 34, 99 gregarious, 9, 10 hyperlipaemia, 84 insecticide design, 105 metabolic substrates, 97 ‘Monday morning’, 106 outbreak, 53 phase characteristics, 41-3 phase polymorphism, 1-55 plague, large-scale, 52 see also Locusta; Locustana; Nomadacris; Schistocerca solitary, 9, 10 Lorusta behaviour/activity, 32, 35, 36, 37 colouration, 13, 16, 17, 18, 19, 20 cytology, 31 endocrine control, 85 endocrine organs, 38, 39, 40, 44 hopper development, 28 insecticide design, 105 male sexual behaviour, 22 metabolic substrates, 96, 97 morphology/morphometrics/anatomy, 8, 9, 10, 11, 12 phase polymorphism, 4 pheromone, 21, 50, 52 physiology/biochemistry/molecular biology, 28, 29, 30 ,

21 7

INDEX

pronotum, lateral view of, 9 reproductive parameters, 23, 24, 25, 26 VG, 45, 46, 47, 48,49 Locusta danica, 4 Locusta rnigratoria (migratory locust), 4, 6-7, 14 Locustana colouration, 13, 18, 19 cytology, 30 morphology/morphometrics/anatomy,9 physiology/biochemistry/molecular biology, 29 Locustana pardalina (brown locust), 4, 7 locustol, 5 1 lycopods, 174

malate dehydrogenase (Mdh), 137, 139, 141 Malpighian tubule, 96, 97 fight or flight response, 102 flight, 99, 100 paralysis/insecticide poisoning, 10 1 Manduca sexta, 17 marching, 32 mating behaviour and honey bee genetics, 120-1 maturation, 21-2 mayflies (Ephermeroptera), 173, 175 Mdh see malate dehydrogenase median neurosecretory cells (MNSC), 34, 35 melanins, 15 Melanoplus, 6 metabolic substrates, 92-3, 95, 96, 97, 98 metathetely, 10 methoprene, 131, 132, 135 MF 1/11, 92 migratory grasshopper, 6 migratory locust (Locusta migratoria) 4, 6-7, 14 Miomoptera, 173 MNSC see median neurosecretory cells molecular biology, 28-30 ‘Monday morning’ locusts, 106 monophyletic groups, 173 Monura, 174 Moroccan locust (Diciostarus maroccanus), 6 morphology, 8-12 morphometrics, 8-12 mosquito (Anopheles stephemi], 90

moth, 85, 91 multiple-stimulus-threshold complexes, 156 muscle effects, 9@-2

N selection, 155 natural selection, 154-6 Nauphoeta, 100, 106 NCAI, 25 nectar, 1 4 5 4 foraging, 158, 159, 161, 162 Neoptera, 172, 173, 174 neoteny, LO nerve and muscle effects, 90-2 neurohormones, 136 peptidergic insect, 54 neuromodulation cardioacceleration, 102 neuropeptide, 82, 8 6 7 cessation of activity, voluntary, 104 endocrine control, 83, 85 neurosecretory cells (NSC), 17, 18, 20, 21 CC factor, 20, 21 see also pars intercerebralis median neurosecretory cells no-slip condition, 178 Nomadacr is colouration, 13, 14, 19 hopper development, 26, 27 morphology/morphometrics/anatomy, 8, 11, 12 physiology/biochemistry/molecular biology, 29 reproductive parameters, 22, 23 Nomadacris septernfasciata (red locust), I noradrenaline, 37, 82 NSC see neurosecretory cells nurses, 134, 135 overaged, 130

0, consumption, 28 octopamine, 82, 8 6 7 activation continuum, 103 arousal response, postulated, 86-7 arousal syndrome, extended, 90, 91, 92 behaviour/activity, 36, 37 cessation of activity, voluntary, 104 endocrine control, 83, 84, 85 fight or flight response, 102

21 8

octopamine-contd insecticide design, 105 metabolic substrates, 94, 97, 98 paralysis/insecticide poisoning, 101 Odonata, 173 Oedipoda coerulescens, 18 Oedipodinae, 6, 7, 17 ommochromes, 15 ontogeny, 146 oocyte development, 24, 25, 46 growth, 35 length, 37 proximal, 38 vitellogenic, 39 Ordovician, 174 organizational structure in honey bee societies, 1 5 M O Orthoptera (grasshopper/cricket), 6, 20, 173 ovarioles, 23

Palaeodictyopteroidea, 173 Palaeoptera, 172, 173 Palaeozoic, 172 paleopterous, 172 paraglider theories, 176 paralysis, physiological correlates of, 10&1 paranotal lobe hypotheses, 175 parasitic wasp (Habrobracon juglandis), 119, 120 pars intercerebralis neurosecretory cells (PI-NSC), 48 pars intercerebralis (PI), 34, 35 parthenogenesis, 119 peptidergic insects, 54 performing tasks, genotypic variability for, 1 3 7 4 0 Periplunefu americana, 82 activation continuum, 102 arousal response, postulated, 8 6 7 , 88-9 arousal syndrome, extended, 85, YO, 91 cessation of activity, voluntary, 103 DDT poisoned, 101 endocrine control, 84, 85 Right, 99 insecticide design, 104, 105 ions, 98 metabolic substrates, 92, 95, 96, 98 paralysis/insecticide poisoning, 100 water content, 96

INDEX

permethrin, 105 Permian, Early/Lower, 173 PG (prothoracic glands) see VG phase characteristics and endocrine effects, 8-37 behaviour and activity, 31-7 colouration, 12-21 cytology, 3&1 hopper development, 2G8 morphology/morphometrics/anatomy , 8-1 2 physiology/biochemistr y/molecular biology, 28-30 reproduction, 2 1-6 phase polymorphism, 4-8 locusts, 1-55 phase transformation and endrocrine organs/hormones, 3 7 4 9 phenol, 52 phenotypic plasticity/integrity, 55, 156, 157 pheromone, 21,49-52, 91, 142 alarm, 105, 133 endocrine organs, 38 gregarization, 51, 52 maturation, 21-6 moth, 85 Schistocerca, 21, 22, 50, 51, 52 physiology, 28-30 PI see pars intercerebralis pigments, 15 pitch stability, 199, 207 plasticity, 55 colony-level integration of individual behaviour, 144-6 division of labour, 13&1 hormonal regulation, 13poisoning, insecticide, 10&1 Polistes (wasp), 142 pollen foraging, 145, 147, 158, 161, 162 pollen storage and colony frequency distribution, 159 polyandry, 124 genotypic variation, 160, 161, 162 honey bee genetics, 126 sex determination, 1 2 1 4 polychromatism see polymorphism polymorphism, 1-4 genetic, 161 green-brown, 13, 14, 15, 20, 21 homochromy, 13, 15, 16 insect, 1 4

INDEX locust phase, 1-55 phase, 4-8 phase/density dependent colour, 13 superimposed, 14 postulated arousal response see arousal response, postulated potassium, 98 precocenes, 54 precocious foraging, 130, 131, 134, 141 primers see pheromones proctolin, 90, 92 progymnosperms, 174 pronotum of Locusta, 9 prothoracic glands see ventral glands prothoracicotropic hormone, 48 protocerebrum, 17 Protodonata, 173 protopterygote, 172, 174, 206, 207, 208 early insects, 1 7 4 5 glide angle improvement, 20 1, 203 glide characteristics, 197, 198 glide speed reduction, 204, 205 gliding cylindrical bodies, 188 possible, 176 stability/control, 198, 199, 200 Protorthoptera, I73 protowings see winglets pteridosperms, 174 pterins, 15 Pterygote, 172, 173, 174 gliding, 196 pyrethroids, 105

quasi-specialist, 161 queen bee brood, 142 brood viability, 123, 124 homozygous, 120 sperm use pattern, 122

red locust (Nomadacris septemfasciata) 7 releasers see pheromones reproduction, 2 1 4 maturation and pheromone, 21-2 parameters, female, 2 2 4 sexual behaviour, male, 22 reserve bees, 130 reserve workers, 147-8

21 9

resolved-flow analysis, 187, 189-90, 191, 192 resolved-flow equations, 196 response thresholds, 148, 154 Reynold’s number, 181, 207 aerodynamics, basic, 180-2 constant, 190-3 force coefficients, 182, 183, 185, 186 glide angle improvement, 201,202, 203 glide characteristics, 194 glide speed reduction, 204, 205, 206 gliding, 188 stability/control, 199 Rhodnius, 85, 99, 100 paralysis/insecticide poisoning, 101 roll instability, 200 roll stability, 199 RPCH, 92 running jump/flying leap theory, 177

Schistocerca, 6 arousal syndrome, extended, 91 behaviour/activity, 32, 33, 34, 35, 36 colouration, 13, 14, 16, 18, 19, 20 cytology, 30, 31 endocrine control, 85 endocrine organs, 37, 38,40 F/C ratio, 11 haemolymph, 17 hopper development, 27 metabolic substrates, 98 morphology/morphometrics/anatomy, 8, 9, 12 paralysis/insecticide poisoning, 101 pheromone, 21, 22, 50, 51, 52 physiology/biochemistry/molecular biology, 28, 29 reproductive parameters, 22, 23, 25, 26 VG, 46,47,48 Schistocerca americana, 6 Schistocerca americana gregaria see Schistocerca gregaria Schistocerca cancellata, 6 Schistocerca gregaria (desert locust), 6, 7, 91 Schistocerca nitens, 91 Schistocerca paranensis, 6 Schistocerca piceijorms, 6 self-organization and division of labour evolution, 149-53 sex determination and honey bee genetics, 119-20

220

sex determination and polyandry, 1 2 1 4 sexual behaviour, male, 22 short-horned grasshopper, 6 sigmoid functions, 123 silverfish (Thysanura), 172, 174 single-cohort colonies, 133, 134 division of labour plasticity, 135 genotypic variability, I41 sink speed, 195, 196, 197 skin friction, 179, 192, 197 drag coefficient, 182 laminar. 182, 186 viscous, 184, 185, 202 ‘sleep’, 103 sodium, 98, 99, 102 solitaria, 4, 5 solitarized, artificially, 39 specialization, 156 sperm use pattern for queen bee, 122 spermatogenesis, 120 sphenopsids, 174 stability and control, 198-200 steady state attractor, 152 stimulus level regulation, 154 ‘streams’, 33 stress, 88-9 stress syndrome, generalized, 8 1 subfamilies, 137-9 behavioural variability, 144 given tasks, 138 ‘super sisters’, 124 swarm, 33, 130 switching net, 150, 151 sympathetic nervous system, 82 tangential drag, 204 tangential flow, 189 taurine, 100, 101, 104 terminal velocity, 204, 205, 207 termites, 127 lower, 3, 4 test genotypes, 121 tetrapods, I74 Tettigoniidae, 8 thanatosis, 103 thermogenesis, 104 thermoregulatory capacity, 175 threshold functions, 152, 153, 155 Thysanura (silverfish), 172, 174 tongue-lashing, 95

INDEX

transfer, pathways of, 92 transiens, 4

transmitted effects, 26 trehalose, 92, 99 trigonometric expressions, 186-7 undertakers see corpse removal Van Scoy strain, 139, 140 velocity gradient, 178 ventral glands see VG veratrole, 52 vertebrate syndrome, 82 VG, I8 acridids, 45, 46 behaviour/activity, 34 and CA, 46,48 CC, 48, 49 colouration, 18 endocrine organs and hormones, 45-9 Locusta, 45, 46, 47, 48, 49 morphology/morphometrics/anatomy,10, 11 Schistocerca, 46, 47, 48

vibration reaction, 50 viscous force, 178, 179, 182 viscous realm, 197 vitellogenin, 24 voluntary cessation of activity, 1 0 3 4 von Karman wake, 182, 189 wasp (Polistes), 142 water and arousal syndrome, extended, 94-8 water balance, 83 wetted area, 180 ‘wild-type’ workers, 140 winglets, 207 aerodynamic function, 2 0 M enlarged, 207, 208 first flights, 176 glide speed reduction, 204, 205 protopterygotes, 174, 175 stability/control, 198, 199 Xylocopa capitata (Carpenter bee), 94

yaw stability, 199