SOME ASPECTS OF THE POPULATION ECOLOGY

OF THE

GRASSHOPPER ACRIDA CONICA FABRICIUS

This thesis is presented for the degree of

Doctor of Philosophy of Murdoch Universi

1 9 8 5

Submitted by

Michael Charles Calver B.Sc. (Hons) Murdoch

I ! Under carefully controlled conditions of temperature, humidity, light and any other important variable, the experimental animal will do as it damn well pleases.

- Harvard Law of Biological Experimentation FRONTISPIECE

Dorsal view of adult female Acrida conica� Original

drawing by Mr. Mike Bamford.

DECLARATION

I declare that this thesis is my own

account of my research and has as its main

content work which has not been submitted

previously for a degree at any university.

Michael C. Calver

August 1985

i I I I TABLE OF CONTENTS

PUBLICATIONS (i)

ACKNOWLEDGEMENTS (ii)

ABSTRACT .. (iii)

LIST OF TABLES (vii)

LIST OF FIGURES (xi)

1. INTRODUCTION - AN APPROACH TO POPULATION BIOLOGY 1

2. BASIC BIOLOGY 11

2.1 The genus Acrida 11

2.2 Growth and development 11

2.3 Colour patterns and colour change 13

2.4 Natural enemies 15

2.5 Study sites 15

3. POPULATION DYNAMICS 20

3.1 Introduction 20

3.2 Materials and methods 21

3.3 Results .. 40

3.4 Discussion 44 4. ANTI-PREDATOR DEFENCES 53

4.1 Introduction 53

4.2 Crypsis, distribution and anti-predator

behaviour in the field 61

4.3 Background colour matching 73

5. ARTHROPOD PREDATORS 77

5.1 Introduction 77

5.2 Methods .. 78

5.3 Results and discussion .. 82

6. VERTEBRATE PREDATORS

6.1 Prey choice in the field 85

6.2 Prey choice in the laboratory 89

CONDITION, FECUNDITY AND DIET

7.1 Introduction .. 101

7.2 Methods .. 103

7.3 Results and discussion 105

8. A NOTE ON SEXUAL SELECTION

8.1 Introduction 108

8,2 Methods 109

8.3 Results and discussion 110

I I 9. GENERJl.cL DISCUSSION 114

REFERENCES 124

APPENDICES 154 (i)

P U B L I C A T I O N S

Some of the work reported in this thesis has been published.

Calver, M.C. (1984). A review of ecological

applications of immunological

techniques for diet analysis.

Aust. J. Ecol. 9, 19-25

Calver, M.C. and Wooller, R.D. (_1982).

A technique for assessing the taxa,

length, dry weight and energy

content of the arthropod prey of

birds.

Aust. Wild. Res. 9, 293-301 (ii)

A C K N O W L E D G E M E N T S

I am grateful to Dr. Ron McKay and my supervisor,

Dr. Stuart Bradley, for permission to use their recent

unpublished model for analysis of capture-recapture data.

Dr. Bradley also performed the analyses for fitting both of the population models reported in Chapter 3, and maintained unfailing good humour throughout the span of this work.

Several people gave valuable assistance in field work, especially my father, Mr. John Calver, who helped in the difficult preliminary trials in the first year of field work and also made some of the equipment. At other times Mr. Mike Bamford, Dr. Toni Milewski and Ms. Pauline

Duncan also lent a hand. Dr. Ron Wooller was a frequent source of advice, encouragement and perceptive criticism,

Dr. Barbara Porter drew most of the figures and the late

Mr. Fred Marshall translated some papers from the original

German. I would also like to thank the Town Clerks of the City of Canning and the City of Melville for permission to work. on properties in their jurisdiction.

Finally, my mother, Mrs. Kay Calver, typed the final manuscript with great care, speed and accuracy. (iii)

ABSTRACT

The population ecology of the grasshopper Acrida

conica Forskal was investigated at sites near Perth,

Western Australia, and interpreted in the light of data

collected on the species' predators, growth and develop­

ment, fecundity and reproductive behaviour.

Basic Biology A. conica has a univoltine life history

with eggs hatching in mid-November and most individuals

maturing by eight weeks later. Females pass through a

total of seven instars and males six. A marked sexual

dimorphism develops with females measuring 1.5 times the

length of the male and weighing two to three times as

much. The species is polymorphic in colour, and individuals

may range from yellow and brown through to green. Laboratory

experiments indicated that the polymorphism was under

environmental control, with humidity and diet being the

regulating factors.

Population Dynamics Data on hatching, senescence, moulting

rates and survival rates in juveniles were collected by applying Read and Ashford's maximum likelihood models to sequential sampling data, because capture-recapture data are inappropriate where animals lose marks through moulting.

A second analysis was made for juveniles using a new model which combines capture-recapture data with the Read and

Ashford model. Capture-recapture techniqu�s were applied for the adults, and analysed using the Jolly-Seber model. (iv)

Both sexes hatched in similar proportions. However,

either the third or fourth male instar was extended

considerably in each season compared to the females,

and this led to synchronization of adult emergence

despite the difference in initial hatching times and

the extra female instar.

Survival rates between the sexes were dissimilar,

with juvenile female survivorships being less than those

of males, although adult females had higher survivorships

than adult males. The differential survival produced

unequal adult sex ratios which varied between 2 : 1 and

13 : 1 males to females at different sites.

Predation Observations of A. conica in the field

indicated that flight, startle displays and crypsis were

the principal defences, and were supported by the adoption

of an aggregated distribution. The incidence of regurgit-

ation was insignificant. Laboratory experiments indicated

that grasshoppers were capable of matching their backgrounds.

Birds took longer to catch a grasshopper on a matching background, and long backgrounds conferred protection irrespective of their colour. Similarly, capture times were increased when larger numbers of live grasshoppers were presented, but not when the grasshoppers were dead, suggesting that appropriate behaviour in aggregation has a defensive value. Calculations based on handling time and biomass for bird predators indicated that specific instars gave an optimum return of biomass/unit time. In

I I (v)

the case of magpies, Gymnorhina tibicen, which are predators

in the field, fifth and sixth instar females represented the

optimum, coinciding with the increased mortality rates of

these instars observed in the field.

Predation in the field was estimated by serological

analysis of invertebrate predators, examination of prey in

spiders' webs and examination of droppings from vertebrate

predators. Of the invertebrates spiders took the most

grasshoppers, these being mainly green females from the

earlier instars. Larger grasshoppers were attacked primarily

by birds, and disproportionate numbers of green grasshoppers

were caught.

Growth, Development and Fecundity Grasshoppers reared

in the laboratory on well-watered grasses of Avena fatua

(wild oats) and Zea mayes (maize) showed improved growth

rates compared to other grasshopper,s whi.ch were reared on poorly watered plants of- wheat and A. :fatua. Better nourished females also laid more eggs/pod. Comparison of condition factors among female grasshoppers collected in the field showed that sixth instar individuals from areas with lusher grass were in better condition thari others, and that females tended to aggregate in such areas. Male densities were higher in drier zones.

Reproductive Behaviour In laboratory trials heavier males were found to be more successful in gaining access to females, although females did reject males who were slow

I I (vi)

in establishing copulatory union. Copulation varied in

length from 30 minutes to over two hours, and was

followed by a period of mate guarding of similar length.

Conclusion It is suggested that female life histories are designed to maximize the fecundity of individuals at a possible cost in higher juvenile mortalities. By contrast, males aim to accentuate the sexual dimorphism to increase their survivorship relative to the females. They also regulate their development to ensure maturation at the optimum time to maximize mating success. (vii)

LIST OF TABLES

Table 1 Mean pronotal lengths (mm), total lengths (mm)

and dry weight (mg) for male and female

grasshoppers of each instar.

Table 2 Common plants at the four study sites.

Table 3 Mortalities of marked and unmarked second and

fifth instar grasshoppers over a period of

three days in the laboratory.

Table 4 Chi-squared test of the influence of marking on

the loss of grasshoppers, using data collected

in 1979/1980.

Table 5 Tests for random distribution of marked

grasshoppers.

Table 6 Catchability of grasshoppers.

Table 7 Tests for association with the factors of

grasshopper colour, grasshopper sex and

background colour at site I, site II and

site III.

Table 8 Tests for association with the factors of

grasshopper colour, grasshopper sex and

background height at site I, site II and

site III. (viii)

Table 9 Tests for association between grasshopper sex

and distance from the lake at site I.

Table 10 Tests for association between morph and sex for

adult A. conica at site I in the summer of 1979/

1980, 1980/1981, 1981/1982.

Table 11 Tests for association between morph and sex for

adult A. conica at three sites in the summer of

1981/1982.

Table 12 Regurgitation frequencies for a sample of

grasshoppers of all instars of both sexes,

caught during the distribution work in the

summer of 1981/1982.

Table 13 Number of leaps made in 30 seconds by disturbed

grasshoppers, the incidence of burrowing

behaviour (B) and the.number of grasshoppers

tested (n).

Table 14 Tests for association between the occurrence

of second leaps and the colour of the background

landed on at the end of the first leap in

A. conica.

Table 15 Results of serological tests on potential

arthropod predators collected at site I in

1980/1981 and 1981/1982. (ix)

Table 16 Sex, instar and morph of grasshoppers caught in spiders' webs at site I in 1980/1981 and

1981/1982.

Table 17 Height above ground and background colour of spider webs found at site I.

+ Table 18 Optimal prey sizes (- standard error) of

lycosids and adult mantids preying on A. conica.

Table 19 Birds caught at site I which could have been

feeding on A. conica.

Table 20 The sex and morph of adult A. conica caught

by magpies at site I.

Table 2l Times taken for magpies to capture live

grasshoppers under controlled conditions.

Table 22 ANOVA table for log-transformed data in Table 21.

Table 23 Times taken for magpies to capture dead

grasshoppers under controlled conditions.

Table 24 ANOVA table for log-transformed data in Table 23.

I ! (x)

Table 25 Number of days to maturity for grasshoppers

raised from the second instar on three

different diets.

Table 26 Data on the number of eggs and egg pods laid

by females on three different diets.

Table 27 Condition factors of female grasshoppers

collected at site I in 1981/1982.

Table 28 Condition factors of male grasshoppers

collected at site I in 1981/1982.

Table 29 Weights of males, number of copulatory

attempts, duration of copulation (± standard

error) and duration of mate guarding

(- standard error). (xi)

LIST OF FIGURES

Fig. 1 Colour patterns and sexual dimorphism of

A. conica.

Fig. 2 Lateral view of an adult male A. conica

showing the main zones of possible colour

variation and striping.

Fig. 3 Dorsal view of an adult female A. conica

showing the abdominal startle display and

the main zones of possible colour variation

and striping.

Fig. 4 Predators of A. conica.

Fig. 5 Differences in vegetation colour at site IV.

Fig. 6 Scheme for marking A. conica.

Fig. 7 Pattern of plots used for MRR sampling.

Fig. 8 Survivorship curves for grasshoppers based

on the Read and Ashford maximum likelihood

models.

Fig. 9 Survivorship curves for grasshoppers based

on mark-release-recapture analysis. (xii)

Fig. 10 Distribution of juvenile A. conica .

Fig. 11 Distribution of adult A. conica.

Fig. 12 The proportion of brown grasshoppers of

each instar caught at each site.

Fig. 13 Changes in the proportion of brown

vegetation at three sites during the

summer of 1981/1982.

Fig. 14 Number of grasshoppers out of 10 settled

on the matching background at 15 minute

intervals.in a background choice experiment.

Fig. 15 The pattern of wells for gel diffusion

tests.

Fig. 16 The handling time in seconds for each of

three magpies fed male and female grasshoppers

from third instar to adult.

Fig. 17 The energy return in kJ/minute for each of

three magpies feeding on particular grasshopper

instars.

I I 1. INTRODUCTION AN APPROACH TO POPULATION BIOLOGY

The features of life history timing, fecundity and mortality

which together form a population�s observed demography arise from

a bionomic strategy developed to maximize the fitness of individuals.

Emphasis on the elucidation of these strategies arose from the need

to expand and generalize on the results of studies of ecological

genetics. These concentrated traditionally on organisms with

easily detectable variations which could be studied readily in the

field. Initial studies demonstrated strong selection forces in

animals as diverse as the snail Cepaea nemoralis (Cain and Sheppard

1950} and the moth Biston betularia (Kettlewell 1955). Many more

recent examples appear in the review of Jones et al. (1977), and the

work of Endler (1980, 1982} and Reznick and Endler (l982). Because

many of these studies demonstrated correlations between environmental

variables and allele frequencies in populations, several workers

applied multiple regression techniques to develop equations predicting

allele frequencies from measured environmental factors. They were

prompted in part by calls from increasing numbers of ecologists

(Janzen 1973 and Matthews l976, for example) for ecology to be based

in natural selection theory. However, the approach ignored the

mechanism of operation of selection and could not distinguish between

factors that have a causal influence on selection or simply covary

with allele frequency (Bishop et al. 1975). In this context Jones

et al. (1977) indicate that while a single cause of a polymorphism

is invoked usually, in each population the genetic structure could

be maintained by different single methods, or several methods in

combination. They concluded that complex and unique explanations

may be needed for each population.

I I 2

While these attempts to predict morph frequencies from the environment were developing, an alternative approach was concentrating on explaining the origin and persistence of polymorphisms in populations by the action of "evolutionarily stable strategy" or ESS. This had the advantage of placing the results of selection in the total context of an organism's complete biology. A rigorous definition of the concept arose from the application of games theory to the problem of intraspecific fighting (Maynard Smith and Price 1973, Maynard Smith 1974,

Parker 1974, Maynard Smith and Parker 1976, Serpell 1982).

Essentially it argues that instead of comparing the success of individual organisms in an activity it is better to concentrate on the success of "strategies" averaged across the intraspecific groups which use them. A strategy that is successful will tend to increase in a population, and rare strategies will be lost because they cannot compete successfully. Consequently, the proportion of organisms displaying the successful strategy increases in the population. For it to persist, it must then be capable of competing against copies of itself. Such long-lived strategies can be identified and studied. The ESS concept can be extended into other areas in which a strategy approach is used, including optimal foraging (Kamil and

Sargent 198l, Hoddap and Frey 1982, Pyke 1984), mating behaviour

(Eaton 1978, Parker 1978, Thornhill 1980, O'Donald 1980}, competitive resource sharing (Milinski 1984) and sex allocation (Trivers and

Willard 1973, Trivers 1974, Charnov 1982).

The great attraction of the ESS approach is its powerful ability to integrate diverse fields of biology through what has been called "selection thinking" (Charnov 1982). Using this, problems are phrased in the general form: "under what

I I 3

social or life-history conditions natural selection favours one or the other form of (the phenomenon under study)" (Charnov 1982).

This leads to the identification of the ESS for that problem, and has the extra advantage of allowing the solution to be expressed in genetic terms. For example :

At what age do we expect a cohort of protandrous shrimp to change from 6 to�? The answer is at an age when, under the prevailing demographic and growth

conditions, genotypes which switch at some other age

can contribute less genetic material to future

generations. Framed in this way, the tools of

population genetics can be used to provide answers

and allow us to calculate the appropriate value

(Charnov 1982).

This attitude integrates such diverse fields as behaviour and physiology by relating them to the central concept of fitness, which determines the ESS. To illustrate this point Charnov

(1982} refers to the Shaw-Mohler equations for sex allocation, and develops these fitness-based ideas as a background to specific cases in the field. Q l Donald (1980) gives a further example of the usefulness of �• selection thinking'' in his account of the development of modern sexual selection theory. He cites numerous early studies in which failure to base observations in the context of fitness left their significance confused.

Of course, ESS theory does not imply that all individuals in a population should be following a single behavioural pattern. As pointed out by Maynard Smith (1979), Charnov (1982) and Dawkins

(1982) the ESS can be a complex of actions, each with a definite 4

probability. These can arise in a number of ways. If the environment is heterogeneous the ESS may be an amalgam of behaviours, with each one suited to a particular microhabitat.

They could be in a stable polymorphism, with some individuals using one strategy exclusively and others another, with the relative proportions maintained by frequency-dependent selection.

Alternatively, the ESS may be the same for all individuals, but requiring them to use all the possible actions with probabilities assigned to each. Dawkins (1982) suggests that the "polymorphism" and "no polymorphism" states are opposite ends of a continuum, and that the ESS could be reached by any number of combinations of pure and mixed individual strategies which summed to give the stable proportion of strategies in the total population.

Currently, much of the emphasis of ESS theory has been on intraspecific examples, especially fighting {Bishop and Cannings

1978, Anderson 1980) and theories of sexual selection {Thornhill

1980, Charnov 1982, Cohen 1984). However, the concept ought to be extended into interspecific interactions as well. If frequency­ dependent selection is capable of maintaining a balanced polymorphism of strategies to form an ESS for a prey species, then the feeding pattern of the predators should itself be an ESS. There is a clear need to understand the complex interactions of prey and

predators, rather than regarding prey simply as a resource or

predators only as agents of selection. The growing view that

optimal foraging behaviour and frequency-dependent selection a.re

essentially the same phenomenon expressed from the point of view

of predator and prey respectively (Hubbard et al. 1982, Sih 1984)

shows this integrative trend. Similarly, the argument of

Lawrence and Allen (1983) for discussions of search image to be 5

qualified by a thorough understanding of the biology of crypsis emphasizes the need to understand the interactive situations from the perspective of both parties. Price et al. (1980) extend the integrationist argument even further by suggesting that many ecological phenomena can be understood only in the complete context of plant-herbivore-predator relationships.

Many of the ESSs acquired by predators will be in response to such factors as variable prey distribution both spatially and temporally. Maynard Smith (1982) indicates that in such cases the ESS must be learned and cannot be under direct genetic control.

Of course, the capacity for different types of learning is itself genetically regulated, and so forms a basis for selection. The model of Harley (1981) showed that the learning patterns arising under these conditions will bring the proportions of different strategies used within a population to those predicted as the

ESS levels. This emphasizes the value of the ESS concept in understanding predatory behaviour.

Despite the integrationist potential of ESS theory, there is a dearth of studies which relate demographic data on survivorship and fecundity to produce estimates of the relative fitnesses of morphs in a population. Instead, emphasis has been placed on the interaction between morphs and their environment without a total assessment of fitness, or on recording changes in gene frequency with time (Bradley 1975). For example, in a field study of the wasp Sphex ichneumonus Brockmann et al. (1979) and

Brockmann and Dawkins (1979) invoked ESS theory without the backing of survivorship and fecundity data, but were forced to assume that choice of strategy during their observations was

I I 6

independent of survival and fecundity at other times. By contrast, Clutton-Brock et al. (1982) recognized the need to measure total lifetime reproductive success rates in their work on red deer.

The biology of grasshoppers makes them ideal animals for use in a study to relate demographic data to selection in a strategies-based approach to population biology. Some features are largely intraspecific and concern the relationship between polymorphism and sexual dimorphism on the one. hand and sexual selection on the other. However, these aspects can be extended to interspecific concerns as well because they determine a grass- hopper's susceptibility to predation. Additionally, they are key aspects in the natural history of the grasshoppers. In the case of colour polymorphism many grasshoppers show a fixed genetic pattern of inheritance, while others have an environmental colour control and can change colour at a moult (Novak 1975). This position lends itself to an ESS approach, since the two broad strategies of a fixed colour on the one hand and pasticity on the other are clear. The genetic systems are often complex, as Gill (1981) has pointed out that the many epistatic loci involved in colour control in some U.K. grasshoppers ensure wide variations in colour patterns in each generation to take maximum advantage of environmental variability.

In environmental colour change a number of factors including background colour, humidity and food have been implicated (Fraser­

Rowell 1971), but their interplay and precise role in many species is still uncertain. The protracted debate between Ergene (1950,

1952) and Okay (1953) over the relative importance of background 7

colour and humidity in regulating the colour of Acrida turrita is only one example. However, both Fraser-Rowell (1971) and

Ibrahim (1974) note that individuals of Acrida have been reared that maintain constant colour under conditions that would change others of their species. This indicates an underlying genetic basis for colour change, and suggests that strains of genetically fixed colour could be bred. This genetic component makes the assessment of both fixed and variable colour systems by an ESS approach valid.

Grasshopper sexual dimorphism offers scope for developing an ESS with both intra and interspecific components. Females tend to be larger and heavier than males, and the common relationship in insects between body size and fecundity (Mccaffery

1975, Lee and Wong 1979) suggests that it improves their reproductive

success. One consequence of this is an increased potential for

sexual selection, as the larger and more fecund females ought to be preferred by males. Demographic data are valuable in assessing

the consequences of mate choice, and can highlight differences in the population dynamics of the sexes that reflect different life history strategies. These differences are meaningful in the

interspecific context of predation pressures.

The need to avoid being eaten is important for grasshoppers.

Together with demands of thermoregulation and intraspecific

communication it is a major determinant of colour, and sexual

dimorphism may have consequences in the susceptibility of sexes

to predators. Schulz (1981) documented different antipredator

strategies of male and female grasshoppers, and attributed them

to the particular predation pressures each sex suffered because

I ' I I 8

of sexual dimorphism. Glen et al. (1981) suggest that predation may cause unequal sex ratios in some insects, which in turn implicates strategies of mate choice and, at the developmental level, sex allocation. This agrees with the notion of Polis

(1984) that different developmental stages and sexes of arthropods may function as ecological "species" and be perceived as such by other organisms. Indeed, the threat of predation pervades the life history of grasshoppers. It influences their shape and colour (Gillett and Gonta 1978, Schlee 1983), their behaviour

(Schulz 1981, Carlberg 1981), their population distribution

(Gillett et al. 1979) and both directly and indirectly their habitat choice (Joern and Lawlor 1980). An appreciation of these predation pressures emphasizes the interactive aspects of population biology, and utilizes data on stage-specific and morph-specific mortality and fecundity.

Acrida conica is a useful subject for an evaluation of the interrelations among polymorphism (including the special cases of environmentally controlled colour change and sexual dimorphism), fecundity, sexual selection and predation. The morphs are distinctive and can be recognized throughout the life history, and the univoltine life cycle eliminates the problem of over- lapping generations. Finally, the population densities are high so that large samples can be collected.

Critical to an assessment of the resolution of conflicting pressures on the species' life history strategy is an evaluation of the forces of selection acting on the populations. Because selection arises from "differences that the environment causes in the age-specific fecundity and mortality of individuals of

I I 9

different phenotypes" (Bishop 1973) the measurement of these

factors using the techniques of population ecology has major

significance. In particular, population ecology has the

advantage of placing fecundity and mortality data in ecological

context, where underlying factors such as mating preference and

dietary constraints on fertility are considered. In the case

of grasshoppers, Dearn (1984) highlighted the need for studies

on the microdistribution of morphs in relation to habitat

heterogeneity, and emphasized the value of estimates of the

fitness of different morphs.

This study of the population ecology of A. conica centres

on a core of population dynamics data which illustrate stage-

specific mortality within each sex. Dietary data were collected

because of the role of diet in environmentally controlled colour

change and its possible significance in fecundity and sexual

dimorphism, and mating behaviour was studied as an indicator of

the role of sexual dimorphism in sexual selection. These were

all intraspecific factors, but they operated within the overall

constraints of predation.

The impact of predation was assessed partly through a

study of grasshopper behaviour involving factors such as

microhabitat choice, jumping, regurgitation and selecting matching

backgrounds. A variety of vertebrates and invertebrates were

screened as potential predators, and their influence on the

grasshoppers assessed on the basis of selection experiments in

the laboratory and identification of grasshopper meals in

predators collected in the field. Taken together the data

indicated the major predators and the features of grasshopper

I ' ! ! 10

behaviour that would make different stages and sexes more

susceptible to attack. The survivorship estimates from

population dynamics data indicate the relative success of

the different options for grasshoppers, and are the basis

of an assessment of the optimal life history strategy.

! I 11

2. BASIC BIOLOGY

2.1 The genus Acrida

The genus Acrida is included in the sub-family Acridinae

of the family Acrididae, which contains the true, short-horned

grasshoppers. The genus is widespread, with species occurring

in Europe (Ergene 1952), Africa (Whellan 1973), Asia (Ram 1978)

and the Middle East (Ibrahim 1974). In Australia it is

represented by only one species, A. conica, which is believed

to be of geologically recent entry (Key 1974). This is common

in Australia, where approximately one third of all grasshopper

genera contain only one species, while many congeners occur

elsewhere.

Acrida spp. have a characteristic elongated appearance

which aids crypsis on long grasses, where they are often

abundant (Sinclair 1975). They are slow, clumsy fliers,

trailing their long legs behind them and making a distinctive

crackling noise. Some species have prominent anti-predator

displays on the dorsal surface of the abdomen or the hind

wings which appear in flight. Several species have been used

to study aspects of morphological colour change (Ergene 1950,

1952, Okay 1953) digestive efficiency (Ram 1978), anti-predator

defences (Whellan 1973) and cytogenetics (Kumaraswamy and

Rajasekarasetty 1976, 1977).

2.2 Growth and development

Since I was unable to hatch eggs of A. conica in the

laboratory, my information on its growth and early development

is based largely on field populations. It is a univoltine 12

species, and the juveniles hatch in November. The precise time of hatching varies slightly from year to year, and is probably regulated by temperature. By emerging in November the grasshoppers can feed on freshly grown spring grasses, especially in areas of regrowth after fires. Excluding the short-lived vermiform larva, grasshoppers pass through five juvenile instars in the male and six in the female, maturing six to ten weeks after hatching. The rate of development is influenced by environmental factors such as temperature and diet (chapter 7), and a detailed account of duration of instars appears in the population dynamics data (chapter 3). Table l records the mean lengths, pronotal lengths and dry weights for each instar. The rates of increase in the linear dimensions consistently approximate the ratio of l.4 l called "Dyar's law" found in many insects (Norris 1974). However, the extra instar in females allows a marked sexual dimorphism in size, with adult females approximately l.5 times longer than males and three times heavier in dry weight.

Both males and females may copulate several times, and there is evidence that males compete for mates while females also show some selection (chapter 8). The eggs are laid in batches in discrete egg-pods. Grasshoppers in laboratory cages oviposited consistently in grass tufts rather than in the soil provided, and this may be the case in field populations as well. The number of eggs laid by each female is related to the quality of her diet, with those receiving a variety of fresh grasses laying significantly more e_ggs (chapter 7). Several egg-pods mc1.y be laid at

I I (rrnn) ( mm) Table 1. Mean pronotal total

and dry weight (mg) for male and female

grasshoppers of each in.star All ·,v7c:_lues

are given with standard errors and

sizes are in parantheses.

The grasshoppers were collected from site I

in November - January 1984/1985. I

M a 1 e s

Pronotal length Total length Dry weight + + - 1 1.31 ± 0.03 (5) 11.6 0.4 (5) 3.0 0.2 (5) +- +- 2 1.98 ± 0.05 (9) 15.7 0.4 (9) 5.5 0.4 (10) -+ +- +- H 3 2.55 0.04 (10) 21. 4 0.4 (10) 13.5 0.8 (10) ,.µ + � - +- +- H 4 4.06 0.05 (10) 27.0 0.4 (10) 21.l 2.2 (10) +- +- 5 5.90 ± 0.06 ( 10) 35.7 0.5 (12) 57.7 2.5 (8) + - + +- 6 6.25 0.20 (10) 39.2 - 2.3 (5) 72.3 2.6 (8)

F e m a 1 e s

Pronotal length Total length Dry weight + + + 1 1.55 0.22 (2) 11.0 1. 0 (2) 7.5 0.4 (2) + + + 2 1.95 0.05 (10) 15. 6 0 .4 ( 10) 8.7 0.4 (10)

+ + 3 3.01 0.10 (10) 23.2 - 0.7 (10) 17.1:: 1.0 (10) + + + 4.00 0.13 (10) 27.3 1.0 (10) 32.0 1.6 (10) + + 5 5.98:: 0.18 (8) 38.l 0.8 (8) 62.6 6.5 (8)

+ + 6 8.62 0.09 (5) 48.8 0.9 (5) 129.8 :: 28 .0 (.4)

+ + + 7 10.09 0.16 (10) 58.0 - 4.2 (5) 234.5 - 16.0 (_5) 13

intervals by each female in the laboratory, and sequential rises and falls in the weights of individual marked females in the field show that it occurs there as well. Because the species is univoltine and healthy eggs could not be hatched readily in the laboratory, it appears that there is an obligate diapause.

2.3 Colour patterns and colour change

A. conica shows a diverse range of colours, a deep green as well as shades of grey r brrn,vr1 and stra,,v.

Most types can be found in a single population, and of the different shades are shown in the colour plates in

Figure 1. Over the basic body colour may be laid a number of contrasting stripes and patches. Figures 2 and 3 show the areas of the body that may carry striping 1 the terminology of Bradley (1975). The principal areas of variation include the dorsal and lateral surfaces of the head, pronoturn and elytra, although less obvious occur on the antennae and the hind femurs. may be in any of the possible body colours, or also in black, white and purple. Different colours may occur within the same stripe, creating a mottled appearance Adult females have a prominent purple flash display on the dorsal surface of the abdomen, while males may have a yellow a very faint

and broken purple one or none at all

Colour in acridids may be regulated or

determined by the environ..rnent. The phase of

locusts is well knovm (Uvarov 1966) , and Figure 1. (opposite and overleaf) Colour patterns and sexual dimorphism

of A. conica.

(a) Green adult female with almost no striping. (X 1.5)

(b) Green adult female with extensive striping on the

head, pronoturn and elytra. (X 1.5)

(c) Brown adult female with extensive striping on the

head, pronoturn and elytra. (X 1.5)

(d) Copulating pair, with male above. The female is

approximately 1.5 times the male's length, and

three times heavier in dry weight. (X 1.1)

(Photographs (a) - (c) by Dr. Ron Wooller, photograph (d)

by Dr. Geoff Shaw.)

I I 1 ( a) 1 (c)

1 (d) Figure 2. Lateral view of an adult male A. conica showing the main

zones of possible colour variation and striping. The

terminology is af ter Bradley (1975).. (Original drawing

by Mike Bamford.) pronotum I

frons Figure 3. Dorsal view of an adult female A. conica showing the

abdominal startle display and the main zones of

possible colour variation and striping (original

drawing by Mr. Mike Bamford). The terminology is

after Bradley (19751. pronoturn

elytron

coloured abdomen 11------femur 14

also change colour rapidly in response to an environmental stimulus ("physiological" colour change) or show a slower

"morphological" change that involves the construction or destruction of pigment (Norris 1974). Mechanisms of morphological colour change in the genus Acrida have been controversial. In work on A. turrita Ergene (1950, 1952) argued for a response to the colour of the background, but this was disputed by Okay (1953) who suggested that humidity and food plant were the important factors. This is supported by the work of Ibrahim (1974) who implicated these factors in the control of the colour of A. pellucia. Similar mechanisms operate in A. conica, where I found that replication of

Ibrahim's rearing conditions induced green colouration under high humi�ity and a diet of maize, while low humidity and a diet of wheat led to brown grasshoppers. Colour change occurred at a moult, although green grasshoppers sometimes developed a noticeable decay in the pigment colour before moulting. Change could be made from green to brown and vice versa in grasshoppers of both sexes and all juvenile instars, although mortality under changed conditions was sometimes high.

Both Fraser-Rowell (1971) and Ibrahim (1974) note that some individuals maintain their colour under conditions which would change others of their species. This implicates an underlying genetic mechanism which controls the threshold level of colour change. The striping shown is likely to be controlled genetically as well.

Subsequent discussions of colour refer to either green or brown grasshoppers, with brown grasshoppers including_all 15

the shades of brown, yellow and straw possible. No distinction is made for striped animals. This avoids the difficulty of differentiating the fine shades within the brown category, and the many types of striping possible. Further it emphasizes the environmentally controlled aspects of colour change rather than the genetic ones.

2.4 Natural enemies

Acridids are attacked by a wide range of vertebrate and invertebrate predators, as well as parasites and parasitoids

(Norris 1974) . However, many of the parasites and pathogens noted as killing acridids in eastern Australia (Milner 1978) are far less important causes of mortality in south-western

Australia (McFadden, pers. comm.). Some of the more common predators of A. conica are shown in Figure 4. These include the Australian Magpie (Gymnorhina tibicen, race dorsalis), several species of praying mantis (subfamilies Orthoderinae and Mantinae), wolf spiders (Lycosidae) and some spinners of orb webs (Argiopidae). The relationships between A. conica and these predators are discussed in chapters 4-6.

2.5 Study sites

Although found in a wide range of habitats, A. conica is most numerous in areas where there is either permanent surface water or abundant ground water, ensuring that some of the grass cover remains green through the dry summer.

This provides adequate feed. The four study sites I used

(hereafter designated with Roman numerals) were qll

I I I ! Figure 4. Predators of A. c.onica

(a) Adult male grasshopper trapped in the web of a

spider (Argiopidae).

(b) Australian Magpie (Gymnorhina tibicen) with an

adult female grasshopper.

(Photographs by Dr. Ron Woollerl 4 (a)

4 (b) C- - - - � ------= --- -- 3 ------� -_; � ------_.:::....::::s.=-:. ------=-- - - � ------·- 7 - - - �---�---,,..-�------_;-=------� -_::- ______----� - --t ______�---'-"" .; _

16

recreational reserves within the Perth metropolitan area.

Of these, site I was adjacent to a lake, sites II and IV

close to the Canning River, and site III on the Swan River

foreshore. This gave a suitable moisture regime for the

grasshoppers. Although the sit�s were recreational reserves,

they were all areas used for walking and were not weeded,·

mowed or otherwise tended. A common feature at all sites

was marked contrasts between fresh green and dead vegetation.

These could be interspersed irregularly (sites II, III and

IV}, or in discrete zones (site I). These were presumably

caused by differences in water levels, distribution of

different species of grasses, and regrowth in areas of

bur_ning. Figure 5 illustrates some of the most striking

differences, and a list of the principal plants at each site

is given in Table 2. Most of the work reported was done at

site I, with some.supporting data collected at other sites.

The exact areas worked in varied -in size between half and

one hectare. A description of each site follows.

Site I

The ground at this site sloped gradually away from the

lake's edge up to a road approximately 100 metres from the

water. The soil was loose and sandy, and the largest trees

were scattered Banksia attenuata and Acacia saligna (Port

Jackson willow) . The main grass cover was thick Cynodon

dactylon (couch), with a few small scattered patches of bare

earth. It r�mained partly green throughout the summer,

suggesting a good availability of water. The grass was low,

very rarely exceeding 30 cm. Interspersed were emergent

· 1 17

dead annual grasses of Ehrharta sp. and Avena fatua (wild oats), reaching up to twice the height of C. dactylon.

There were also sparse plants of the low annual grass

Lagurus ovatus, and near the lake's edge dense patches of the pennywort Hydrocotyle sp.

The greenness of the grasses varied with distance from the water. There was a belt of approximately 10 - 15 metres width at the lake's edge where the grass was particularly green, and patches of Hydrocotyle sp. occured. Its boundary was marked by a narrow trail of bare earth and tramp·led grass, and above it the grass became drier and·browner in late summer, and Ehrharta and A. fatua were more abundant. As well as the lake and the road, the site was bounded by tended lawns on one side and a thicker cover of trees includi_ng B. attenuata and

Eucalyptus sp. on the other.

Site II

This was a level site of open grassland about half a kilometre from the Canning river foreshore. Unlike site I this area extended for several kilometres from east to west along the river, being bounded to north and south by the river itself and the access road, which separated it from nearby homes. The actual area of approximately orie hectare from which grasshoppers were collected was not sharply defined as was the case with site I. Further, since the distance from water was greater than at any other site, h_igher proportions of the cover were brown, and greener grasses tended to be clumped in low-lying pockets. -===-=--=--=--=--=--=--=--=--=-·-=--�------= ------:-----_;:-

18

The soil was loose and sandy, with occasional depressions

and few bare patches. It was covered thickly with tall annual

grasses (Ehrharta sp. and A. fatua) up to a height of 80 cm,

although much of this cover was dead or dying. Some green

plants of C. dactylon grew patchily in isolated areas of bare

ground and at the base of the occasional eucalypt tree. The

site has since been cleared.

Site III

This was a narrow strip of land along the Swan river

foreshore between the beach and a steep retaining bank one

to three metres high which separated the site from adjacent

playing fields. It varied between five and 20 metres in

width, and similar vegetation continued for several kilometres

in a westerly direction, while to the east it was bounded by

a marsh. The cover was nearly complete, consisting mainly

of C. dactylon 10 - 30 cm. high. Along the beach there were

clumps of Juncus (reeds) up to a metre high, and behind them

patches of Cyperace-ae (sedge) and the salt-loving Halosacia

(Chemopodiaceae) . Interspersed with the C� dactylon were

sparse plants of Ehrharta and the semi-annual sedge Scirpus.

Overall, the cover was far greener than at the other sites,

reflecting the availability of water. The retaining bank

was thickly covered with Pennisetum clandestinum (kikuyu

grass), also very green.

Site IV

This was an area of approximately half a hectare of open,

low-lying grassland close to the Canning river foreshore. It

! I -1 ------7 ------:::r::=== ,------�c------_-__ ""_--::_-- __- _::·:::::::. -___ -:..:::..--: __--:_�,-::_-·]_ �----�- �-----� ------

19

was bounded on two sides by a woodland of Eucalyptus rudis

(flooded gum) and A. saligna, on a third by marshy ground

with cover of Typha angustifolia (bullrushes), Hydrocotyle sp.,

and Melaleuca trees, and on the fourth by a steep embankment

six metres high thickly covered with the grass P. clandestinum

and the castor bean Ricinus communis. The soil was rich and

loamy, and the persistence of green grass at the height of summer suggested abundant ground water. The cover was complete, consisting mainly of C. dactylon to a height of

20 cm. There were isolated clumps of dead seeding heads

of emphemeral grasses and dicotyledons including Rumex

(dock) and occasional emergent weeds (Asteraceae). The

site was burned in late summer 1982.

All sites showed changes in the relative abundance of

the _common plants each year, presumably in response to

rainfall, temperature and water tables. This fluctuation

would be characteristic of the areas where A. conica is

abundant. -- -- -::r------.. �_Lr ------�--- -�---- �-.-_] ___ ------,-:;-cc-_�_-::-______::....J -- .�------�

Table 2. Common plants at the four study sites.

SI'rE I

Family Species

Gram:Lnae Cynodon dactylon

E:hrharta Eip.

lwena fa.tua

Lagurus ovatus

Proteaceae Banksia attenu�ta

LE:!guminosae (t-'limosaceae)

Myrt.aceae

Apiaceae Hydroco�y.�� sp.

SI'l'E II

Grarninae

Avena. fa t:.;J2

Myrtaceae

Gramin,1e , Cynodon______dactylon.. ... ,,...... , ,,...,,. Ehrharta sp.

Pr::nliisetum c lt1nd1.::s ti num

Cyperaceae

ChenopocUaceae

Juncaceae Juncus sp.

\ __------::._-- =--"':::::::r:�•�--�-----"�""'�=:---=--"------�:--c.L__•------_ -----:.--:---::-- _-:���...-'."".: �------_-_..... :.. ------�------_---- '------..------_ L_ - . - - - -::::-::·� . _ -' - ..- ..1--::...-::.:

'rable 2. Common plants at the four r,tudy sites - continued

Sl'rE IV

Species

Gr:mlinae Cyr,odon da.ctyJon

Myrtaceae F:t1calyptl1S y::udi_:':l

Mela.leuca sp.

Legurninosae (Mimo::Liceae)

�ryphaceae Typha angustifolia

Euphorbiaceae Ricinus comrnun i ::, ======��===-=-=---=--=--=-=--=--=--c.;__• �-=---�-�---�---�-- �-:-- -_-_::--__-_c:_-- ---::- �------0.:c-:;l__ i;--_---_--_-_-..-_-_-��

Figure 5. Differences in vegetation colour at site IV. The

sharp contrast between green and brown was caused

by regrowth after burning. All sites offered

contrasts in colcmr, not always as extreme.

(Photograph. by Dr. Barbara Porter.)

=--"'======�======---=--=-::::--.--.-.-.::-.·-·------

20

3. POPULATION DYNAMICS

3.1 Introduction

Survivorship functions are vital in assessing the extent

and direction of selection on a population. Although it seems

superficially that comparing the frequencies of individuals at

different ages or stages of development will give the same

answers, the differences noted could be caused by changes in

the rates of such events as hatching and development, and not

just from differential mortality. By contrast, survivorship

functions estimate mortality at each stage in the life cycle

and highl_ight different development rates, estimating precisely

the source of frequency changes. Because the timing is known

the data on mortality and development can then be related to

environmental factors.

The obvious features to study in•A� ·conica were the colour

and the extreme sexual dimorphism. However, the ability to

change colour at a moult precluded any study of colour as a

fixed influence over the whole lifespan, and the swi tchi_ng

threshold for colour change was not identifiable readily in

individual grasshoppers. Accordingly, I concentrated on

survivorship in relation to sex to detail the consequences

of the sexual dimorphism, and, if possible, the selection

pressures sustaining it. There are studies of the ecology

of sexual dimorphism in grasshoppers and other arthropods

(Schulz 1981 and included references), and Polis (1984)

suggested that different sexes and instars of arthropods

could function as ecological "species". Consequently, the

,I 21

ecology of sexual dimorphism has great significance.

Preliminary field studies in the summer of 1979/1980 suggested that the adult sex ratio was biased consistently to males, with observed sex ratios ranging from 2 : 1 to

13 : 1. This emphasized the importance of considering the survivorship function of each sex. Further data were collected on antipredator defences and the impact of particular predators (chapters 4 - 6), diet and fecundity (chapter 7) and sexual selection (chapter 8), to explain the survivorship functions observed and their significance.

3.2 Materials and methods

Techniques for estimating population parameters

Because of the impracticability of total counts in nearly all population studies, investigations of population dynamics usually resort to either mark-release-recapture programmes

(MRR), sequential random sampling, or to methods based on catch per unit effort. Each of these broad types of techniques has its own special advantages and limitations, and consequently Southwood (1978) recommends using more than one method simulataneously whenever possible.

The basic principle of MRR models is that animals are caught, tagged and returned to the population. Subsequent samples are taken and the relative proportions of marked and unmarked individuals in them are used to estimate the total population size, as well as such factors as mortality and migration. A wide range of statistical models is available ======�======�=- ===-=------==-:c_c--c=:__c•-=- --=•-=--=----=---=••·SCI =-=c==== �=�-�--~--~-�=�-c------�----==------=-===--�---_J______J-'.:__,

22

for interpreting the data, and they are reviewed by Southwood

(1978) and Began (1979). Potentially, they are powerful

techniques because the data on population size, mortality

and birth they supply are vital in demographic work and

st�dies of selection. Further, all models except the

elementary Lincoln Index use data from successive samples

to refine the accuracy of the prediction, making maximum

use of information. Their major disadvantage is that each

model has its own set of assumptions which are often difficult

to meet in real biological systems. For example, Southwood

(1978} outlines a series of assumptions common to MRR models�

Excluding those unique to a single model, they are :

marked animals are not cha_nged in behaviour or

longevity, and marks are neither gained nor lost;

marked animals mix at random with others in the

populations;

sampling is at discrete intervals, with sampling

time small in relation to interval time;

all individuals are equally catchablei

sequential capture does not affect catchability;

every marked individual has the same probability

of survival.

The biol_ogy of different animal groups and even different

species can tend to violate particular assumptions. For

example, the durability of metal bands as individual markers

for birds makes them more reliable than the enamel or oil - ---� ------_-_- . --:::c:::::: ! - - - •

23

paints used frequently to tag insects (Tanner 1978).

Against·this are the disadvantages that birds may become

net-shy after being caught once (Lovejoy 1974), while

Pollock (1981) and Nicholls et al. (1982) highlight small

but consistent biases associated with common methods of

bird trapping and patterns of band wear. While these

problems are predictable, others are harder to foresee.

For example, some insects show increased movement after

capture and release (Greenslade 1964) while others hardly

move at all (Edwards 1961), and sex and age-related

differences in movement rates and catchabilities occur in many animal species (Furnell and Schweinsburg 1984). further, the act of marking can itself change catchability

(Fairley 1982, Readshaw 1982) .

Statistical problems occur with analysis of MRR data as well. Despite the wide applications of the jolly-Seber model, computer simulations reported by Roff (1973) showed that the original formulae used to ,determine the variances of population parameters were unreliable for calculating the confidence limits on real parameter values. Most disturbing was the observation that underestimates of parameters and their variances were correlated, so that if the parameter estimate was low its variance would be low as well. Consequently, even the upper confidence limit of a parameter estimate could still be below the real value. Manly (1984) noted that the problem is caused by the use of estimates of unknown parameters in calculating variance estimates, and suggest�d that 24

transformation of estimates so that the variances of the transformed values were independent of the parameters would solve the problem. Using his recommended trans- formations still gives symmetrical confidence limits to estimates of survivorship, but population size estimates will have asymmetrical limits. Alternatives to the transformation approach include the simulation method suggested by Buckland (1980) and an application of the jackknife method used by Manly (1977), but transformations remain simpler to use. The continued development in this area highlights both the potential and the problems of the

MRR statistical analyses.

One approach to these problems has been to test each assumption of a MRR method in turn, and seeking an alternative or treati_ng results with caution if an assumption is violated.

Southwood (1978} discusses some statistical and sampli_ng procedures for testi_ng assumptions, and their interpretation.

Began (_1979, 1983) takes a very stro_ng attitude supporti_ng r_igorous testi_ng of assumptions, arguing that failure to do so is abuse of the methodol_ogy and makes results at least suspect and possibly misleading. However, his implication that a large proportion of workers are either unaware of the assumptions or fail to test them may be exaggerated, as researchers who found the assumptions valid may have felt it unnecessary to refer to them in publishing their results. While agreeing on the importance of testing assumptions, others including Tanner (1978), Blower et al.

(1981) and Montgomery (1985) concede that validation of 25

all of them is a form of biological utopia. Instead, they

emp�asize choosing the most appropriate analysis available

for the biology of the animal studied, and noting the likely consequences of any violated assumption. These

may include directional movements of a parameter estimate such as the depression of population size estimates that result from unequal catchabilities (Southwood 1978), or

an increase in the variance of an estimate without moving

it directionally (Bradley 1975). Often, new simulations

reveal further possible problems. For example, Pollock

and Otto (_1983) point out that if many individuals in a

population have a low capture probability, estimates of

population size will have a severe negative bias.

Nevertheless, with careful sampling design to minimize problems of differential behaviour of population sub-groups

(White 1975), a practical approach to assumptions can broaden

the applicability of MRR models. An example of this is

the work of Carothers (1973a,b, 1979} on the problem bf

catchability in MRR models. He suggested that equal

catchability was an unattainable ideal because of the

chances of trap-shyness or trap-addiction, and because

the probability of capture varied among individuals

irrespective of previous capture history. However, in

a series of simulations he found that the biases on

estimates of survivorships were negligible, although there

was a considerable amount of heterogeneity. Jolly and

Dickson (1982) agree with these results, although they

emphasize that there are still problems in estimating ----======----=-=· -=-=-=--=· ·=·-=--�- =--=··''-'-'---"'-0-·-=--s_-1==--=--=--=--=--=--=- -=-=--=--======-ccc:--=--=-�==r:e--,7=:::,,.-,-_ =-="===

26

population size when catchabilities are unequal. In a

case where survivorships rather than population sizes are

being sought, small variations from the ideal of equal

catchability appear unimportant. This illustrates the

degree of robustness given by MRR models. Montgomery

(1985) summarized this perspective neatly by writing :

"The effective use of MRR depends not so much on satisfying

the full requirements of the MRR model but on the objectives

of the study".

However, the trend in development has been towards

more general models with fewer assumptions (Bradley 1985).

For example, Jolly (1981, 1982) points out that the standard

Jolly-Seber technique in widespread use is easy to apply,

but requires estimation of many parameters. If some of

these such as survival rate and probability of capture

can be assumed constant over the study, models with a

reduced number of parameters are applicable and desirable.

To some extent this situation is under the researcher's

control, because probability of capture is a product of

the sampling method used. However, survivorship functions

are a biol_ogical characteristic and cannot be adjusted for

the purposes of the study.

As an alternative to MRR, techniques of sequential

random sampli_ng are free of assumptions relati_ng to marking,

altho_ugh they make others relating to sampli_ng techniques.

Further, they do not ·combine data from several successive

samples in calculating estimates of density and survivor­

ship, and therefore may not use data as efficiently as 27

MRR models. Nevertheless, they often allow coverage of a greater sample area and the data convert readily to life tables. Technically, they involve counting all the animals in a unit of the habitat, and estimating how many such units the population occupies. Comparisons of the density estimate for animals of different ages (or instars if arthropods are studied) are used to calculate survivor- ships. A number of units should be sampled, usually more than 30, and they should be selected at random. Complete counting of the animals within each unit is essential_, and escapes or failure to count animals will bias the results.

Several models are available for analysis, and they are reviewed by Birley (1977), Southwood (1978) and Bradley

(1985) . Although they make fewer assumptions than MRR models, they can still be restrictive. For example, early models assumed that survivorship followed a negative exponential function, with all individuals passing £ram stage to stage in synchrony. Further, it was assumed that each stage, age-class or instar was sampled with equal efficiency. Synchronous development clearly does not apply to many species, and differences in size and behaviour of different stages necessitate very careful sampling to ensure that all stages are equally likely to be caught, and to be found and counted in each sampling unit.

These difficulties were highlighted by Birley (1977), who added the further problem that the models assumed constant survivorship within each instar. He pointed ------:r:=-= J.'.'--:::....--_ , ------�------_-_-_- --=-c:=:==r;;,'-::":'-�- ;";e-:======-�-::----...:- :-_------�..:----.:.------��----::-----'--_---::----1 ------I

28

out that this was only one of three likely possibilities,

the others being that freshly-moulted animals had higher

mortality because of a softened cuticle, or that they had

lower mortality because they were concealed and inactive

during moulting. In either case the assumption would be

violated, and the estimate calculated questionable. To

avoid this, Birley's model treats each instar data set

separately from all others, allowing consideration of

survivorship functions within each instar. Further, by

avoiding comparisons between instars in calculating

survivorships it sidesteps assumptions of equal catchability

of different instars. However, the method does need

additional data on the nature of the recruitment distribution,

and analyses where these are unavailable are described as

"speculative". Consequently, experimental field trials need

to be run prior to or concurrent with the_ sampling programme.

Some line transect techniques used to collect information

for population esttmates (Onsager 1977, Onsager and Henry

1977, Eberhart 1978) are adaptabie to collect data for

sequential random sampling analysis, as well as information

on the distribution of animals. Applications of sequential

random sampling models to grasshopper populations are given

in Ashford et al� (1970) and Bradley (1975, 1985).

A final possibility for estimating population size is

to apply models working on a basis of catch made per unit

effort. These are most popular in fisheries biology

because of their ready applicability to large aquatic

populations not assessable by MRR or line transect 29

techniques, and several models are reviewed by Roff (1983) and Dupont (1983). In reality they are limited because

data often come from a wide variety of sources, although

some of the more recent models can be used to predict

future population sizes, and are free of restrictive

assumptions concerning birth rates and juvenile survivor­

ships.

For this study I chose a MRR approach to gain

maximum return from an extended period of sequential

sampling over a summer. One ·analysis. was done using

a new.model (McKay and Bradley, unpuplished) which

fits Read and Ashford direct count life tables to ..

MRR data using a nOJ:?--linear least squares fit. This

uses da.ta maximally. It is described in detail in the

section on creation of life tables. For comparison, a

second analysis based on a sequential random sampling

model (Read and Ashford 1968) was made as well.

Preliminary sampling in 1979/1980 and tests ot assumptions

The final sampling design was adopted on the basis of results of a preliminary survey at site I in 1979/1980.

All subsequent work was done there. Initial concerns were the establishment of a suitable sampling programme and the checki_ng of the validity of a number of assumptions of the

Jolly-Seber MR.R model against it. I searched the site for newly emerged grasshoppers weekly from mid-October, and began sampling after finding the first hatchlings in late November. Initially, sampling involved marking 30

2 square plots 9 m in area and collecting grasshoppers from these over a period of 15 minutes each. Sequential samples were taken every third day.

However, recapture rates were low (5% - 1 2%), suggest� ing that grasshoppers were dying as a result of marking, moving out of the plots, or developing so rapidly that most had moulted and lost their marks before they were recaptured.

Marked animals did not show significantly greater mortality than unmarked controls in the laboratory (Table 3), so the

2 sampling plots were increased to 16 m in area and search times extended to 30 minutes each, which increased recapture

2 ratesi' to at least- 25%. .A further increas:e in area to 2 Sm was needed for grasshoppers in the fourth instar and older,

2 and a greater area of 1110 m was used for the adults.

Sampli_ng began between 0930 and 1000 hours each day so grasshoppers could become active, and the order of sampling plots was determined randomly each time in case it was an important variable. All plots were sampled before grasshoppers were marked. and released, with the time between capture and release varying between two and three hours. All grasshoppers from a plot were released in its centre. For each grasshopper caught its sex, instar, colour and individual number were recorded.

Grasshoppers were marked individually using enamel modelling paints. This is a widespread approach in insect population studies, and many examples are discussed by

Southwood (1978). Some general considerations he notes are : = ---=====-====cc=-=--=--=--=-=- -=•---"---c-·-�·-.cC-··-=-=-' ---=---=-•1 =--=--=--=--=-=--=--=:_c-_c:======- -- -�c::=-:-=:-:-'C-C--'."-".:-Lc::_-_c_-_·_-_:-�-:-:-:·c..:·..:-:-:.;i_=-::;

31

the effort of individual marking is usually only

justified when recapture rates are high;

missing marks should be recogniza�le as such and

not lead to assigning a recapture a mistaken number;

the sites of marking should not interfere with either

crypsis or display, nor be subject to loss during

normal cleaning;

methods reducing handling time are preferable.

Recapture rates with A. conica justified individual marking,

and the grasshoppers were tagged in three places on the

ventral surface, as shown in Figure 6. The ventral surface

of the thorax indicated the hundreds mark, the anterior

ventral surface of the abdomen the tens and the posterior

ventral surface the units. Use of the ventral surface

would not have altered the cryptic patterns of the grass-

hoppers. By using 10 colours each representing one digit,_

individual marks between 0 and 999 inclusive could be made.

Although the use of such a range of colours might appear

clumsy, it was in reality fast since no more than three

colours ever had to be applied to a single animal. Fine

paint brushes of size 000 were used, and two marks were

made side by side on the same spot as a precaution against

loss of marks. After marking, grasshoppers were lightly

breathed on to dry the paint before transferring to a

gauze holding-cage prior to release. The paint dried

quickly and adhered well if applied thinly, but blobs

of paint flaked readily when dry, or stuck small grasshoppers ,_ _ --::-. ------�� ;_-:.,:- -_�---��:.:::!:.�:::." -:. :::��-�: :: - --·_�---- =-:.---er=-= - -c:c: --c:c:c:c:. ---� -=------=--:c:c:--c:c::--c:c::--:c::--=-II ----- :---.-_.-::---=---==::- � - ; _ -c · . -� - =-=--======-=- ======-==-=-=---.:-::_-_-..::-.,:-_-:-_-:�-----�:------_-- -:--_- _� -

0

Thorax

0 00

000 Ab domen 000

00

Figure 6., .Scheme for marking A. conica. Marks were

applied on the ventral surfaces of the

thorax and abdomen using combinations of

10 paint colours. Each digit was shown

by two marks, so that if one was lost the

grasshopper was still identifiable. The

unit spots are shown (0}, the ten spots

(0 0) arid the hundred spots (0 0-0}. Ten

different colours coded digits from zero

to nine. r:::::::=::::,______�---- �.-.,-_-_ ------� "·--- -.- -��- - - =.r::--::.-.--.-- �- _,.._,.-•.•:·- •-c._.• -.---·---·.---.:_c.-�l ------

32

to the cage or to grass blades. Gloss enamel was always

used in preference to matt and metallic colours which

flaked readily. The number of marks possible and their

ease and speed of application and reading made painting

preferable to a system of mutilation marking (Gangwere

et al. 1964).

The effect of marking on the mortality of grasshoppers 2 was determined by x analysis (Blower et al. 1981) for

adults and second instar individuals, thus covering the

size range of animals marked. Marking had no significant

effect on mortality (Table 4). On the basis of this

evidence and the similar survival rates of marked and

unmarked grasshoppers in the laboratory (Table 3), marking

does not alter grasshopper mortality. Loss of marks was

a very small problem in all years. For example, in 1981/

1982 86.7% of a total of 476 recaptures had not lost any

of their six marks, 7.6% had lost one mark, 5.6% had lost

two marks and only one had lost three marks. It was most

unlikely that a recapture would have lost all six marks,

so the main problem was loss of two marks at one point

leavi_ng an individual's identity in doubt. These losses

were rare, and in all cases identity could be confirmed

by checking previous records.

As well as assuming that marking has no effect on

the mortality and catchability of animals and that marks

are not lost, the Jolly MRR model assumes that marked

individuals mix at random with others in the population.

The need to enlarge the size of the plots sampled with __ , -· ._·------,======:·------·-:-�------·..... -�.- -·-.·----, ·------·-·--

33

later inst�rs gave some evidence of grasshopper movements,

but distribution of marked animals was checked further

2 with x tests (Begen 1979, Blower et al. 1981). During

the first week of December 1979 and again in January 1980,

two of 'the four plots being sampled wex-e divided i.nto

four equal subplots and the proportions of recapture in

2 each of them assessed using x analysis. Of the four

tests' done, none showed an a,ssociation betw�en

recaptures and subplots, suggesting that random mixing

usually occurred (Table 5).

In a detailed study of the suitability of populations

of Alpine grasshoppers for MRR analysis, White (1975) found

that population subgroups varied in terms of catchability

and mixing, and had to be treated separately in calculations.

Consequently, the above analysis of mixing can be criticized

because it does not consider the subgroups of sex, morph

and instar, which could have quite different distribution

characteristics. However, allowing for these would have 2 reduced the numbers in each cell of the x contingency

tables to the point of being meaningless. Accordingly,

the sexes and instars were always treated separately in

MRR analysis. The factor of morph colour was ignored,

because consideration of the full range of possibilities

would have created a large number of small and unwieldy

groups. The use of separate analyses for different

sexes and instars also corrected for differential catch�

ability, which is certainly a problem over the. wide size

range of grasshoppers handled. A final assumption of

1.·.:_-_ I I - �.:---..:�------.-----:.�-:·�---=--=--= -=--c::c---:c:c:--:c:c:--:c:c::-=====i--=--= =- �-- -=--=--=--=--=--=-=-=--=-===--:::::- -:::::- -�-'Z--�--�--�='Z'Z'Z�:::I- :=::.1::::--2--::::--2-2---c:c:-:c:c:--·�-::_:-·=--_:::::_- --_:_:c--- -cc:::---�-:::::-·.- --::-:---z-:!'"_-3_-=!-=---:�------

34

MRR that sampling be at discrete intervals and small in

proportion to interval time was met by sampling every

third day.

A substantial reference collection of grasshoppers

of both sexes and all instars was made during the summer,

and using it I became proficient at recognizing the instars

and sexing the youngest grasshoppers. Misidentification

was not a problem in later years.

Sampling in 1980/1981 and 198 2/1983

The procedure used was that devised during the

preliminary exercise in 1979/1980. I began searching

for grasshoppers weekly in October, and sampling began

when the first hatchlings were found in mid-November

each year. The arrangement of the sampling plots is - - 2 shown in Figure 7. Four square plots each of 16 m

were used for the first three instars, and expanded to 2 25 m for older juveniles. The plots were positioned

so that the larger plots shared common boundaries,

2 forming a square of 100 m , excluding a narrow trail

approximately 75 cm wide separating two of the plots

from the others.

This meant that the total perimeter of the smaller

plots used for the first three instars was larger in

proportion to their area, although the possibility of

"edge effects" from increased immigration and emigration

would be offset by the reduced movements of the younger

grasshoppers. Further, the gaps between plots prevented ----: · -,Cf=· -:c:c:--:c:c:--�--�--:C::C--�--::C:C-:c'::c--c:=--::=-======-=--=--=:c:---=--c::=::-=1======�- =�-=-�·.:-=·.:.:.-_--:_·::-.-:-·:·••-c -_,------• ----,-----�f � ------C:::C--C:::C·-�--::c:c--::c:c--==:c--=-=:-:=---=:--c:::----c::: =====

0 1 L--J lm

------1- - ·,

I I I ii ------___ t' 1---- - �-L -� _,_ � l

L

Figure 7. Schematic drawing of the layout of the sampling plots.

The smaller plots were used for sampling the first

three instars, and the larger ones for older

grasshoppers. ---_--.,..:l . ------.!

35

sampling in one from disturbing grasshoppers in adjacent plots. With a larger ratio of area to perimeter the four combined large plots would not have marked problems from edge effects. Two of the plots were approximately

4 m from the lake's edge in the zone of greener vegetation, while the adjoining ones above the walk trail were in drier grass. This represented the two broad types of vegetation available to the grasshoppers. The problem of heterogeneity of vegetation types was insurmountable in areas where the grasshoppers were abundant, and the sampling des_ign compromised the need for a larger area with minimal edge effects and the possibility of having more smaller plots each with uniform vegetation. The sampli_ng and marki_ng procedures followed those described for 1979/1980. Sampling was repeated every third day and continued from first emergence of juveniles until late January, when fewer than ten were bei_ng caught on each sample.

A different sampling schedule for adults began with the first sample in January each year, and for seven samples was concurrent with sampling for juveniles.

Thereafter only adults were collected. The two large areas used for adult samplying consisted of two vegetation 2 zones, one of 370 m adjacent to the lake and the other of 2 740 m beyond the walk trail that marked the boundary between the_ greener and the drier vegetation. The zones were rectangular with the long sides of 35 m parallel to the lake. They were searched for a total of one hour 36

and grasshoppers caught with a net and removed. Captured grasshoppers were given individual marks, their mark, colour and sex recorded, and released in areas of 2m x 2m in the. centre. of -their sampli_ng zone after the whole

· sample had been processed.

Creation of life tables

Life history parameters were estimated using the maximum likelihood model proposed by Read and Ashford

(1968), and secondly using a technique developed by

McKay and Bradley (unpublished} for fitting Read and

Ashford life table models to MRR data. The basic Read and Ashford models estimate mortality in each s�age of the life history, as well as the duration of the stages.

The models predict the density of each stage at any period during the sampling programme, and thus the parameters can be estimated by optimizing some function which describes the goodness-of-fit of the observed densities and those predicted by the model. In the case of random quadratting, this is a maximum likelihood function.

The structure of the models is sufficiently flexible for them to be modified to suit the biology of specific organisms, and such modified models have been used successfully with grasshoppers by Ashford et al. (1970) and Bradley (1975, 1985). A further

advantage is that no independent �stimates of parameters such as hatching rates are required. The mathematics �----=-..=------"'��---·------,·---.------·I ------•I ------I

37

involved in the solution of the maximum likelihood functions when data are provided from random quadrats is discussed in detail by Bradley (1985). Overall, the models describe population dynamics in terms of a mean individual. The mean time this individual spends in each stage, its times of entry into the stage and leaving it by moulting, and its probability of death within the stage are all determined.

In common with other population models, those based on maximum likelihood functions make a number of assumptions about the biology of the species under study. These are :

counting of animals is complete during sequential

random sampling and not biased towards any stage;

the site is uniform;

animals are distributed independently;

destructive sampling gives a negligible decline

in numbers;

within an instar death is independent of time;

hatchi_ng, moulti_ng and senescent death are

described adequately by Erlangian functions. --j

38

The first assumption represents a major problem in the application of the maximum likelihood model. The quadrats were not cleared completely on each sampling occasion, nor were the catchabilities for each stage of the life cycle equal. Although these problems are corrected partly by treating each sex and instar set separately in analysis, they preclude any other than general assessments of the trends observed in the data.

Uniformity of site and grasshopper distribution are likely to be related. Neither ��tion was true as distinct vegetation types occurred and grasshopper distributions were aggregated (see chapter 4). Bradley

(1985) found similar problems in a study of some U.K. acridids, but argues that aggregated distributions would increase the variances of population parameters rather than shift the parameters themselves directionally. Since destructive sampling was not used assumption four presents no problems.

Birley (1977) showed clearly that the assumption of equal probability of death throughout an instar was questionable because of changed behaviour and vulnerability at moulting. However, the time period of several hours covering moulting is small compared to the total duration of an instar, and suggests that any deviation in mortality is likely to be unimportant. Possibly the increased vulnerability caused by the soft cuticle is balanced by hiding behaviour so that mortality is still similar to that of active individuals. The fit of Erlangian functions

'! j I - -_ ------�[ -:--�--.

39

to hatching and moulting distributions of acridids was studied in detail by Bradley (1985), using both his own data on U.K. acridiqs and those from other sources. He found a third order Erlangian function to fit the distributions best, and those functions were used in this analysis.

Application of the model gives these parameters for constructing life tables :

N the actual number of individuals hatching t 0 the date at which the hatch begins

\0 the rate parameter of the hatch•

the rate parameter of moulting in each juvenile instar I AI

the death rate in each juvenile instar I µ1

the rate parameter of "senescent death" in the adult AA

the rate parameter of "random death" in the adult. µA

Rate parameters have no biological meaning, but are used to calculate the mean duration of each stage. Death rates can be used either as the probability of individuals dying in a stage, or alternatively as the proportion of grass- hoppers dying in a stage. Separate life tables were made for both males and females in the seasons 1981/1982 and

1982/1983.

Because of the potential problems caused by violation of some of the underlying assumptions of the maximum likelihood models, a second analysis was made using the model of McKay and Bradley (unpublished). This fits - - - -J � -

40

Read and Ashford life table models :to MRR data using a

non-linear weighted least squares fit, and this was

applied to my data. Population densities at each stage

were estimated at each sampling period using the Jolly­

Seber MRR model with Bailey's correction applied. The

Read and Ashford (1968) third order Erlangian model was

then fitted to the data by the process of non-·linear

weighted least ·squares cptimization (Gill and Murray 1978) ,

using routine E04FDF of the NAG algorithm library (NAG

1975}. This routine optimizes a weighted least squares

function, and in this case the weights used were the

reciprocal square roots of the estimated variances of

population size produced by the t'iRR model. This allows

the differences in the sampling intensity on the different

instars to be corrected by application of MRR techniques.

Although neither the Read and Ashford model nor the McKay­

Bradley approach is a perfect solution because of the

_ difficulty of fitting all the assumptions, by using them

together general trends in the survivorship curves for

both sexes should be clear.

In both analyses the survivorships for adults were

determined using Jolly-Seber estimates with Bailey's

correction. This was because adults leave that stage

of the life history only by death or migration, without

the added complication-of moulting.

3.3 Results

Tests of some assumptions of MRR techniques are

reported in Tables 3 - 5. 'I·hese show that marking does ======- e=c-=--=_..c._-____= - . -.. ------

41

not increase the mortality of grasshoppers kept in the

laboratory (Table 3), nor does bearing a mark change

the survivorship of adults in the field (Table 4)-. Tests

of random mixing of marked individuals show that this

occurs in nearly all cases (Table 5).

Catchabilities of grasshoppers in each sex and instar

for both seasons are shown in Table G, together with the

mean number of times i1dividuals in each group were

handled. Catchabili ties were calc1.1lated by dividing the

number of days grasshoppers were handled by the number of

days they were known to be at risk because they were taken

on days both-before and after a capture. Values ranged

from 0.12 (adult females in 1980/1981) to 0.70 (fifth

instar females in 1981/1982). In each season they were

lowest for first instar and adult grasshoppers, and highest

for fifth instar females. For a given instar males had

slightly higher catchabilities than females, exceptions

being the fifth instar in 1981/1982 (0.70 for females to

0.54 for males), the first instar in 1980/1981 (0.33 to

0.25) and the fifth instar in 1980/1981 (0.64 to 0.58).

The mean number of times individuals were handled

ranged from 1. 08 (first instar males in 1981/1982) to

2.37 (fourth instar males in 1980/1981). Overall, older

grasshoppers were handled more often, although adults

were handled less frequently than the last juvenile

instar. The total numbers af grasshoppers caught for

each sex and instar in each season a1�e shown in· Ap1-1e11di�x I ..

Survivorship curves for each sex calculated using

I - ) \"------J.

42

each method of analysis are shown in F'igures 8 and 9,

together with the duration of each instar. Variances . , for the McKay-Bradley MRR technique were calculated by application of NAG library routine E04YCE (NAG 1975), which calculates variances appropriate to weighted: least square fits. Variances can also be estimated for both

the survivorship and the duration of each stage in the

Read and Ashford direct .__;aunt model. However, they are su�pect because the factors used in their calculation are dependent and rely upon the grasshoppe!s being distributed randomly, which is untrue for A. conica (see chapter 4).

Accordingly, variances are not shown in these figures.

Using the Read and Ashford direct count life tables, the survivorships of females were less than males in the juvenile ·stages in both seasons. In 1980/1981 20% of females reached adulthood compared to 28% of males, and the figures in 1981/1982 were 5% and 8% respectively.

Assuming an initial sex ratio of unity, these values predict an adult sex ratio of approximately two to one in each season. These are reflected in the overall sex ratios in the samples, which rose from near unity to substantial male�biased ratios after 60 days. The total numbers of adults handled show a ratio of five males to each female in 1980/198l, and this result was repeated in 1981/1982. The most·marked differences in survival rates between the sexes were in the fourth instar in

1980/1981 and the fifth instar in 1981/1982. Despite the extra instar in the female the periods from hatching

! I - ! I - 43

to maturation were similar for both sexes (SO days for

males and 53 days for fcmaJ..es in 1980/1981, and 25 days

_and 37 days respectively .in 1981/1982) . This was caused

partly by rapid female development, and by protract;ed

instars in the males, the third in 1980/1981 and the

fourth in 1981/1982. These variations in survivorship

from instar to instar and in instarduration stop the

survivorship curves corresponding exactly to any of tl1e

generalized types descibea. in ecology texts (:Krebs 1978,

for example), although they approximate a situation where constant nLL.rn.bers Of individualS die per unj_t time.

Similar trends are shmm in the survivorship cur,.res

based on t:.he McKay-Bradley technique, based on MRP.. In

both seasons male maturation preceded that of the females,

the timespans being 84 days for males and 92 days for

females in 1980/1981, and 53 days and 68. days respectively

in 1981/1982. Al though the McKay-Bradley approach

predic;:ted longer development times than the direct count

method, bow¾ models indicate faster development of

grasshoppers in 19Bl/1982. Once again 1 10ale. in.star

duration lengthened i!-lith maturation in both ve:1rs.

Female instars were of more even duration. :?ema.le

survivorships 1,vere less than male survivorshi!)S in

1981/1982, with 2% of females maturing co�r2� 0 d to

37"o of males. Eowever, in 1980/1981 a ::ceve:rse resu1t

showed 2G?o of females and 45'., of males maturing.

conflicts with a predicted adu2-t sex ratic, of fi,;_;•e na.J"-s

to every £err.ale predicted for J. 98O/1081 frcr::. the Jo 2. J \.

\"_- �------__-_- _ __-_- ----·I

Table 3. Mortalities of marked and unmarked

second and fifth instar grasshoppers

over a period of three days in the

laboratory.

No. of grasshoppers No. alive after three days

Marked 20 14

Second Instar

Control 22 16

Marked 19 19

Fifth Instar

Control 20 19 Table 4-. Chi-squared test of the influence of marking on

the loss of grasshoppers, using data collected

in 1979/1980. The tables test for association

between recapture rates and the actual marking

of grasshoppers, at a 5% level of significance.

Adult males and second instar grasshoppers of

both sexes were used to cover the size range of

gras-sh.oppers- marked, and these groups were large

enough for analysis. -- ""'""' _s::r:== ------· �- -l ------,1 -- ---c-_ ------j

Adult Males ·

Number not Number

recaptured recaptured

Rate after 47 74 first capture

Rate after first 23 45 recapture

2 x 0.470, n.s.

Second Instar

Number not Number

recaptured recaptured

Rate after 126 49 first capture

Rate after first 43 recapture

2 x = 3.175, n.s. - ,,.. ______--�.- - ---

Table 5. Tests for random distribution of marked grasshoppers.

Each test shows the numbers of marked grasshoppers

caught in four equal subdivisions of a 5 m x 5 m quadrant.

Total

Observed 6 7 4 4 21 Test 1 Expected 5.2 5.2 5.2 5.2

2 X = 1.3, n.s.

Observed 13 8 7 12 40 Test 2 Expected 10 10 10 10 2 x = 2.6, n.s.

Observed 11 6 5 25 Test 3 Expected 6.2 6.2 6.2 6.2

2 x = 5.60, n.s.

Observed 8 12 7 5 32 Test 4 Expected 8 8 8 8 2 x = 3.25, n.s. Table 6. Catchability ?f grasshoppers. For grasshoppers with a multiple capture history the number of

times they were hand led (Ba,ndlesl was divided by

the number of days they were known to be at risk

because they were captured both before and after

(Days at risk) . This gave the catchability.

The number of individuals used in the calculations

(N} is smaller than the total number of captures

for each sex and instar because not all grasshoppers

had a suitable capture history for inclusion. The

female sixth instar has no juvenile equivalent in

the male, and insufficient first instar grasshoppers

in 1981/1982 had a suitable capture history for

analysis. This is the approach of Ehrlich et al.

(1984)

1::"-:----- = ------·- --- -�=··c..c..c·-=·--=--=- --.:c::c- --::e::,--CL·I=-=-=·-=-=-=--=-=-- = ==- =-- -=-====--- �- -=====-c:=::-C:C::--::::::C-�1======2-:C::C-::c::c:.�- �::=:- -::• -_- -·••-,--•_-,---,-:,1_

1980/1981

Instar Males Females

Days at Handles Catchability N Days at Handles Catchability risk risk

1 4 1 0.25 2 6 2 0.33 4

2 28 16 0.57 22 20 8 0.40 12

3 90 39 0.43 33 270 103 0.38 80

4 151 81 0.54 47 77 42 0.54 27

5 100 58 0.58 44 139 89 0.64 67

6 62 26 0.42 32

Adults 312 55 0.17 50 97 12 0.12 28

1981/1982

Instar Males Females

Days at Handles Catchability N Days at Handles Catchability N risk risk

1

2 10 4 0.40 8 32 9 0.28 16

3 28 16 0.57 22 27 7 0.26 16

4 20 11 0.55 26 31 11 0.35 18

5 48 26 0.54 29 23 16 0.70 17

6 5 2 0.40 5 4 Adults 116 21 0.18 45 6 1 0.17

·1 ------�-- - .c-=-======::-o======-=�= J ·------�-�_-:_ __-_ -_-_ -::·------�------�=------:.-_--

Figure 8. Survivorship curves for grasshoppers based

on the Read and Ashford maximum likelihood

models for 1980/1981 (above) and 1981/1982

(below). Survivorships (log) are plotted

on the vertical axis against duration of

instars on the horizontal axis. Standard

errors are not shown because the factors

used in their calculation are dependent,

making the results questionable. 1.0

0.8

0.6

0.5

0.4

0.3

0.2

�-

P-! .,., �

0 20 40 60 :> .,.,:>

tJ'I 0 1.00

0.50

0.30

0.20

0.10

0.05

0.03

0.02

15 30 '45 60

Days Since Sampling Began �=:�-���::::-=�_,c�----" ------,ec--_-____- ,ce, ==••======-=--= -=--=·-=-=- =

Figure 9. Survivorship curves for grasshoppers based

on mark-release-recapture analysis for

1980/1981 (above)and 1981/1982 (below).

Survivorships (log) are plotted on the

vertical axis against duration of instars

on the horizontal axis, and all values

are given with standard errors. - ______� _-� -,c _-_ _;--_:-_J ___r;:-:.--"'--=-=--�-----_:-_� ---::

J..00

0.50

0 . .30

0. 20

0.10

0.05

0.03

0.02

30 70 110 lS!J

1.00

0.50

0.30

0 .. 20

0.10

0.05

0.03

0.02

-- _6T _i= 30 70 110 150

. Days Since Sampling BPgan-­

- 1- 44

MRR analysis of the adults. Finally, �arked differences

in survival rates between the sexes occurred in the first three instars in 1980/1981, in contrast to the predictions of the direct count models which show similar survivorships at these qges.

3.4 Discussion

The principal problem in applying either method of analysis to the A. conica population was the unequal catchabilities, which meant that sampling effort could have been distributed unequally across the instars and between the sexes. However, this imbalance was more apparent than real. Sex differences were overcome by analysing each sex separately, and the synchronized development of the grass­

hoppers restricted the range of instars present at each sample. Morph-related differences in catchability were

unlikely because grasshoppers were caught when they moved,

minimizing crypsis. Similar arguments justify MRR analysis,

since the basic assumptions excepting equal catchability

were met. Further, estimates of survivorships using MRR

are robust with respect to catchability (Carothers 1979)

and survivorships were more important than population sizes

in this study. Although the Read and Ashford direct count

survivorship curves are most likely to be biased by this

problem, they nevertheless share many features in common

with those predicted by the McKay-Eradley MRR model, based

on MRR. For instance /' both models show declines in

survivorship in 1981/1982 relative to 1980/1981, and they

both predict male-biased sex ratios in each season. The ____--.-_,:.,::---_- ___ ------

45

differe?ces are likely to result from the influence of

·unequal catchabilities, especially·on the Read and

Ashford direct count method.

The most striking feature of the survivorship curves is the lower survivorship of females, leading to an unequal sex ratio in the adult populations in each season.

The anomalous result given by the McKay-Bradley technique in 1980/1981 is clearly ,n artifact of the analysis, since the Jolly MRR analysis.for. adults showed a male- biased sex ratio of•S : 1. Consequently, the trend of higher male survivorships showed in the first four

instars would be correct, and the crash in male survivor-

ships in the fifth instar untrue. Comparing male and

fem.a.le survivorships more closely, particular female

instars appear to suffer disproportionate mortality.

The Read and Ashford direct count model shows these to

be the fourth and fifth instars each year, while the

McKa:Y-Bradley method shows the early female instars to·

suffer in 1981/1982, and the fourth and fifth in 1980/

1981. Since females are heavier than males of a given

instar, the differential survivorship may be caused by

size:_related mortality such as the size selection of

prey by predators.

Of course, the extent to which the different survivor­

ships produce a disparate adult sex ratio depend_s on the

sex ratio at hatching. This is difficult to determine

accurately from either model because of the small numbers

of first instar grasshoppers caught, which caused high 46

variances on estimates of initial population numbers.

Small numbers of adult females cau·sed a similar problem in analysing the adults, al though numbers of· males were' adequate. Howe·ver, without clear contrary evidence there. is no reason to doubt an initial sex ratio of unity, or to reject the survivorship estimates of unequal sex ratios biased to males at maturity. Ehrlich et al. (1984) suggest three other explanations for disparate sex ratios in populations where the. initial.sex ratios in each generation are 1:1. These are differences in the relative catchabilities of adults of both sexes, differential migration of adults, and unequal adult mortality.

In A. conica the relative catchabilities of the adult males and females were close in both seasons, and would not have been a factor. However, the low values for adult catchabilities meant that grasshoppers would have to be present for long periods in the study site to have a high probability of capture. These residence. periods can be calculated for a 95% probability of capture from the equation

}{ probability of no capture = 0.05 where x = the number of samples needed for a 9.5% probability of capture. Assuming daily sampling, the times required to catch 95% of all grasshoppers ·ln 1980/1981 were 15.1 days for males and 16.1 days for females, and in 1981/1982 were 18.4 days for males and 24.7 days for .females. These

figures far.exceed the average expected lifespans for adults

! I \_-,_- ---======-=-~=--==�-=--=·-=--=.·-=-·=·-=··-· ·-•------•·•-LC_ ···-•c_c•· . - -_.-_----- =-:c------�-r::-:::::--�--::--_--:

47

predicted by survivorships, so only a small proportion

of the total adult population was -- handled in each season,

especially when considering that samplii1g was done only

every third day. Individuals migrating shortly after the

last moult were unlikely to be handled, and differential

migration could account at least partially for the-unequal

adult se·x ratio apparent in sampling.

Differential migration can be determined from the

relative proportions of recaptures taken in different.

sampling areas, but the small numbers of adult females

2 precluded any comparison of these by x analysis. An

indication of time of migration is given by noting the

capture histories of individuals which moved a"'.'.'ea..

Those which moved soon after the final moult would have

been handled only once before being detected in a new

area, while others would have a longer capture history.

However, female numbers were again too small to be

meaningful. The adult survivorships give some indication

of d:ifferential migration as well, although they cannot

distinguish it from mortality. In both seasons adult

female survivorships were very similar to males,

inconsistent with either a greater rate of feuiale migration

or a higher mortality, These possibilities are inseparable,

but the small differences in survivorship suggest that they

are slight and would, if anything, shift adult_sex ratios

back towards unity. Overall, the disparate adult sex

ratios seem to be caused by differential juvenile

mortality, but may be modified slightly by differences

in catchability, migration or mortality of adults.

! I I ... 48

A second unusual feature of the survivorship curves

is that despite the extra instar in females, male and

female maturation is nearly synchronous, allowing for

the variances on the estimates of instar duration. Females develop more quickly than males, perhaps

because of improved digestive efficiency or different

diets. Consequently, males may extend one or more

of the third to fifth instars, although the variances

involved prevent particular instars within this group

be'ing recognized individually as extended. This was

so unusual that at first it seemed that they were

artifacts of incorrect identification of grasshoppers

in the field. However, third instar and older grass-

hoppers are recognized easily from their wing bud

development, so the extended instars are real phenomena.

If different instars a.re involved each season the

. mechanism should be largely an environmental one.. The

overall result is that males mature only a few days

before females, despite the extra female instar.

Adult emergence times are critical because of their

rela,tionship to the success of adult breeding. For

females, there must be sufficient time to mate and lay

as many eggs as possible before death from senescence

or the onset of winter. Females maturing too late can

die before laying, a situation analagous to the problem

of some female butterflies whose larvae must reach

diapause size before the first frosts {Murphy et al.

1983). Phytophagous insects must coincide with their

! I \_-._-- ! I =- --======•=--=-•=•=••=-••=•=••=•="-•-�----• =-•=- -�--=----=-•=•-=•=••=-•=- =•=--=•======-=-�--=--•C::::::-c::=:==:r:ar..:-=--: �::"'---===�-=- ='??C:::r::== ------.C�

49

major food plants as well (Singer 1972). Males face

similar pressures comp_ounded by the need to gain access

to mates. Early male emergence is found in many insect

species (Ehrlich et al. 1984) and means that a proportion

of males emerging early will die without ever encountering

a female. Conversely, those which emerge too late after

most females have matured may have a low probability of

encounteri_ng a receptive and fertile female, especially

in those species in which females are plugged after mating.

This problem of the coincidence of maturation underlies

recent attempts to redefine sex ratios in terms of individuals

available for mating. Examples include the "realized sex

_ ratio" of Ehrlich and Brown (1980), a ratio of males to

females in a population of a single generation; and the

"operational sex ratio" of Emlen and Ori_ng (1977), the

ratio of males to fertilizable females at an instant of

time.

Scott (1977) described a deterministic model of male­

female emergence lags in invertebrate populations that

predicts that it is slightly advantageous for males to

precede females in emergence. The model uses the

rationale that a male's lifespan should bracket the time

with the greatest probability of mating, which is when

the la�gest mean number of females per male is available.

Similarly, females should have a lifespan that encompasses

the time of maximum probability of mating. Male selective

advantage is then defined as the number of females a male

mates with in a lifetime, while for females it is the ======·-=--=--=--=--=--=--�--=-"�======'?:?�-�----�---_-C'?:""".=�-=r----:::.-_---_-_,-_,-_, __ -�•--:------:�,

50

number of progency produced� This approach has the

considerable advantage of defining the difference in

emergence times in terms of the individual fitness of

males and females, rather than invoking a group

selectionist argument. The optimal interval varies

according to factors of survival rate and standard

deviation of emergence time, and for animals such as

A. conica with low survival rate and synchronous

development the difference in maturation is predicted

as less than a week. Allowing for the variances in

estimates of instar duration, this prediction is supported

by the survivorship curves. Similar arguments have been

made by Wiklund and Fagerstrom (1977) and supporting

data are given by Boggs: (1981)_ and Singer - (1982) for

butterfly populations.

In this light, the extended juvenile instars in

males help to synchronize male and female development,

compensating for the extra female instar and reducing

the period of adult males waiting for females to emerge.

This approach would be successful only if it improved

male survivorship overall, since from the point of view

of reproductive success it makes no difference if a

male dies as a juvenile or as a virgin adult. The fall

in female survivorships during the extended male instars

suggests that males are prolonging instars of comparatively

higher survivorship to females. The result would be

that at a single time the largest grasshoppers in the

population would be females, a product of the sexual

_I 51

dimorphism and the adjusted development rates. This

could lead to predators concentrating on the larger

females, and would mean that males gained the advantages

both of synchronized development and improved survivor-

ship. Two predictions based on this argument are

examined later in this thesis: that there is

disproportionate predation on females, increasing in

the later instars (chapters 4 - 6), and that males

can regulate their development rates behaviourally

(chapter 7) .

Once rea.ched, the unequal sex ratio has major

consequences for the adult population. Genetic

arguments for 1 : 1 sex ratios are established (Eshel

and Feldman 1982)., but they refer to initial sex ratio

and do not preclude differential mortality leading to

unequa_l sex ratios in later life (Ehrlich et al. 1984) .

Firstly, only a limited number of adult males would

mate while virtually all receptive females would, a

product of both emergence times and sexual selection.

Eme_rgence times might mean that some males live their

entire lives in a population where the sex ratio is

effecti:vely 1 : O, while those that have a chance to

-mate must contend with severe competition from other

·males. Under these circumstances genetic drift may be

influent.ta! in small populations with less than 30

females, especially considering that the eggs of a

single pod are likely to share the same fate, either

hatchi::ng successfully or dying of a common cause such -�=---=��======-==--====··=··=--2-r=-"·'.c:c--'.c:c--:c:c--:c:c--==:c-:c::c--:c::c--

52

as exposure, predation or parasitism.

The following chapters explore the possible

causes of the main features of the life tables

(chapters 4 - 7), and assess the consequences for

populations of A. conica (chapters 8 and 9). "-=--======-t---: ·······-�----.·_-_._._·.:_-�c; -· · --c,f ------·- - -- �

53

4. ANTI-PREDATOR DEFENCES

4.1 Introduction

The anti-predator defences of animals can be classified as

primary if they operate continuously regardless of the location

of predators, or secondary if they operate only when the animal

is threatened (Edmunds 1974). The major primary defence ot orthopterans is crypsis (Gillett and Gonta 1978), although some

species are aposematic (Chapman and Page 1979,, Rothschild 1973,

Price et al. 1980). Secondary defences include deimatic or

startle displays (Carlberg 1981), biting, kicking and scratching

(Edmunds 1972), regurgitation (Lymbery and Bailey 1980), toxic or

irritating secretions (Bernays et al. 1977, Rothschild et al. 1977,

Whitman 1982), deflection and thanatosis (Edmunds 1972), flight

(Kevan et al. 1983), stridulation and crepi tation (Whel\lan 1973) ,

and general features of shape and texture that make them difficult

for some predators to swallow (Sherry and McDade 1982}, Schlee

(1983) or facilitate escape from snares (Nentwig 1983). In

addition to these, orthopterans may adopt either dispersed or

aggregated distributions, each of which provides a defence against

certain sizes and types of predators (Gillett et al.. 1979). The

defences used by a particular species may change throughout the

life history, because developing individuals pass through a range

of different predatory pressures caused by changes in size and

lifestyle associated with maturation (Law 1979). In cases of

sexual dimorphism, the anti-predator defences may come to vary

between the sexes as the dimorphism develops (Schulz l98l).

Overall, crypsis remains the most universal defence. It

applies usually to body colour (Wiklund · 1975, Kettlewell 1973), ======-=ic-�----�------::-c---_:_:c-:-:::a=------·· ----

54

shape (Otte and Joern 1977) and pattern (Endler 1978), although

it may occur in smell or texture (Edmunds 1974) depending on

what sense the predator uses in hunting. The cryptic function

of colouration must be accomplished within the overall constraints

of thermoregulation and interspecific communication .(Anderson,--�- R� V.

Multiple functions for colour patterns can be

achieved by exploiting £actors of visual acuity, colour vision

and hunting distance on the part of the predator. Thus prey

need to resemble the background at the predator's level of

perception rather than absolutely (Endler 1978). Consequently,

colours which cannot be seen will not be included in the

camouflage pattern unless by way of ------reflectc1,_:09_�----- (Kaufman 1974) ,

and it is possible to have colours cryptic against one predator

species, but not another which is colour-blind (Hinton 1976).

Striped colour patterns may blur into solidity at speed (Pough

1976, Jackson et al. 1976) to create a different "colour" or

animals may be conspicuous in flight and cryptic when at rest

(Kettlewell 1973), a feature sometimes enhanced in swarms

(Milinski 19771. Striping may create an illusion of greater

speed as well, disnouraging pursuit (Deiner et·a1. 1976).

Differences in perception between predators and conspecifics

will allow the same colours and patterns to function for both

courtship and predator avoidance (Robertson and Hoffman 1977).

The demands of crypsis are believed to be important in grass­

hopper microhabitat choice (Joern and Lawlor 1980).

If crypsis is successful, predators are more likely to

encounter and feed on the most conspicuous prey, forming a

search image that improves their effi0�ency in finding the

conspicuous animals. Extensive investigations of search 55

image formation have been made with birds in both the laboratory

(Dawkins 197la,b) and in the field (Craze 1970, Murton 1971,

Alcock 1973). These revealed that birds learn to feed on particular colours of food, and that these searching images are formed more readily for conspicuous prey. Although search images could be formed for cryptic prey, these were of shorter duration. A further advantage of crypsis was demonstrated by

Erichsen et al. (1980), who found that the inclusion of a

"discriminating cost" of finding cryptic prey in optimal diet models reduced the profitability of cryptic forms.

In the case of polymorphic prey such as A. conica, factors of the relative frequency of the morphs, density distribution and the innate colour preferences of predators can all lead to the formation of search images for specific morphs that increase their mortality relative to their proportions in the total prey population. Frequency-dependent selection occurs when the relative mortalities of the morphs depends on their frequencies

(Willis et al. 1980, Allen.and Anderson 1984). Fullick and

Greenwood (1979)- explain frequency-dependent selection as a result of predators learning more readily that common prey are food, how to select them if they are cryptic, where to find them, and so on. Frequency-dependent selection in which predators take mainly rare varieties of prey tends to eliminate variability and cause stabilizing selection, while apostatic selection, which is the disproportionate elimination of common forms, maintains variability (Clarke 1962, Elton and Greenwood

1970, Greenwood and Elton 1979, Hubbard et al. 1982). Numerous studies have suggested that the type of selection which occurs is density-dependent, with stabilizing selection occurring at

I I 56

high prey densities and apostatic selection at low prey densities

(Allen 1974, 1976, Allen and Anderson 1984, for example). This can lead to different populations of the same polymorphic species experiencing different types of selection (Harper and Whittaker 1976).

However, the response observed may vary with the age and experience of the predator (Cook and Miller 1977, Willis et al. 1980), and with innate colour preferences of the predator (Allen 1976).

Further, Harvey et al. (1974) advised caution in interpreting the results of field experiments on selection because wild predators have feeding experiences beyond the experimenter's control.

Allen (1972) suggested that stabilizing selection occurs at high prey densities because search costs are low, and predators may take disproportionately more of rarer forms because of their conspicuousness or novelty value (Mueller 1971). The deliberate selection of rarer prey is seen by Westaby (1978) as sampling the food resource, which is necessary if the predator is to be able to decide on an optimal diet. When search costs rise at lower prey densities, attention is concentrated on the more frequently encountered forms (Allen 1972). Because of the interactions between density-dependent and frequency-dependent selection, there is a strong relationship between density and fitness for different morphs (Manly et al. 1972, Clarke 1972) . Slatkin

(1979) develops the genetic argument further by arguing that frequency and density-dependent selection in a polymorphic species will tend to equilibrate the phenotypic classes at an evolutionary equilibrium, while phenotypic parameters that affect directly features of the phenotypic classes will not, under similar selection pressures, tend to equilibrate fitnesses. Hubbard et al. (1982) and Sih (1984) take an integrative approach to the 57

area, arguing that frequency-dependent selection responses can be predicted from optimal diet models, a.nd that the two responses are essentially the same. If this is so, frequency-dependent selection appears to emphasize the prey's perspective of predation, while optimal foraging is a more predator-centred approach.

The tendency for prey species to clump at favourable food concentrations can create artificial pockets of high density, leading to density-dependent selection. In the case of such patchily distributed prey, predators will learn to search in specific areas rather than form search images for cryptic prey

(pawkins 197la, Smith and Dawkins 1971, Lawrence and Allen 1983), and the intensity of search for prey will rise when they are dense (Brower 1958). This can occur irrespective of search image in the case of the _aggregation response of arthropod predators

(Hassell and May 1974, Bernstein 1982). Consequently, cryptic morphs close to a single conspicuous animal are at a greater risk than those at a distance, as shown by Kettlewell (1955,

1956} in studies of cryptic Lepidoptera. Further data on this area presented by Tinbergen et al. (1967) suggest that �t high prey densities the area-restricted searchiQg behaviour of predators allows them to find items overlooked normally.

Nevertheless, clumping in either space or time can be a defence against predators. Animals that clump in time by synchronous emergence or breeding can swamp predators and reduce the losses tha.t would occur with a more regular distribution

(Schaller 1972, Taylor 1976). Aggregations in space can inter- fere with a predator's attack efficiency (Brock and Riffenburgh ======-

....------� - _ _ -- -= - --::,.7 -_ _ _ ---- :=;---____,.c-; _ c ------__- __ - ., - -·!------·I

58

1960, Olson 1964, Cushing and Harden Jones 1968, Kenward 1978,

Tostawaryk 1972), or with its search success (Paloheimo 197la,b)

while reducing the chance of a particular individual being

killed if a successful attack is made (Breder l967, Hamilton 1971).

Aggregated prey may also scatter when attacked, and so confuse

a predator (Brown 1974) and predators which aggregate at high

prey densities may come to interfere with each other. In extreme

cases predators may be prevented from even attacking at all

(Gillett et al. 1979, Joern and Lawlor 1980), or even mobbed

(Lorenz 1968, Curio 1975). The defensive value of groups is

enhanced further by improved sensory awareness to detect danger

(Pulliam 1973, Siegfried and Underhill 1975), and the transmission

of visual and aural warnings (Smythe 1970, Edmunds l974).

Treisman (1975a,b) argued that if predators are capable of

taking only very few prey on encountering a group then group

formation is favourable, but if they can kill the entire group

then dispersion is a better strategy. The capacity to make

multiple kills underlies the conclusions of Tinbergen et al.

(1967) and Craze (1970), who found no advantage in group

formation. Stewart-Oaten (1982) argued in favour of a Poisson

distribution as a predator avoidance mechanism. If such a

distribution were adopted predators should search each patch

for a constant time to maximize their hunting success. As

this conflicts with the tendency to search intensively in

patches of greater success, it reduces prey losses. However,

this argument is based on theoretical grounds and ignores

reasons such as food distribution and mating demands which

influence prey distribution. Although much of the work on

the value of clumping has been concentrated on vertebrates,

- I

! I' -- -- -I

59

especially fish and birds, several recent studies have demonstrated the same principles in aquatic invertebrates (Neil and Cullen 1974,

Treherne and Foster 1980, 1981, 1982) and in grasshoppers (Gillett et al. 1979, Chapman and Page 1979:, Joern and Lawlor 1980, Kevan et al. 1983) .

Of the secondary defences used by grasshoppers, deimatic

displays are aimed at vertebrate predators in the same way as crypsis (�oppinger 1970). Their value is greatest when they are a novel stimulus to the predator, and declines with increasing

predator familiarity (Key 1974). Vertebrates can be avoided by the characteristic behaviour of hopping or flying as well.

Grasshoppers may le�p and then either freeze or burrow into the

undergrowth (Schulz 1981), or continue in a series of jumps

before settling. Flight may carry a grasshopper beyond the

reach of an individual predator or onto a matching background

to improve crypsis, while other resting grasshoppers may be

disturbed, confusing the predator by synchronized jumping

(Kevan et al. 1983) . If a grasshopper is seized it may escape by autotomising legs, especially the hind ones, while the spiny

hind tibiae may deter small birds (Sherry and McDade 1982) and

the hard pronotal shield may also cause difficulties (Schulz

198l). Regurgitation (Lymbery and Bailey l980) may discourage

small vertebrate predators and spiders by gumming up ·their mouths

or creating an unpleasant taste, but they are less effective

against raptorial arthropods (Chapman and Page 1970).

A. conica uses a combination of these primary and secondary

defences. The presence of distinct colour morphs from the first

instar suggests that colour has a cryptic value from that stage,

and this may be reinforced by the elongated body shape. Leaping

-- 1.·._:_·_----·_-_------,

60

and flight are further defences, and in adults they are

supported by a flash display of pink/purple or yellow on the

dorsal abdominal surface. Seized grasshoppers may regurgitate,

and they can autotomise hind limbs. The distribution in the

field may also be significant in reducing predation risk.

This chapter describes A. conica's defences, concentrating

on distribution, microhabitat choice in the field, background

colour matching in the laboratory, jumping behaviour and

regurgitation., Distribution was significant because of its influence on interactions with predators, and its importance

in both crypsis and diet� Accordingly, data were collected

not only on grasshopper distribution but on the type, height

and colour of the associated vegetation. Since the vegetation

changed over the summer and the grasshoppers had the option of

moving or changing colour to match it, the interactions were

significant. Distinction always had to be made between the

sexes and among the instars to highlight sex or stage related

differences that could have important life history consequences.

A study of background colour choice developed from this, as it:

was important to distinguish between selective predation and

deliberate background choice as causes of any association

between grasshopper colour and background colour. Jumping

behaviour is important because it is a major secondary defence,

and also because a general knowledge of it is needed to guard

against bias in the line transect techniques used to estimate

grasshopper distribution. Regurgitation was easy to note,

and any differences in frequency between sexes or among the

stages may have reflected the major predators.

J \_-_:_ �------_- �------. --- --· ·-----� - l ------. --•,c==:::f:__- --

61

Overall, these aspects are most significant in their

interactions with predators, and in· their potential conflict

with such factors as reproduction and diet selection. The

subsequent chapters will develop these areas, and place the

descriptive material in the wider context of the integrated

life history strategy.

4.2 Crypsis, Distribution and Anti-Predator Behaviour in the Field

Methods

Detailed data on anti-predator defences were collected at

three sites and supplementary data from a fourth site over the

summers of 1980-1981 and 1981-1982. At each site the distribution

of grasshoppers from the time of the first emergence to maturity

was studied using an adaptation of a line transect technique

described by Onsager (1977) and Onsager and Henry (1977). A

square metal frame enclosing an area of 0.lm2 and covered with

a fine cloth mesh was used for sampling. At sites I and II

10 parallel transects 30 metres long and three metres apart were

marked. At site III two parallel transects 150 metres long and

three metres apart were used because the grasshoppers were in a

narrow belt along the river foreshore at this site.

Line transect methods can give biased estimates of density

and distribution if the sampling area is variable, as can occur

if it is determined by eye (Onsager 1977 , Titmus 1983), if

· grasshoppers leap before the sampling device is placed, or not

all trapped animals are scored (Gandar 1982). My method had a constant sampling area, while the distance from which the

quadrats were placed was considered adequate because of the short

mean leap of juvenile grasshoppers (see results). To check that · all trapped animals were being scored three test qua dra. t s.:.were cleared and then searched again five minutes later, when no

further grasshoppers were found.

Further assumptions of line transect techniques are discussed

by Franzreb (1981). They are :

animals are distributed randomly;

behaviour of animals in one region of the transect

does not alter the behaviour of others in a

different region of the transect;

animals do not move in response to the observer

before detection;

no animal is counted more than once;

animal responses remain unchanged across a series

of sequential samples;

there are no intraspecific differences caused by

sex or age.

Because the first assumption is often violated in reality (Gailes et al. 1968, Eberhart 1978) it is customary to approximate it by placing transects at random (Eberhart 1978) or by using a systematic grid with a randomly placed starting point (Anderson, D.R. et al. . J-9.79) . The second approach was used in this study. The second and third assumptions were considered met because of the short leaps of the juveniles (see results), and the fourth was guarded against by removing animals during sampling. The fifth assumption is unlikely to be violated by invertebrates, and no 63

evidence was found for sex-related or age-related differences

in the escape behaviour of juveniles.

At each site eight samples we.re taken at weekly intervals

from late November when the grasshoppers first emerged until

mid-January, when most of them had matured and required different

sampling methods. I began sampling at 1000 hours because by

then the grasshoppers were active. I followed a transect and

placed a quadrat every three metres by holding it at arm•s

length and dropping it. The,nurnber' o.f grasshoppers-/quadrat was noted, an,d ���.s��ples scored £qr sex, j.nstar,_· mo:r:ph and regurgitation. Grasshoppers were released where they were ca�ght.

On the first, third and fifth sampling occasions the

vegetation characteristics of each quadrat were scored to monitor

changes in grass colour over summer. More frequent samples were

not taken because I had no interest in the rate of colour change.

For each quadrat the proportion o� green and non-green cover was

noted by reference to a commercial paint colour chart. The

proportion of bare ground was noted as well, and what proportions

of the total cover were less than 15 ems tall, between 15 and 30

ems tall, and over 30 ems tall. These heights covered the main

grasses. I did not note overhang from trees or bushes.

The distribution of adult grasshoppers was monitored using

a modification of the method used for juveniles. The same transects were followed, but each was treated as a belt one metre wide. Walking slowly along the transect it was possible to note grasshoppers leaping when disturbed, and to monitor their position by keeping a mental count.of the number of paces walked along a particular transect. Males and females could be distinguished 64

easily because of their size differences, but colours were far more difficult to spot accurately, and no attempt was made to record them. Five samples were taken at weekly intervals at each site, beginning the week after the last sample of the juveniles. The vegetation was sampled again on the third occasion in the same manner as before, to provide an indication of any changes occurring during the adult phase.

This approach is analagous to the fixed-width line transect techniques used to assess bird population densities (Franzreb

1981) and has similar problems. These include the unequal sighting efficiency across the width of the transect (Eberhart

1978) and the fact that the grasshoppers' movements are dependent on the observer and fast in relation to him (Anderson, --:cD.R- ! .. et- al �J.?)79) .

There are further problems in ensuring that grasshoppers are not counted more than once. When such difficulties occur Hilden

(1981) has cautioned against the technique for estimating absolute densities or studying population changes with time. However, they are less critical in studying distributions, the purpose for which data were intended here.

Escape behaviour in juveniles was investigated by disturbing a resting individual with a grass blade and noting its behaviour over the next 30 seconds. For animals which jumped, the length and number of the jumps and the colour of the background landed on were noted, as well as any attempt to burrow into the grass.

The grasshopper was then caught and its sex and instar determined.

This work was done at site IJ, and was always begun at 1000 hours to ensure that the grasshoppers were active. 65

Results

The distribution of juveniles at the three sites is plotted in Figure 10, with the appropriate coefficients of dispersion.

These are useful for ready assessment of the degree of randomness, aggregation or dispersion of observations (Sokal and Rohlf 1969).

In all cases the grasshoppers were aggregated, with the coefficients of dispersion ranging between 2.15 and 4.78. This meant that although the mean densities of grasshoppers in the areas sampled were between four and eight per square metre, they occurred mainly in dense pockets where there could have been many more per square metre. At sites I and III the coefficients of dispersion rose with sequential samples, indicating greater aggregations of grasshoppers occurred as they matured. This trend did not occur at site II. Adult grasshoppers had aggregated distributions as · well, despite their greater mobility (Figure J_J;). The distributions given include both males and females, because meaningful distributions were difficult to prepare for the small numbers of females.

Distribution of grasshoppers in relation to the colour and height of the vegetation was assessed by analysis of the appropriate

multiway contingency tables using the G statistic (Sokal and Rohlf 2 1969) Although used less frequently than x analysis, the G

statistic enables an investigation of interaction effects in multiway tables. Tables 7 to 8 show the results of multiway

analysis usi_ng the factors of grasshopper sex, grasshopper colour,

background colour and background height. Interesting significant

associations are those between grasshopper sex and grasshopper

colour, and grasshopper colour and vegetation colour. There

was a greater tendency for female grasshoppers to be green, ______:::::::r::=:::,.. ------,·-__-_-;. ___ ! ------I ------c=r___:_:-----...

66

rather than the proportions of the colour morphs to be the same in both sexes. However, grasshoppers of particular colours tended to match their backgrounds, rather than being distributed irrespective of background colour.

The association between grasshopper colour and sex is also evident in Figure 12 which shows the percentage of brown grass­ hoppers of each instar caught at site I during 1980-1981 and

1981-1982, and at sites II, III, and IV during 1981-1982. For each instar, more females than males were green. At site I the greater tendency of the females to occur in green vegetation led to a noticeable segregation of the sexes in juveniles, with the females aggregating in the greener vegetation near the waterline while the males were further away (Table 9 ) . Adults at site I also showed a significant association over the seasons

1979-1980, 1980-1981 and 1981-1982 (Table 10), although the relative proportions were different each season. More females than males were green. The same association occurred in adults collected at site II in 1981-1982, but not in samples from sites

III and IV in the same season (Table 11).

The vegetation at sites I and IV became increasingly brown as summer progressed (Figure 13). The trend was less marked at site III, which abutted the Swan River. However, in all cases the proportion of brown grasshoppers in each sex declined over the summer.

Regurgitation frequencies grouped by sex and instar are shown in Table 12. For both sexes, there was a significant association between instar and the number of individuals which regurgitated. However, when first and second instar grasshoppers 67

2 are removed from the tables the resulting x values are in­ significant, indicating that the first two instars regurgitate more readily. On the whole regurgitation frequencies were low, and excepting the first two instars were approximately

10% for both sexes.

Data on the jumping behaviour of juveniles appear in

Table 13. Although the largest mean jump was only 1 2 . 2 cm for fourth instar females, the standard deviations were large, in one case exceeding the mean. The influence of background colour on the likelihood of a grasshopper making more than one 2 jump was investigated by x analysis (Table 14). For both males and females the likelihood of making more than one jump depended on the colour of the background landed on, with grass­ hoppers being more likely to leap again on a contrasting background.

Discussion

The most striking feature of the anti-predator behaviour of A. conica is the aggregated distribution of both adults and

juveniles. This is found in other orthopterans as well (Gillett et al. 1979, Schulz 1981). In the case of the aposematic

Zonocerus variegatus reported by Chapman and Page (_1979) and

Chapman et al. (1979) aggregation was most marked in the early

instars, and less so in later ones. Bradley (1975) found a

similar response in several species of U.K. acridids, and

suggested that the aggregation of early instars was an artifact

of synchronous hatching of aggregated egg pods. However,

subsequent _aggregation of adults was regarded as a definite

response to biological or environmental factors. Gillett

------:: , ·-=-======�·------

Table 7. Tests for association with the factors of

grasshopper colour, grasshopper sex and

background colour at

site I (opposite)

site II (overleaf)

site III (overleaf)

using the G statistic.

* significant at 0.05

** significant at 0.02

*** ;::; significant at 0.01

n.s. not significant

7----c------

i I ------·t ------. ------j .c: - -- �------_____ .,;-- ---1 ._ __ _:�- _-..c:--:_ :-_ ,:,:-_,.._-_ -.-.. ---c-,-,-��====-� ------_:;��------� - -

SITE I

Background Colour (% green}

<. 25 25-50 50-75 7 75

cl'' 13 2 10 23 � 0 Green r. 0 \J 0 + 20 2 l9 43

77 13 26 27

Brown

27 8 19 25

Hypothesis tested d.f. G

Grasshopper colour x 3 33.4*** background colour

Grasshopper colour x 1 26.4*** grasshopper sex

Background colour x 3 15.2*** grasshopper sex

Threeway independence 10 6-·o *** ======-·=--=--=-=--=--=--=--=·-=--=--·=·-=--·===·-=--=-=--=- =- =--=-======-=--=--===l"ZLd-":':'::C:'======>cr - �,-.::------·-·------.-:.-..,-:-'C,-,sL-

SITE II

Background Colour (% green)

< 25 25-50 50-75 ) 75

d' 12 5 16 20 H 0 Green 0 0 u + 21 6 29 53

65 21 25 26

CJ) Brown

0+ 41 28 28 25

Hypothesis tested d.f. G

Grasshopper colour x 3 48.2*** background colour

Grasshopper colour x 1 16.6*** grasshopper sex

Background colour x 3 9.6* grasshopper sex

Threeway independence 10 72.8*** .,c ______·--. -- -=•------"�-- ======;_;_-_____-_-_-,:_:_-,. - �-,I ------· -- .-

SITE III

Background Colour (% green)

1.. 25 25-50 50-75 '7 75

c:Jr 99 27 42 59

Green

0 +0 57 19 24 25

20 5 9 72

Brown

+0 27 7 33 45

Hypothesis tested d.f. G

Grasshopper colour x 3 52.6*** background colour

Grasshopper colour x 1 13.0-*** grasshopper sex

Background colour x 3 8.8 n.s. grasshopper sex

Threeway independence 10 72.0 *** Table 8. Tests for association with the factors of

grasshopper colour, grasshopper sex and

background height at

site I (opposite)

site II (overleaf)

site III (overleaf)

using the G statistic.

* significant at 0.05 ** = significant at 0.02 *** significant at 0.01 = n. s. not significant -=--=--=--=--=-�=====--=--=--=--=-=--=-======�--�-zc-�--;;r:::_,=-=------·/·•-c:-_---�------,c=:_;------·------

SITE I

Background Height (mm)

L 15 15-30 ? 30

d" 17 11 20

i,..i Green

0 s:: + 33 31 20

d1 61 47 35

Brown

16 31 32

Hypothesis tested d.f. G

Grasshopper colour x 2 0.4 n.s background height

Grasshopper colour x 1 26.4*** grasshopper sex

Background height x 2 4.6 n.s. grasshopper sex

Threeway independence 7 44.8***

I:= -- SITE II

Background Height (mm)

< 15 15-30 7 30

cl' 19 17 17

H rd Green 37 28 ·r-l 44

rd d' 48 36 53 >4 Brown

39 47 36

Hypothesis tested d.f. G

Grasshopper colour x 2 1 n.s. background height

Grasshopper colour x 1 16.6*** grasshopper sex

Background height x 2 1 n.s. grasshopper sex

Threeway independence 7 23.6*** SITE III

Background Height (mm)

<... 15 15-30 7 30

d' 70 69 88

Id Green

·r-l � 41 33 51

Id 40 31 35

Cl) Brown

0 + 71 41 40

Hypothesis d.f. -G

Grasshopper colour x 2 1.8 n.s. background height

Grasshopper colour x

grasshopper sex

Background height x 2 0.6 n.s. grasshopper sex

Threeway independence 7 19. 0 ***

I I. Figure 10. Distribution of juvenile A. conica at

site I (opposite)

site II (overleaf)

site III (overleaf)

during the summer of 1981/1982 . The vertical axis

shows the percentage of 100 quadrats sampled and

the horizontal axis shows the number of grasshoppers/

quadrat, ranging from none to eight. Each graph

represents a weekly sample, taken over the period

mid-November to mid-January. The mean number of 2 grasshoppers/quadrat (x), the variance (s ) and

the coefficient of dispersion (CD) are shown, as

2 well as the chi-squared value (x ) for comparing

the observed distribution to a Poisson distribution, 2 The x values all have two degrees of freedom

despite the number of classes because of the

grouping of several classes with low expected

values. All are significant at the 5% level. �--:c:.c--:=:c--�--:cc:--c:c:-:cc:--:c::c- -:::c-':::::--':::::--C::C:--:::::-:c::::--:c::::--zc--z::--====-:::;::--:cc--�--':::::-':::::-:::::---= -=--=--=-=-=====�==�==�'cc--:c::--cc::--2--2-�--=--" ---,-c--; ______"' ------

DISTRIBUTION OF JUVENILES AT SITE I IN l98l/l9_82

(i) (ii)

2 2 x = o.58 s = 1.47 X = 0.75 s 1.95 2 2 CD = 2.55 x = 21.17 CD = 2.60 x 31.20

50 50

(iii) (iv)

2 2 X = 0.42 s 1.44 X = 0.34 s 1.08

X 2 2 CD = 3.45 = 22.96 CD = 3.16 X = 24.34

50 50

�

Ii-I 0 (v) (vi)

X 2 2 p:; = 0.41 s = 1.96 X = 0.42 1. 34

2 2 � CD = 4.76 X = 35.53 CD = 3.18 X = 16.03 �

50 50

(vii) (viii)

2 X 2 = 0.4 s 1. 58 X = 0.22 s = 0.76 2 2 CD = 3.94 x 27. 24 CD = 3. 45 x = 17.74

so 50

3 5 3 5

NUMBER OF GRASSHOPPERS PER QUADM,T ------I =-...- -_-..:--_--..:-..:--=---=------, ------j .: - --�--.;-___ ------:_- --1 e;...------=- - - _-_-:- --: ----.-..:£ - -- - -:__-_: - 0-- _------.c--=.:-�-"'-=-�-=�-'"�-"•J____L_ .,--_-_,. ___ -:____ ·- ----= -_- - - :_ _.-:;_-_-_:- _:-=------,

DISTRIBUTION OF JUVENILES AT SITE II IN 198l/1982

i { ) (ii) 2 = 2' x = o. 81 s 2.39 x = o.66 s = 1.86 2 = 2 CD = 2. 94 x 36.02 CD = 2.82 x = 24.59

50 so

(iii) (iv)

2 2 x = o.64 s = 1.44 x = o.49 s = 1.48 2 2 CD = 2.26 x = 15.09 CD = 3.04 x = 26.33 50 50

(v) (vi)

2 2 x = o.41 s = 1.s6 X = o.s3 s = 1.06 2 2 CD = 3. 80 x = 25.13 CD = 3.50 x = 21. 97

50 50

(vii) (viii)

2 2 x = o.31 s = 0.74 x = o. 36 s = 1.40 2 2 CD = 2.39 x = 6.75 CD = 3.98 X = 16.94

50 50

3 5 3 5 NUMBE-;R OF GRASSHOPPERS )?ER, \ QUJillAA.T 0 ce- =· -=--=--=---"7'-·=� =c_�_-_c::=::_--CZ.-�J=-:c::c-- :c::c--:c::c--:c::c--c--:C::C--:C::C- :C::C-:C::C·-=--=--=- =--=cL____!=caz:----, -�- ·======E--2-2--2--=------:.----- . -�'.'."---�----_·_-_:'."",:f ------,

DISTRIBUTION OF JUVENILES AT SITE III IN 19-81/19-82

(i) (ii)

2 2 X = 1.44 s = 4.03 x = 0.10 s = 2 . 27 2 CD = 2 .80 x = 75.o CD= 2.92 / = 30.75

50 50

Ciii) (iv)

2 2 X = 0.49 s = 1.42 X = 0.4 2 $ = 0.99 2 2 CD= 2 .91 x = 19.35 CD= 2 .36 x = 6.14

50 50

(vl (vi)

2 2 � X = 0.8 s = 2.71 X = 0.5 2 s = 2.15 2 2 CD= 3.38 x = 35.75 CD= 4.14 x = 28.68 �z 50 50

(vii) (viii) 2 x = o.a1 s = 3.55 x = o.44 s = 1.63 2 2 CD= 4.38 x = 45.37 CD= 3.70 x = 29.75

50 50

3 5 7 1 3 5

NUMBER OF GRASSHOJ?PER,$ :PER QUADMT

)_-._:_·.·---.-_--- Figure 11.. Distribution of adult A. conica at

site I (opposite)

site II (opposite)

site III (overleaf)

during the summer of 1981/1982. The vertical axis

shows the percentage of 300 paces which disturbed

X grasshoppers, and the horizontal axis shows the

number of grasshoppers disturbed/pace, ranging

from none to seven. Each graph represents a

weekly sample, taken over the period January/

February. The mean number of disturbed/pace .(x),

2 the variance (s ) and the coefficient of dispersion

(CD) are shown, as well as the chi-squared value

2 (x ) for comparing the observed distribution to a 2 Poisson distribution. The x · values all have two

degrees of freedom despite the number of classes

because of the grouping of several classes with low

expected values. All are significant at the 5%

level. ======-:c::=--cc:c-::cc-�-c:::c--::c::c-..:=:c-c:,::_ ..,::c,. ======:-1----· ·· ·---,:c:::�:c._;- :L:··c=.:·==�-�·-C:C:-I·f =-�--=--�-:C::C--::C::C--�--=--=-======1-c:c::-=-===·=··C::C:--C::C:-:C:C-:C:C--::C:C--=-==-."?:. ==

SITE I

(i) (ii) 2 X = 0.32 s 0.60 2 X = 0.36 s 1.07 2 CD = 1.85 :l 25.44 CD = 2.98 x 64.77

50 50

(iii) (iv) 2 2 x ·= 0.22 0.50 X = 0. 29 s 0.51 2 2 CD = 2.29 X 23.47 CD - 1.78 X 2 3.82

50 50

SITE II 0Ji.J (i) (ii) � 2 2 µ:J X "' 0.52 s 1. 24 X = 0.36 s 0.7� � 2 2 ·o CD = 2.37 X 45.32 CD - 2.11 X 44.22

50 .50

(iii) (iv) 2 2 X = 0.44 s 0.93 X = 0. 33 s 0.42 2 2 CD = 2.12 X - 45.90 CD - 1. 28 X 20.96

50 50

0 2 4 0 2 4

NUMBER OF GRASSHOPPERS DTSTURBED

·- ! DISTRIBUTION OF ADULTS IN 1981/1982

SITE III

(i) (ii)

2 2 x = o.35 s == 0.88 x = o.38 s = 0.86

2 2 CD= 2.48 x == 35.60 CD= 2.26 x = 36.11

50 50

(iii) (iv)

2 2 x = o.43 s = 0.68 x = o.47 s = 1.01

2 2 CD == 1. 60 x = 30. 27 CD= 2 .15 x == 46.19

50 50

3 3 5

NUMBER OF GRASSHOPPERS DISTURBED Figure 1,2. The proportion of brown grasshoppers of each

instar caught at site I during 1980/1981 and

1981/1982 (opposite) and at sites II, III,

and IV during 1981/1982 (overleaf). All points

shown with binomial standard errors, and the

smallest sample size was 17.

••----• males

• - - - • females -====--=--=::c::--c:c::c--::c::c--:=::--======--1=------· >-C------��------,I ------:- --�L�'-----····------_

PROPORTIONS OF BROWN GRASSHOPPERS

SITE I

1981/1982

90

10· \ \ \ 50 \' A---+---,� \ / ...... 30 t/ "t---l 10

1980/ 1981 90 P-i

P-i

70

50 'I-- f 30 - --+-+- ', 10 I

1 2 3 4 5 6 7

GRASSHOPPER INSTAR ------=-:c=�=-=··=··=·===-=·-·=··=·=====-= -,=-! =-.t--�- ·=··=··=· -=�==·=--�--f=-�-=--:=::::--=c::---�---�--=--=====--=--=···'Z··='··=--:=::::-=-·-C::C::-::C:C-=··:C::C:---=-c:=:r•=�=--'""'3•-Z--:--:c::"..,..-'?:".--0".:··-':".:'--'.':2:-=.,.--=i-

PROPORTIONS OF BROWN GRASSHOPPERS

SITE IV

90

70

so

30

10·

SITE II

90

:z;

70

� H 50 ' 'k 'l "' P-i 30 ''1---+--+/-l 10-

SITE III 90

70

50

30 ------+--+--1 10

1 2 3 .4 s· 6 7

. GRASSHOPPER !NSTAR Table 9. Tests for association between grasshopper

sex and distance from the lake at site I.

Separate analyses are presented for fifth

and sixth instar females against fourth and

fifth instar males, and for second and third

instar females against second and third

instar males. The analyses are given

for both 1980/1981 and 1981/1982. The

numbers shown are the total numbers of

grasshoppers of the appropriate sex and

instar handled during MRR work. ..r:__=--r,::-- .. · ---======- .-···-· -- - -·_...... ••. , -,- -- - .... ------� ----�.:------.:---:� -;

1980/1981

< 10m from lake ;> 10m. from lake

5 + 6 0+ 0 + 1to 45

4 + 5 65 79

2 X 2 0. 517 _; p ( 0. 01

t.. 10m from lake "! 10m from lake

+ � � 2 3 83 63

1 2 + 3 0 39 57

(j 2 X = 6:699 p < 0. 05

1981/1982

I.. 10m from lake 710m from lake

5 + 6 � � 28 7

4 + 5 d'd' 29 57

2 X 4 7 ...1_9 0 I p(.0.01

"- l0m from lake 7 10m from lake

4 2 + 3 � � . 61,

2 + 3 6 g 3 77"

2 x = 0.451 . n.s. ---======-=--=-=-=--====--=-=··=--=-=--=-=··=-�-- -=·=-�---�--1·J=-=---=--=--=--=--=--=·-=-=--=-=====�---=-=--=-=-·=-=--=--=·--=--=-�·===··--� ���--�:�--�-- -��-�-�==- ==��

Table 10. Tests for association between morph and sex

for adult A. conica at site I in the summers

of 1979/1980, 1980/1981, 1981/1982. Figures

are the total numbers of individuals marked

in each season.

1979/1980

Green Brown

166 176

56 34

2 X 5.341 p <-. 0.025

1980/1981

Green Brown

l' 82 70

0+ 24 4

2 X 9.856 p (._ 0.01

1981/1982

Green Brown

cf' 119 90

0 + 32 9

2 X 6.387 p � 0.025 -----======' -=--=-=--=--=-=--=- -=--�=-=··=·=---=----=·-=··=·=--=--=--=--=- =···======-1------_:_--_-_- .-:�-----�----_ -

Table 11. Tests for association between morph and sex

for adult A. conica at three sites in the

summer of 1981/1982. All grasshoppers at

each site were collected in one trip.

Site II

Green Brown

18 28

28 14

X 2 = 6.672 p <, 0.01

Site III

Green Brown

64 24

0 + 28 5

2 x 1.935 n.s.

Site IV

Green Brown

40 55

0 + 22 26

2 x 0.180 n.s. ------...±:.=:..::..= ------·- -.--_---i

Figure 13. Changes in the proportion of brown vegetation at

site I (opposite)

site II (overleaf)

site III (overleaf)

during the summer of 1981/1982. The vertical

axis shows the percentage of 100 quadrats, and

the horizontal axis shows the percentage of the

total colour that was brown. Each graph

represents a single sample taken at three-weekly

intervals over the period November - February. _--- ' ------(------1-..,------� ------, ____ - :___ --.:-_-_:-;: =·------.---·--.• ...J -j - -I

.. CHANGES IN VEGETATION COLOUR·

1981-1982:

SITE I

(i) ( ii)

50

0

(iii) (iv) �

50

10 50 100 10 50 100

% GREEN COVER, �-; --:-"' -- - �--- - -,_ ------1 ;_-_:_-__-_-_-_-.:-::�- �--- -· -.=-:--..;f -----'---·_-----,

CHANGES IN VEGETATION COLOUR

1981-1982

SITE I I I

(i) (ii)

50 50 .'

liii) (iv)

50 50

D

.-I SITE I I 0

Ii-I (i) 0 {ii)

D z 50 50

(iii) Civl

50 50

10 50 100

% GREEN COVER

I I ======-=-:r;------3 ___ ·------•--•------�---'

Table 12. Regurgitation f;requenci\es for a sample of

grasshoppers of all ins tars of both. sexes,

caught during the distribution work in the

summer of .198.1/J.982. The propo·rtion of

grasshoppers regurgitating was not the 2 same in all ins tars, using. the x -· test.

However, if the first two instars are

excluded, similar regurgitation_ frequencies occur in the older instars.

I ! Males

Instar Number regurgitated Other Total

l 76 141 217

2 32 119 151

3 8 94 102

4 11 102 113

5 6 54 60

Adult 14 117 131

2 x (all instars) 60.28 . p < 0.01

2 x (excluding first two instars) = 0.55 n. s.

Females

Instar Number regurgitated Other Total

1 42 100 142

2 19 66 85

3 4 78 82

4 7 60 67

5 9 72 81

6 2 56 58

Adult 8 79 87

2 x (all instars) 43.99' p <.. 0. 01 2 x (excluding first two ins tars) = 0.55 n.s. ·=--=----=-"•-�--=-·=-··=----= ·-=-·==--=---=--=--=-=-·=·======·-=-=·-=- -=·=-===...- �=-=��� --- __ _ �..,.�------�---!------�------

Table 13. Number of leaps made in 30 seconds by disturbed

grasshoppers, the incidence of burrowing

behaviour (B) and the number of grasshoppers

tested (n). Mean jump distances are ± standard

errors. Adults are not included.

Males

Instar Number of grasshoppers making

1 leap 2 leaps 3+ leaps X distance (cm) B n

1 25 7 4 4.2 + 0.45 4 36

2 18 3 3 5.8 + 0.75 0 24

3 31 2 0 5.6 + 0.54 2 33 + 4 17 1 1 10.8 - 1.00 1 19 + 5 27 2 1 9.2 1.48 1 30

Females

Instar Number of grasshoppers making

1 leap 2 leaps 3+ leaps x distance (cm) B n

+ 1 32 8 3 3.1 - 0.30 2 43 +- 2 28 5 3 6.7 0.65 3 34 + 3 19 1 2 -9.8 1.51 1 22 +- 4 27 9 1 12.2 1. 22 7 37 + 5 17 1 3 11. 7 - 2.02 0 21 +- 6 38 6 4 9.6 1.53 6 48

-_- 1:-.:_·_··--.-_ -- !:�----_-•. -_---. Table 14. Tests for association between the occurrence

of second leaps and the colour of the back­

ground l�nded on at the end of the first leap in A. conica.

Males

Contrasting background Matching background

1 leap 43 75

'"7 1 leap 14 10

2 X 3.98 p <._ 0.05

Females

Contrasting background Matching background

1 leap 66 95

> 1 leap 27 19

2 X 4.53 p < 0.05 ======· ------======--�--�--�--�--c::c:--c:c::--c::c:-=--=--======

68

et al. (1979) argued in favour of different strategies of

aggregation and dispersal in relation to colour in locusts

displaying phase polymorphism. A. conica 1 s sustained

aggregated distributions seem extreme in view of other acridid

distributions.

One of the consequences of such aggregation is that

predators will concentrate on prey clusters. vertebrates

will tend to search longer and more intensively in areas of

high prey aggregation (Brower 1958, Smith and Dawkins 1971),

and invertebrates will congregate because of their own

aggregative responses (Hassell and May 1974, Bernstein 1982).

While the prey aggregations may be harder to find (Paloheimo

197la,b) there is potential for heavy mortality once they are

located.

High density aggregations influence the nature of.

predation as well. Because encounter rates of predator and

prey will rise, prey which present a novel stimulus in terms

of colour, shape or size will tend to suffer disproportionately

heavy predation, in accordance with ideas of stabilizing selection

at high densities (Alcock 1973, Erichsen et al. i980). Further,

the protective benefits of crypsis are lessened at high densities,

as pointed out by Kettlewell (1955, 1956) who showed that cryptic

morphs close to a conspicuous one have a much higher risk than

those at a distance.

However, aggregation can confuse and delay a predator by

providing a plethora of targets and making it extremely difficult

to follow a single animal from first spotting to capture. When

a disturbed grasshopper leaps it is likely to set others jumping, ---·---�---.----�� f ======-=--- . ·-, -··--·- - \� - - ..:- -

69

and, as the jumping analysis showed, is unlikely to come to rest

until it is in the best position for crypsis. In maintaining

aggregations grasshoppers may be forced into areas of predominantly

unsuitable background colour, but this is offset to some degree

by the le$sening of the importance of crypsis at higher densities.

Conspicuousness under such circumstances may even be an aid to

survival, as Gillett et al. (1979) in a study of Schistocerca

locusts, argued that gregarious locus·ts ought to be conspicuous

to enhance the confusion effects caused by h_igh densities.

The deimatic display of A. conica functions within this

confusion effect. It is unsuitable for frightening predators

because all grasshoppers have a similar display, the only

difference being a sexual dimorphism that makes the female's

display purple and the male's yellow. With all grasshoppers

in an aggregation displaying this way the element of surprise

in repelling predators, which is essential for the success of

this defence, (Coppinger 1970) would be lost. Rather, predators

would be able to form search images for the displays that would

make it easier to recognize grasshoppers in flight. The

accompaniment of the flash of colour with a loud crackling

noise in flight aids this.

The defensive quality of the display may come from its

power to distract a predator when a large number of conspicuous

animals leap in synchrony, so that it is hard to decide which

to follow. The ease with which the grasshoppers can be followed

in flight permits a better assessment of their speed, which in

itself could discourage pursuit (Deiner 1976) . Further, predators

form search images for the grasshoppers in flight where they are 70

much harder to catch, while they appear very different when they are settled and more vulnerable. This approach has been used by many animals, including fish (Thresher 1977) and lepidopterans (Kettlewell 1973}, and it has been attributed to

Acrida spp. by Whellan (1973}.

However, in aggregations rar� forms eould be at a disadvantage, since oddity has been shown to be as important as specific search image in prey selection (Mueller 1971).

On this basis, the sexual dimorphism would be expected to lead to different mortality patterns between the sexes, as can be seen in the population dynamics data. The initial heavier mortality of females, which produces the disparate adult sex ratios, appears to be based on size. In the adult populations females therefore would be in danger of suffering further "rare form" excessive predation on the basis of their size alone.

This is counteracted by having a different deimatic display to the males, which makes it useful as a true startle display because of its comparative novelty. Deimatic displays are reported in other Acrida species, and in some cases these include the hind wings as well as the dorsal abdominal surface

(Fraser-Rowell 1971, Whellan 1973}. Similar factors may be involved.

The tendency of both sexes to increase the proportions of green individuals during development is in contrast to the increasing brownness of the vegetation at the study sites.

Given the connection between colour and the type of food eaten as well as the humidity of the microhabi tat, this s_uggests juveniles are choosing greener backgrounds deliberately. The ------.- -.�--- --

71

alternative explanation of differential mortality is discussed in the chapters on predation by invertebrates and vertebrates.

Choice of green background could occur because of advantages in growth and development arising from different diets, and there are many examples in the literature of grasshopper sizes, development rates and fecundities being influenced by food

(Waldbauer 1968, Bailey and Mukerji 1976 for example) ,. Some data for A. conica are given in the chapter on diet. Females are more likely to be_ green and to occur in green vegetation, perhaps because they can gain more in tenns of fecundity by being well fed. The association between sex and distance from the water at site I is further evidence of this. Females may choose to move into the lusher vegetation for the nutritive benefit and possible improvement in fecundity. This has consequences for predator avoidance, because predators seeking larger prey would concentrate their efforts in greener areas.

Females therefore may pay in terms of higher mortality for the separation, while males gain a degree of immunity by keeping to other areas.

At site III the vegetation remains very green during the summer because of the level of the water table. The high proportion of green grasshoppers at all stages observed is expected because of this, and need not imply microhabitat selection. However, there was a tendency for more adult females than males to be green at site I in all years, and at site II in

1981-1982. This association was not found at site III, although the analysis there was complicated by the low proportion of brown females. At site IV there was no association between colour and

J 72

sex as well, and this was the only site to record more brown females than green ones. The lack of consistency in these results between sites reflects the complexity of factors that influence colouration. However, since no situations were found where males rather than females were green, it may be that deliberate selection of a green microhabitat is restricted to females.

The high incidence of brown first instar individuals in both sexes at all sites except site III is interesting, and suggests that the environmental conditions of the parents or of the eggs has an influence on the colour of first instar nymphs.

Perhaps genetic factors dictate the colour of the first instar, but are overridden later by environmental effects.

The height of the background vegetation did not have any influence on grasshopper distribution, suggesting that other factors were more important influences. The data on regurgitation show that it is not a universal response, and well under a half of the grasshoppers handled did not regurgitate. The higher incidence of regurgitation in the first two instars may reflect their likeli­ hood of being attacked by spiders or small lizards, which can be deterred by regurgitation (Lymbery and Bailey 1980). Older grasshoppers which are more likely to encounter birds, have less use for this response.

Overall, the anti-predator defences of A. conica exploit the confusion resulting from an aggregated distribution. These aggregations could disadvantage the females in the early stages because their increased size makes them more conspicuous.

Because crypsis is not as essential for aggregated animals, =--=�======--=--=--=--=--=--=--=-�------..···-- -�===- - -=--=--=--=-======�======--::-""".=

73

grasshoppers have some protection until their colour changes

to match shifts in the background. However, one of the

main assumptions of this discussion is that grasshoppers can

detect and respond to their background colour. This

assumption is tested in the next section.

4.3 Background Colour Matching

The aim was to determine whether juvenile A. conica would

choose their backgrounds deliberately. This would explain

whether deliberate background colour selection could contribute

to the association between grasshopper colour and background

colour observed in the field.

The experiements were carried out in cages of unpainted

plywood with clear perspex fronts measuring 37 cm wide x 48 cm

long x 39 cm deep. Illumination was provided by an unshaded

40W light bulb mounted centrally in the rear wall. One half

of the floor of each cage was covered with fresh,_ green leaves

of the grass Avena fatua, and the other with older, yellow-brown

leaves of the same grass. Uprooted green plants of ·A. fatua

were secured in small, water-filled polythene bags to maintain

freshness and placed on the green layer, while similarly treated

yellowed plants were placed on the other side, As much as

possible the size and positioning of plants was the same between

both sides of the cage in an attempt to equalize factors such as

local temperature and basking sites which could have influenced

the movements of grasshoppers, especially younger ones (Kelly­

Stebbings and Hewitt 1972) ..

! I ,______.,. ------::r:::=-----1...: -__-., - --.7 -_ ------. �

74

Third and fifth instar grasshoppers of each sex were used in the experiments. A group of ten grasshoppers of the chosen sex and instar was placed into a central zone 8 ems in diameter on the floor of the cage. The number of grasshoppers resting on the matching background was recorded at 15 minute intervals over the next two hours. -Each trial was repeated four times, using different grasshoppers each time. The first two trials used brown grasshoppers and the second two green ones. Green and brown animals were never mixed in the same experiment in case any desire to clump overrode background preferences.

Results

The distributions of the grasshoppers during the trials are shown in Figure .13. The null hypothesis that the number of grasshoppers matching their backgrounds was unchanged with time was tested against the alternative· that increasing numbers settled on matching backgrounds using the logit function, which is a quantal response model (Snedecor and Cochran 1976). These models use data in the form of x individuals responding out of n, and observations are treated as binomial variates of means which are related to one or more specific variables. An analysis of covariance was done using time as a covariate and sex, instar, and morph as factors, with a GLIM system (Baker and Nelder 1978) on a Sperry 1100 computer. Analysis of deviance showed a significant trend for the number of grasshoppers on the correct background to increase with time (p� .001}, although the slope varied greatly depending upon sex, morph and instar. A complete analysis of deviance table is not presented because it is likely Figure 14. Number of grasshoppers out of 10 settled on the

matching background at 15 minute intervals in a

background choice experiment. The experiment

is repeated for green and brown males and

females. Third and fifth instar grasshoppers

are marked as

o-o third instar

fifth instar ======------·

BACKGROUND COLOUR MATCHING

0 GREEN +

10

8

6

4

2

+0 BROWN § 10 Q 8 8 ,,0...... 8 / µ� / 6 / / 0 Q s 4 µ'...j 0 @ 2 8 0 0 ,::i::

L') 6 GREEN . � z H 10 @J /o- -c, ::r::t3 8 .,..,0..,, / " .0 '-...

6 BROWN • 10 ,,...... _ • --0...._ 8 '() r>-... / I ',o/ 6 I I I 4 ,j I

2

15 30 45 60 75 90 105 120 15 30 45 60 75 90 105 120 ------���======- -=-=-=r___ ::_-_-�---c---:-"_--:-�·-1

75

that the factors are compounded with the initial proportion of ( grasshoppers sited correctly.

Discussion

Deliberate background colour matching by insects can result

from a visual assessment of background colours (Gillis 1982), or

by matching the background reflectance (Kettlewell and Conn 1977).

Although the data collected cannot differentiate these possibilities

in the case of A. conica, they indicate deliberate choice of back-

ground colour by the grasshoppers. This.could in itself explain

the association between grasshopper colour and background colour

in the field, without invoking differential mortality of the

morphs on different backgrounds (Cox and Cox 1974). It also

explains the tendency of grasshoppers to jump again if they

land on contrasting vegetation.

However, background colour matching must conflict with other

aspects of predator avoidance such as aggregative behaviour, and

with eating the most suitable foods. Grasshoppers may reduce

the first problem by exploiting the tiny variations that occur

in plant colouring to obtain a degree of matching in otherwise

contrasting areas. In any case, they may be compensated for

conspicuousness by the advantages of aggregation, where

conspicuousness may even be an asset (Gillet et al. 1979).

Diet is a more important problem because of its regulatory

effect on growth rates and fecundities (Bailey and Mukerji

1976, Bajoi and Knutson 1977, for example). This is likely

to be a greater difficulty for females, since female acridids 76

are larger than males and likely to need more food.

Brown grasshoppers moving into lusher areas to improve their diet therefore would have to rely on aggregation and associated defences to protect them. The relative success of the different defences is assessed in the following chapters. 77

5 • ARTHROPOD PREDATORS

5.1 Introduction

Birds and other vertebrates are regarded as the most

important predators of grasshoppers. However, this under-

estimates the potential influence of an enormous range of

predatory arthropods. For instance, Enders _(1975) noted

that :

... spiders are the numerically dominant

insectivores in many terrestial communities.

After parasitoid insects, spiders may be the

insectivores most likely to encounter any

individual insect.

As well as spiders, mantids (Rilling et al. 1959), some

gryllids and tettigoniids (Key l974), reduvid bugs and ants

(Chapman and Page 1979), asilids (Joern and Rudd 1982),

some Odonata (Greathead 1963, 1966), and scorpions (Barnes

1974) are all documented as attacking grasshoppers. In

relation to the biology of A. conica there were three main

concerns what arthropods were preying on the grasshoppers,

whether they were taking substantial 1 numbers, and whether

there was deliberate or coincidental selection of prey on

the basis of one or more of size, colour, sex, distribution

or age. This information parallels the survivorship data,

showing the impact of arthropod predation. Further, it

highlights the success of A. conica's defences. 78

5.2 Methods

Field sampling

Field work was done solely at site I during 1980/1981 and 1981/1982. The loss of grasshoppers to web-spinning spiders was estimated by counting bodies in webs, the method used by Thornhill (1976a,b) in population studies of mecopterans. Webs were searched for once weekly from the time of first emergence of the grasshoppers, following the same grid pattern laid out for determining distribution.

Searching was done after collecting distribution data.

Because many of the webs were small and spun close to the base of grass tufts it was necessary to crawl the length of

the transect to ensure that no web would be missed. Each

web found was scored for the colour of the veg·etation in

which it was spun, the height from the ground, and the number,

sex, instar and morph of any trapped grasshoppers. Wherever

possible the spider was caught, placed on ice, and returned to

the laboratory for serological analysis. All trapped grass-

hoppers were marked with paint to prevent them being scored

again, although usually all webs were destroyed between

samples.

Other arthropod predators were sampled with 30 6.5 cm

diameter pitfall traps sunk in mid-November and cleared

twice-weekly untii=--��e end of January. The traps were

opened at 1600 hours and cleared between 0900 and 1000 hours

the next day. Centipedes, scorpions and occasionally mantids

were caught in this way, as well as small lizards and frogs.

Predators were placed on ice immediately and returned to the = = ==-=--=---=------_--_----,--_-_-:-:-_:-_:--_:--,1�-�--�--'::'::--'::'::--::C:::- -::c:::--::=--c::c::-=- -c::c::-======::::::�====z:==�:."'."'-"".""-'Z"'--Z'--"."."'-:c:c-I-�=------__ -. -. -_

79

laboratory for serological analysis. Mantids, gryllids

and tettigoniids were usually caught while searching

intensively for grasshoppers. They were placed on ice

and taken for serological analysis. Dragonflies were

caught with sweep nets on an opportunistic basis.

Serological tests

These can be used to identify the foods of animals

which ingest prey fluids rather than particulate matter

(Greenstone 1977) or which chew their food so thoroughly

that conventional gut analysis is inapplicable (Sutton 1970).

The basic principle is that mammals or birds injected with

prey proteins form specific antibodies against them, and

these can then be extracted from the blood and used to

identify antigens identical or similar in composition to

those which stimulated their production. There are many

variations in methodology, and these are reviewed by Service

(1976)_, Southwood (1978) and Calver (1984).

Antigens were prepared from a collection of second,

fourth and adult instars of A. conica in equal proportions

by weight. This ensured that the antiserum was broad enough

to detect all stages, because age-specific proteins are made

during metamorphosis (Zaman and Chellapah 1963, 1965, Smith

and Silverman 1966). The method was a combination of those

described by Pickavance (1970) and Leone (1947). The protein

concentration of the antigen extract was 3.5%, using the

Biuret technique. This was well above the minimum 2%

recommended by Pickavance (1970) for antiserum production.

° The final antigen was stored in 0.7 ml aliquots at -20 c. - - ·=-=-====·=--=--=--=--=--=-=--=-=--=··c-'-"··=- -=---=---=--=-·=> =--=--=--=--=--=---=-·=--=-=--=- ======·-Zi'--"�--'.c'.c--'.:::'-.-:cz---::::---�-�-c::I1::==J,t::.,------: •. - _- . : ___. _- - - -t

80

Six adult male guinea pigs were used for antiserum

production. Although rabbits are used more commonly,

guinea pigs are cheaper to buy and house and produce

excellent precipitating antisera (Crowle 1973). The

injection schedule used four injections of 0.3 ml antigen,

which were given subcutaneously with three clear days being

left between injections. A short injection schedu_le with

one route of injection and no use of adjuvants produces a

specific antiserum, at a possible cost in strength (Wright

1966, Pickavance 1970). Although some authors claim high

strength and specificity for injections in the lymph nodes

(Newbould 1965, Service 1973), Rothschild (1970, 1971) found

no differences in either titre or specificity between rabbit

sera produced by l:Ymph node injection and conventional means,

so the simpler subcutaneous injections were used. The

guinea pigs were bled from the heart 10 days after the last

injection, and the blood processed using the method of

Pickavance (1970).

The specificity of the serum was tested against antigens

prepared from other orthopterans at the study site using the

miniature Ouchterlony plate technique. These included other

acridids, gryllids, tettigoniids, mantids and phasmids.

Cross-reactivity occurred against the other acridid antigen,

and to a lesser extent against the gryllid and tettigoniid

antigens, and was elminated by batch absorption (Dempster

1960) The titre of the specific antiserum was then tested,

and found to be above the minimum recommended by Pickavance

(1970) for field studies. Specific antiserum was divided

° into 0.7 ml aliquots and stored at -20 c. 81

The abdomens of predators for testing were thawed, 0 macerated and soaked for 48 hours in insect saline at s c to extract saline soluble proteins. A sample of this was then run in an Ouchterlony plate test, using the well configuration shown in Figure 15.. This allowed four predators to be tested simulataneously, and provided two controls one of insect saline, and a second of antigen solution as a check for the titre of the antiserum.

Serological tests have been described as "frequency tests" by Martin and MacKay (1982) because they show only the number of prey taken. However, predation rates can be calculated (Dempster 1960, Rothschild 1966, Sunderland and

Sutton 1980), although the required calibrations are extensive. The number of arthropods tested in this study was too small to prevent a large standard error on the estimates, so no rates were calculated as the uncertain estimates did not warrant the effort in calibration.

Optimal prey size

Optimal prey sizes of mantids were calculated using the equation of Holling (1976), based on dry weight.

Similar estimates were made of the maximum prey size of the wolf spiders, using the rule of thumb of Enders (1975) which predicts a maximum prey size of half the body length.

The sex, morph and instar of grasshoppers killed by web­ spinning spiders was obtainable readily from their webs. ---� -.------. -- --::r==v------

82

5.3 Results and discussion

Spiders

All three spider species tested attacked A. conica in 1980/1981 and two species in 1981/1982 (Table 15).

The high proportions of positive results in web-spinners probably reflect the low height of many webs (Table 17) which were set in the areas where grasshoppers were active.

It is unusual for grasshoppers to be caught in webs in large numbers, because their stocky build enables them to crash through the sticky strands (Nentwig 1982). However,

A. conica is more vulnerable than other acridids because of its elongated shape, exposing more surface area for entanglement for the same body weight. This explains why fewer of the larger juveniles are caught (Table 161. In adults, the position is complicated by the wings, which increase greatly the vulnerable surface area. This is more serious for males than for females, since the males strike the web with less momentum and are snared more easily.

The data reflect this, showing that more adult males than females were caught (Table 16), and that more adult males were snared than fifth instar males. Of course, the disparity in the numbers of adults could be also a product of the unequal sex ratio at this stage.

In both years more green grasshoppers than brown ones were caught (Table 16), and most of them were fourth instar or younger. The preponderance of green grasshoppers caught reflects the larger proportion of webs spun in largely green 83

areas. Given the association between grasshopper colour and background colour shown in chapter 4.2, the location of the webs made green grasshoppers more vulnerable. In turn, it also contributed to the disproportionate numbers of females caught (Table 16) because of their higher proportion of green individuals.

The lycosids attacked grasshoppers as well, but the proportion of positive results was low. This probably reflects their maximum prey size, which restricts them to attacking second instar or smaller grasshoppers (.Table 18).

Since the grasshoppers spend only a short time in these instars (see chapter 3) they would soon be beyond vulnerability.

However, it is possible that during the brief period of prey availability lycosids may prey extensively on grasshoppers.

Even then, there is no morphological or distributional evidence to suggest that any sex or morph of grasshopper is prefer:red as prey. The single positive result for the sparassid in 1980/l981 is unusual, because huntsmen live on trees rather than on the ground, and would not meet grasshoppers· often.

Mantids

The optimal prey sizes calculated for the mantids

{_Table 18)_ show that they are likely to concentrate on second and tfiird instar grasshoppers, although females of one spe.cies- may attack fifth ins tar females and adult males. However, it is difficult to estimate any predation - - ---�-,--_-.-.-.---:.�1

84

rate from the proportion of positive results because of the

low density of mantids and their defence of territory

(Mackinnon 1970), and their broad diets. At times of

abundant activity they probably take a wide range of prey,

and the inclusion of A. conica would be part of this,

without greatly influencing grasshopper numbers.

Other predators

The only .other positive result was from a single

gryllid in 1980/1981. This would represent only occasional

predation, and it seems that most of the mortality caused

by the arthropod predators is by web-spinning spiders. The

bulk of this mortality occurs in the early instars, and

disproportionate numbers of females are taken because of

their predilection for greener areas where webs are more

frequent. Of course, other invertebrates not discussed

here may attack A. conica eggs, and gryllids could be

important here. Further, spider predation does not explain

the drop in female numbers relative to males in late instars.

This suggests the involvement of vertebrate predation. 0 GOG 0

Figure is. The pattern of wells for gel diffusion tests.

Each test ran the antiserum against A. conica CAs)

against extracts from the digestive tracts of two

potential predators CAg i and _Ag2)_ prepared in

saline. There is a negative control of saline

only CC J.l and a positive control of A,. conica

antigen in saline (C 21.

,-_ I j •--"------_ Table 15. Results of serological tests on potential

arthropod predators collected at site I in

1980/1981 and 1981/1982. Order Family Species No. tested % positive

1980/1981 1981/1982 1980/1981 1981/1982

Araneida Lycosidae Lycosa sp. 17 25 35.3 24

Sparassidae- 1 1 0 100

Argiopidae Argiope sp. 14 31 57.1 54.8

Scorpiones Scorpionidae Urodacus novaehollandiae 1 3 0 0

- Mantodea Amorphoscelidae 1 2 0 50

11 l Mantidae Orthodera sp. 7 54.5 71.4 r:' 6 87.5 33.3 Archimantis sp. 8 i':: 1)l• 1 1 0 0 0 q ;! :,: ,·,,, ;'i! :,;i 9 Odonata ."Aeshnidae Aeshna brevistyla 4 0 0 i;:i

1 0 Orthoptera Gryllidae 23 16 4.3

1 0 2 0 Scolopendromorpha ======---a===--c======�•------"--°:'•c�_c:.-._.--..-. �- -•-.

Table 16. Sex, instar and morph of grasshoppers caught

in spiders' webs at site I in 1980/1981 and

1981/1982. ======·------=--=---�--=---=--=--·=--::.ai:-·=:=='-�-=--=---=--=:c--=--=--=- -=--=-=---=--=-=--====--=--=-=---=--=--=---:::::c--===��

1980/1981

Instar Males Females

Green Brown Green Brown

1 0 3 15 1

2 20 6 32 8

3 14 5 11 2

4 3 1 16 10

5 0 0 5 2

6 10 4 2 0

7 1 0

1981/1982

Instar Males Females

Green Brown Green Brown

1 3 0 0 0

2 7 1 19 2

3 11 8 16 10

4 0 2 12 4

5 5 1 6 1

6 12 2 2 1

7 1 2 ----==-======:-:::r.=--=--=--=--=- =-=--=---=-""--'--=--=- =-·=- ·=-·-3=- =--=-=- - =--=-�-=- ======_.__=-r:- --=-.c=-, --=-=-==--=--=-

Table 17. Height above ground and background colour of

spider webs found at site I. 1980/1981

Height above % of background green ground (ems) 25 25-50 50-75 75 Totals

15 8 17 64 13 102

15-45 0 14 17 2 33

45 1 8 3 1 13

Totals 9 39 84 16

1981/1982

Height above % of background green ground (ems) 25 25-50 50-75 75 Totals

15 6 20 38 7 71

15-45 1 4 19 3 27

45 0 10 9 4 23

Totals 7 34 66 14

-1 ------·------.-- -- '------======�=:--======

+ Table 18. Optimal prey sizes (- standard error) of lycosids

and adult mantids preying on A. conica. Sizes

· are estimated from the formula of Holling (1976)

for mantids,' and from the generalization of

Enders (1975) for lycosids.

I _ _:_------1 -===-=--=--=- =-=====--=·===--=·· =· -=-=--=-=-=---=-:::_t,.:--�----_-__·c.�---- - � -·- --- �--j � ------_; __ _;_ -_;;;· -_·;:----

Species N Optimal prey Optimal Instar size (mm)

MANTODEA

Amorphoscelidae 1

o>r 1 8.2 1 o"", 1 �

0 +- + 2 11.0 1. 2 1 0-'r, 1 �

Orthodera sp.

1 20.3 2 (Yr ' 2 �

Archimantis sp.

5 25.7 + 1.1 3 err, 3 �

0 + 10 28.4 + 2.9 4 d1"' 4 �

M 1

o..:r 1 28.4 4 d"',

2 44.2 + 5.6 6 (ft" ' 5 �

ARANEIDA

Lycosa sp. 2 15.3 + 1.8 2 �

N No. of individuals used to estimate optimal prey size.

- I

I I ------======·-=--=--=--=---= =--=-·=·-=- -=--·=--=-'=---=--=--=· =--=--=-=-=---=· -======--=--=--=- =:::-:::::===ic=---�-=�======--t--'"-'"-'"----:4

85

6. VERTEBRATE PREDATORS

6.1 Prey choice in the field

Grasshoppers are attacked by all vertebrate classes

except fish. The aim of this section was to document which

of the potential vertebrate predators at site I attacked

A. conica, and to what extent the predation was sex, size or

morph specific. Work was done in 1980/1981 and 1981/1982.

Methods

Lizards and frogs were caught either by hand during

sampling for grasshoppers, or trapped in the pitfalls set

for invertebrates (chapter 5.2). They were placed on ice

immediately after capture to retard digestion as much as

possible, and returned to the laboratory for analysis. They

were then gutted and the particulate contents of the digestive

tract examined for grasshopper remains. The fluid contents

were tested serologically using the procedure given in

chapter 5.2.

Birds were caught with three mist nets 40' long and

6' high and set for two consecutive evenings at fortnightly

intervals from mid-November until the end of February, This

ensured that birds had been feeding before capture, and the

low frequency of trapping minimized the chance of resident

birds learning the location of the nets (MacArthur et al.

1973, Lovejoy 1974). Birds caught were kept overnight to

collect a faecal sample and released the next morning.

Faecal samples were examined microscopically for grasshopper

remains. Although the technique has been criticized because

·- ! 86

of differential digestibility of prey items (Hartley 1948), it allows sequential samples from the same individual

(Bryant 1973) . Further, Davies (1976) found it gave similar results to an emetic technique, while proving less stressful

for the birds.

The number of A. conica eaten was determined by counting heads, which were often voided intact, pronotal shields or the characteristic mandibles. Counting of mandibles was

used as a check on the accuracy of the other methods. Heads

and pronotal shields gave the maximum amount of information

because they enabled the colour of the grasshopper to be

determined, and its size to be estimated by comparing the

length of the fragment to the known dimensions of intact

insects. While male and female grasshoppers could not be

distinguished, it was possible to decide which instars could

have been taken.

Magpies were the most numerous of the avian insectivores,

but were too large to be mist netted. However, the sex and

colour of the adult grasshoppers they fed on could be determined

by using binoculars to watch them feed. They tended to walk

steadily forward, occasionally seizing prey with a sudden

lunge. The larger adults were invariably mandibulated for

a short period ranging from a few seconds up to over a minute,

giving a chance for the sex and colour of the grasshopper to

be determined. Magpies moved across the site twice daily

at approximately 1000 hours and 1600 hours, taking 30 to 45

minutes to move through. The total flock was never more

than eight birds, although not all were seen on the same ======-=--=--=-=--=--=--=- �::c__c_ -_,__:,,:.·=··=·=· =-..,=. ======�-:C::C-c-C:::•:=:=J___=flC",C'?:"""---;,.z;. -:::-7 -a':"'·=- -:,a:,;-_=--'"""--•>?,_--�---�--�--_2"'_2"'.I=�< -_-__..__-__.. ______-.-:_:.:-_.--..--�-

87

occasion. They were unconcerned about the presence of

people, and could be watched from as close as 10 metres

without being disturbed, provided no sudden movements were

made. Magpies were observed during both the morning and

evening feeding sessions on the same days when mist netting.

On three occasions magpies were observed to regurgitate

pellets while feeding. These were collected and examined

microscopically for grasshopper remains.

Results

The birds caught in mist nets are listed in Table 19,

together with the number of grasshoppers they were found to

have been eating and the mean pronotal lengths of the prey.

Observations on the colour and sex of adult A. conica

attacked by magpies appear in Table 20, together with a

comparison against the ratio of the morphs in the grasshoppers

sampled for population studies. Morphs were not caught in

the proportions in which they appeared in the population,

with green ones being eaten more frequently. However, this

analysis was restricted to males because of the small number

of adult females eaten. Analysis of the three regurgitated

pellets found showed the pronotal shields of five green

grasshoppers with a mean length of 6.8 mm, equivalent to

fifth instar females and adult males. Finally, none of

the lizards and frogs tested had been taking A. conica

as prey.

------· · \_ _:_ I -======--=-=-� r------.-.--=·_:cc- -=--=::=--==-=-- =- -=--=--=-======---'<=--=--=-=--=--=-=-�-=�

Table 19·. Birds caught at site I which could have been

feeding on A. conica. The nurr:.ber of birds

caught is shown with the number which had

grasshopper remains in their faeces in

parentheses. The mean pronotal lengths of + grasshoppers are shown - standard errors,

and the number of intact pronota measured

are shown in parentheses. 1980/1981

(mm) Species No. of birds No. grasshoppers x prenatal length caught Green Brown Unknown

6 :!: Singing Honeyeater 8 (4) 5 1 3.0 0.3 (2)

(Lichenostomus virescens)

Silvereye 4 (O)

(Zosterops lateralis)

7 3 :!: Little Wattlebird 6 (5) 2 5.6 0.6 (2)

(Anthochaera chrysoptera)

Magpie Lark 1 (1) 1 2 0 4.8 (1)

(Grallina cyanoleuca)

+ 13 Grey Butcherbird 3 (3) 10 0 5.0 - 0.1 (4)

(Cracticus quoyi)

1981/1982

(mm) Species No. of birds No. grasshoppers x prenatal length caught Green Brown Unknown

Singing Honeyeater 3 (1) 2 0 0

(Lichenostomus virescens)

Silvereye 15 (0)

(Zosterops lateralis)

Little Wattlebird 2 (1) 1 1 2 4.9 (r)

(Anthochaera chrysoptera)

+ Grey Butcherbird 5 ( 3) 8 6 1 6.0 - 0.8 ( 2)

(Cracticus quoyi) ------�------. ---·--· ------=.j ------::r=::·------�

Table 20. The sex and morph of adult A. conica caught by

magpies at site I. The proportions of green

and brown males are compared against expected

figures calculated on the basis of the ratio of

the morphs collected in population studies (see

Appendix 1). The numbers of females are too

low for such analysis.

Summer 1980/1981

Males Females

Green Brown Green Brown

Caught by 47 8 3 1 magpies

Expected 29.7 25.3

2 X 21.91 p <. 0.01

Summer 1981/1982

Males Females

Green Brown Green Brown

CaugI:t by 39 11 6 1 magpies

Expected 29.9 20.1

2 = X 6.89 p <. 0.01 _ ------I------�------�

- - � 88 �

� -

Discussion

With the exception of the Silvereye, all bird species

caught were found to feed on A. concica. Estimations of

the size of grasshoppers taken suggested that the birds

attacked third instar animals and larger, concentrating on

a different size range to the invertebrate predators. The

sizes of prey varied with each bird species, as would be

expected from their variations in mass and bill dimensions

(Hespenheide 1973, Wooller 1984).

Although the number of birds sampled was small and

the catches distributed unequally across the season, the large

proportion of positive results suggests that predation rates

on A. conica were high. This contention is supported by the

incidence of multiple feeds by single birds. Since none of

the other vertebrate predators tested were found to be positive,

birds appear to be the principal predators of third instar and

older grasshoppers, and the attention of the largest of these,

the Magpie Lark, the Grey Butcherbird and the Western Magpie,

would focus on the bigger females. Sub-adult females would

be attractive prey for them, being both large and unable to

escape by flying. Such predation is likely to be a major

factor in the increase in female mortality that commences in

the third instar and continues throughout the life history.

The sample of grasshopper remains identified was too

small to permit meaningful comparisons of the proportions of

the colour morphs eaten, but the observations on the adult

male grasshoppers eaten by magpies showed that green animals

were eaten in disproportionate numbers. This could reflect ------� ------

89

their comparative conspicuousness against the browner vegetation as summer progressed, or an innate colour preference for greens by the birds. However, there is no evidence for or against the last hypothesis. Both hypotheses suggest that suh-adult females would be predated more heaMily because of their tendency to be green, an argument in favour of the general proposition that bird predation is heaviest on females. More detailed tests of the role of size and colour in the prey choice of magpies follow.

6.2 Prey choice in the laboratory

The preferred prey sizes of birds suggested by the field data could result from a number of factors including deliberate prey size selection, or the distribution and behaviour of grasshoppers rendering different indivudals more liable to predation. Of course, these factors could interact considerably. To assess the influence of interaction, the success of magpies at catching grasshoppers at different densities on a range of backgrounds was investigated under controlled conditions. Both live and dead grasshoppers were used in successive trials to differentiate between pure crypsis in the dead ones and the interaction between crypsis and behaviour in the live ones. The size preferences of the birds were assessed separately by offering them choices of pairs of grasshoppers of different sizes, and the optimal prey size was assessed by relating handling time and energy

content for different sizes of prey.

I I ------·'-----l-��------_------_ ::-c "",'-�'CC;�.e-,=�-:c��::i::=,--,, ------_ - - - - -c:cec�::::�:_,�i==.r.,;_::::::::"'.:::-:-:.c:•-�:: : z:-;,�---;;cc-::,J_i,------;--.--:::-d

90

Methods

Test arenas were made using sheets of stiff white card

100 ems x 40 ems, arranged to form a square. Four backgrounds

were used, classed as long brown, short brown, long green and

short green. The short green background was formed from fresh

lawn clippings, and the long green background had a similar

base covered with thick handfuls of fresh Avena sp., a common

grass in the field. Uprooted fresh plants of Avena sp. up

to 15 ems tall were stood on this to cover the base. The

brown backgrounds were formed the same way, but using dried

lawn clippings; arid old, dried plants of Avena sp. Only

grasshoppers of one morph were presented at a time, and fourth

instar females were chosen because they represented an inter-

mediate size, but were unlikely to leap clear of the arena.

Grasshoppers were chosen without stripes, and the colouring

of the brown ones was as uniform as possible.

Three magpies were used, all tame birds raised in

captivity. They were tested separately in a room measuring

3m x 3.Sm and illuminated by overhead fluorescent lighting.

Live grasshoppers were presented first. The order of

presentation of morphs and backgrounds was randomized because

of possible influences of subject search image formation and

learning during trials (Greenwood and Elton 1979). All four

backgrounds were prepared to enable ready switching during

tests. Grasshoppers were presented sing.ly, or in groups of

three or five. The arenas were placed on the floor, and

grasshoppers dropped in from a height of approximately 100

ems. The magpie would watch to see which arena was used

· 1 I::------_...c ,,...:_-..::--_- ... __ -..::-_-_ _:-.:-_ - 1 "I'";:.:�---::_-..= _ ------<---�----�--:----=----=[ ,------,------_____ --;_ __

91

before running up, mounting the wall, and searching. Birds

were timed from when they mounted the wall until a grass-

hopper was seized in the beak. The birds were starved for

24 hours before trials to ensure they were well motivated,

-arid a two hour break allowed after eight grasshoppers were

eaten in case the birds had digestive pauses. Three

replicates of ea6h trial were taken.

The same experiment was repeated using dead grasshoppers.

In this case, grasshoppers were killed by freezing and kept

chilled to pre·serve the colours. They were positioned by 2 superimposing a grid of cells 5 an on the arena, and locating

grasshoppers using random number tables. The grasshoppers

were positioned outside the test room, and the arena carried

in and presented to the test bird.

Size preferences of the birds were checked in a series

of tests in which pairs of female grasshoppers of the same

morph but different instars were presented together. The

pairs presented were : adult and 6th instar, 6th instar

and 5th instar, 5th instar and 4th instar, and 4th instar

and 3rd instar. Each pair was presented 10 times, five

times using brown grasshoppers and five times with green

ones. The grasshoppers were frozen to death� then placed

in a realistic pose on a 15 cm x 10 cm piece of white card

in a bird's cage just before the normal feeding time.

Whichever grasshopper was taken first was noted.

The handling times of magpies were investigated by

feeding live animals of known instar to the birds and

recording the time between seizure and swallowing. Third - i.:---- ..::... - ___ :_-__ - -..:_-.:-:_. ==--=--=--=--=--=--=-======�-=---=--c:c::--c:c:--c:c:-====:::J'-"'"'---"'�--=�==- :,::,--�--�----�---�-c::r-1=------� -_- -�_:---��d-

92

to adult instars of both males and females were used in

the experiment.

Results

The times taken by magpies to capture a single live

grasshopper were analysed with a five way ANOVA, after

logarithmic transformation of the data to correct for

correlation between the means and variances of the cells.

Because there were no higher order interactions involving

subjects, the results from all three subjects were combined

for a single analysis. The ANOVA results appear in Table

22, and the means and variances of the main effects in

Table 21.

Of the main effects, background length and number of

grasshoppers presented were significant (p � 0.001) in both

cases, with grasshoppers on longer backgrounds taking more

time to catch, and capture times falling with the number of

grasshoppers presented. While background colour was

insignificant (F = 3.733, p = 0.057) the closeness of the

result to the critical value would suggest the need for a

larger sample to be taken here to determine conclusively

if brown backgrounds confer any slight advantage to grass­

hoppers.

The two way interaction of background colour by morph

was significant (F = 27.493, p < 0.001), with grasshoppers

on matching backgrounds taking longer to catch. Brown

grasshoppers were better protected on the long backgrounds,

with the background length x morph interaction having an

F of 4.445, p < 0.05. Finally, the background length by 93

number of grasshoppers interaction was also significant

CF = 3. 956, p <. 0. 051 with larger numbers of grasshoppers proving more difficult to catch on long backgrounds than on short ones. No higher order interaction was significant.

To test the effect of increasing grasshopper density on catchability, the data on the catching times at different densiti.es were further analysed with a one way ANOVA, specifiying a priori contrasts of the second and third densities (five and .10 grasshoppers), and of single grass­ hoppers _against the higher densities combined. Separate rather than pooled variance estimates were used in calculating t because of unequal variances (Nie et al. (1975). The results are shown in Table 22. The increase in density from five to

10 grasshoppers did not alter catchability (t = 0.355, p = 0.724), although groups of five or 10 grasshoppers had lower catching times than single grasshoppers ( t = 3.569, p = 0.001).

A similar analysis was done for the data on catching times for dead grasshoppers. However, in this case there were significant differences between the subjects, revealed by subject interactions when the data were grouped in a five way ANOVA design. Accordingly, each subject was analysed independently using a four way ANOVA, and the results appear in Tables 23 to 24. All subjects had grasshopper number as a significant main effect, with larger numbers of grasshoppers leading to a shorter catching time. Background length was also a significant main effect for all subjects, with the grasshoppers being better protected on long backgrounds.

Background colour was significant for subject A alone ======--=-=-=-=-=- =-=-=-==------a- ----0----.-.--:.---:.�f • ------,I

94

(F = 10.624, p = 0.003) with brown backgrounds giving

grasshoppers the best protection.

The pattern of two way interactions varied more

among the subjects. Subject A had only one interaction,

between background length and morph (F = 6.583, p = 0.05)

with brown grasshoppers taking longer to catch on long

backgrounds. In addition to this, subject B had

interactions between background colour and background

length (F = 7.289, p 0.012) and between background

length and number (F 6.981, p = 0.004). Long, brown

backgrounds gave the best protection, and larger numbers

of grasshoppers were less vulnerable on long backgrounds.

Subject Chad only one two way interaction, between

background colour and morph (F = 9.904, p = 0.004}, with

grasshoppers matching their backgrounds taking longer to

catch.

For each subject, the influence of grasshopper

density on time to catpure was further investigated by

one way analysis of variance, using a priori contrasts

and separate variance estimates for t as before. The

results appear in Table 24. For subjects A and B, there

was no �dgnificant reduction in capture times when the

density was raised from five to 10 animals, but for

subject Ccapture times continued to fall with increased

density.

In all trials of the size selection experiment the

birds invariably selected the largest grasshopper presented.

There were substantial differences in the handling times of ======------·)------�------

95

the different instars ( Figure 16). Both.large and small

instars took longer to handle, with minimal handling

times occurring at the last or penultimate juvenile instar.

Small grasshoppers are difficult for the birds to grasp,

while the adults may struggle powerfully and the wings and

hind legs inhibit swallowing. Using the dry weight data

given in Table 1, energy contents for each instar were

calculated from the approximation of Golley (1961) and

Bryant (.1973) that arthropod material contains 23J/milligram

dry weight. Assuming that only 70% of this energy is

metabolizable (Ricklefs 1974), each bird's energy return

in kJ/minute if feeding exclusively on each grasshopper

instar was then calculated for each sex and instar using

the equation:

60/handling time (s) x 0.70 energy value (kJ}

The values are shown in Figure .l7. This shows that females

are usually more rewarding than males of equivalent instar,

and th.at subadult females offer the best energy return per

unit time. In two of the three birds, adult males gave

better energy returns than adult females.

Discussion

Because of the artificiality of the experiments the

magnitude of the differences between.different cells in

the ANOVA is less important than highlighting significant

effects. This is because the actual differences in

capture times have little meaning in terms of events in

the field, whereas the significant effects are factors Table 21. Means and standard errors of the cells

of a 4-way ANOVA on the times taken by

three magpies to capture a single live

grasshopper under prescribed conditions.

- I ------1_-::.• ·_· - BACKGROUND NO. OF GRASSHOPPERS

1 5 10

SHORT GREEN

+- +- - Brown grasshoppers 2.0 0.3 2.5 0.4 2.1 + 0.4

+- + + Green grasshoppers 7.2 2.0 4.6 1. 7 3.6 0.4

LONG GREEN

+ + Brown grasshoppers 7.8 1.7 3.5 0.5 5.6 + l.0

+ + Green grasshoppers 14.3 5.9 4.6 0.6 6.5 + l.3

SHORT BROWN

+ + + Brown grasshoppers 8.3 1.8 5.1 0.9 2.9 0.3 + + Green grasshoppers 4.0 1.0 3.7 + 0.3 2.7 0.5

LONG BROWN

+ + Brown grasshoppers 26.0 7.7 7.7 + 2.2 7 .l 1.0

+ + Green grasshoppers 7.0 + 0.7 3.4 0.4 3.6 0.6

-1 i I ======�--ca-=-=-�-=--=-=--=-==�-=-�.:�.:--�------�------_-__ _

Table 22 (i) ANOVA table for log-transformed data in

Table 21. Only significant effects are

included. The factors are :

BKC background colour

BKL background length

NO grasshopper density

MORPH grasshopper colour

(ii) ANOVA table for log-transformed data from

Table 21 comparing capture times at three

densities. A priori contrasts for

specified groupings of these densities

are given. ---:--======-=- =- =-=--= -""-e::::-===-i-.;- co:-c=.::-==-2::- _::_,-ec:,- :cc- -�- -c:c::-�-:c__:c--:c::c--:c::c--�--ci::::, =-=--=--=-=--:":'--=--=--=--:c::-======-:::c-=-:c;:--=--=-:c:::--:c;c-:=--=--=-=--==-�=-cTi=--::rzz=::c::z::::2':'2�:Z::�

(i)

Source of variation ss DF MS F Significance of F

Main effects * BKC 0.265 1 0.265 3.733 0.057

BKL 2.280 1 2.280 32.152 0.000

NO 1.871 2 0.936 13 .193 0.000

2-way interactions

BKC x MORPH 1.950 l 1. 950 27.493 0.000

BKL x MORPH 0.315 l 0.315 4.445 0.038

BKL x NO 0.561 2 0.281 3.956 0.023

* Not significant, but included because it approaches 0.05

(ii)

Source of variation ss DF MS F Significance of F

Between groups 1.5308 2 0.7654 8.412 0.0004

Within groups 12.8298 141 0.0919

Total 14.3606 142

A priori contrasts OF t, value Significance of

Middle density to highest density 89.0 0.355 0.724

Lowest density to higher densities combined. 71.1 3.569 0.001 Table 23. Means and standard errors of the cells

of 3 4-way ANOVAs on the times taken by

three magpies to capture a single dead

grasshopper under prescribed conditions.

Each bird is shown separately :

A opposite

B overleaf

C overleaf ------1 .::_ -,..-_,_ .:------. ------1 ..._.:-.:-�- _-.,::-�---=------

BACKGROUND NO. OF GRASSHOPPERS

1 5 10

SHORT GREEN

Brown grasshoppers 2.9 � 0.7 3.7 � 1.5 4.4 � 0.6

Green grasshoppers 6.1 +- 1.1 3.9 + 0.1 3.3 + 0.3

LONG GREEN

Brown grasshoppers 56.2 + 52.1 3.1 + 0.8 2.2 � 0.4

Green grasshoppers 9.2 � 0.8 5.1 + 0.2

SHORT BROWN

Brown grasshoppers 9,0 +- 2.2 4.4 + 0.4 2.8 + 0.2

Green grasshoppers 13.1 + 1.0 6.8 + 1.2 5.0 + 1.0

LONG BROWN

Brown grasshoppers 163.0 + 136.4 25.7 � 9.9 9.8 + 0.2

Green grasshoppers 13.8 + 9.6 4.9 � 1.1 7.3 � 5.2 BACKGROUND NO. OF GRASSHOPPERS

1 5 10

SHORT GREEN

- Brown grasshoppers 11.6 +- 7.2 6.2 +- 2.6 4.8 + 0.6

- - Green grasshoppers 3.4 + 1.4 4.4 +- 0.8 2.4 + 0.6

LONG GREEN

Brown grasshoppers 15.7 ± 7.5 3.9 + 0.1 2.6 + 0.4

+ Green g-rasshoppers 14.9 + 5.7 4.9 0.2 14.0 + 6.9

SHORT BROWN

Brown grasshoppers 4.4 + 1.2 2.8 + 0.4 1.9 + 0.1

Green grasshoppers 1.6 + 0.4 2.1 + 0.1 1.9 + 0.3

LONG BROWN

+ Brown grasshoppers 77.0 + 48.0 5.7 2.0 2.2 + 0.0

Green grasshoppers 64.7 + 53.5 5.0 + 1.2 4.7 + 0.5

--1 · - ======· -=--=-=-=---- ,_---CC--cc::.c· -=-·-=--=·-=-- -=--'I-f=-= ---C:C:-=·--�--=--=--=--=-=--=--�-�--Z'--2- z:i::-1======�-::c:r-1===-=· �- �--�---�--ez:--�-�--=Z-3-1:=•;:;;_ --_,-_,-_,-_,-_--.c: ·

BACKGROUND NO. OF GRASSHOPP ERS

1 5 10

SHORT GREEN

+ + +- Brown grasshoppers 4.8 - 3.0 3.0 - 0.1 2.4 0.6

+- +- +- Green grasshoppers 8.1 1.9 5.4 2.3 2.2 0.6

LONG GREEN + Brown grasshoppers 13.0 + 1.8 5.2 0.4 4.0 + 0.0

+ + Green grasshoppers 96.2 83.9 11.0 1.2 6.2 + 0.3

-:�: SHORT BROWN

+ +- -+ Brown grasshoppers 7.9 1.1 7.7 2.9 4.2 0.2

+- +- +- Green grasshoppers 4.3 0.7 3.0 0.9 2.9 0.1

LONG BROWN

+ + Brown grasshoppers 18 .7 - 2.8 6.2 � 1.0 4.6 1.8 + Green grasshoppers 9.4 6.6 3.2 :!: 0.2 -�======-==c-c======" ------��--,,-c

Table 24 ( i) ANOVA tables for log-transformed data in

Table 23. Only significant effects are

included. The factors are :

BKC background colour

BKL background length

NO grasshopper density

MORPH grasshopper colour

(ii) ANOVA table for log-transformed data from

Table 2l comparing capture times at three

densities. A priori contrasts for

specified groupings of these densities

are given.

Each bird is shown separately

A opposite

B overleaf

C overleaf

I

! - 7

Bird A

(i)

Source of variation ss DF' MS F Significance of F

Main effects

BKC 1.099 1 1.099 10.624 0.003

BKL 0.648 1 0.648 6.267 0.020

NO 1.887 2 0.944 9.121 0.001

2-way interactions

BKL x MORPH 0.681 1 0.681 6.583 0.017

(ii)

Source of variation ss DF MS F Significance of F

Between groups l. 887.1 2 0.9435 6.142 0.0044

Within groups 6.9128 45 0.1536

Total 8.7999 47

A priori contrasts DF t value Significance of {

Middle density to highest density 29.5 1.408 0.169

Lowest density to highest density combined 18.8 2.744 0.013 Bird B

(i)

Source of variation ss DF MS F Significance of F

Main effects

BK.L 2.080 1 2.080 29.585 0.000

NO 1. 787 2 0.894 12. 712 0.000

2-way interactions

BKC x BK.L 0.512 1 0.512 7.289 0.012

BK.L x MORPH 0.360 1 0.360 5.126 0.033

BK.L x NO 0.982 2 0.491 6.981 0.004

(ii)

Source of variation ss DF MS F Significance of F

Between groups 2.1755 2 1.0877 6.807 0.0026

Within •groups 7.1909 45 0.1598

Total 9.3664 47

A priori contrasts DF value Significance of t _ t

Middle density to highest density 23.9 0.879 0.388

Lowest density to highest density combined 21.0 3.086 0.006

- i ! ------j � - � ------�------� ---1

Bird C

(i)

ss Source of variation OF MS F Significance of F

Main effects

BKL 1.133 1 1.133 17.281 0.000

NO 1.726 2 0.863 13.162 0.000

2-way interactions

BKC x MORPH 0.649 1 0.649 9.904 0.004

(ii)

Source of variation ss OF MS F Significance of F

Between groups 1.7262 2 0.8631 9.229 0.0004

Within groups 4.2085 45 0.0935

Total 5.9347 47

· A pr�ori contrasts OF t value Significance of t

Middle density to highest density 26.2 2.455 0.021

Lowest density to highest density combined 19.l 3.219 0.005

. ! Figure 16 The handling time in seconds (± variance) for

each of three magpies fed male and female

grasshoppers from third instar to adult. Ten

grasshoppers of each instar and sex were used.

Values for the sexes in the third to fifth

instars were too close to separate, and the

points represent both sexes combined. Sixth

instar values apply to females only, while

handling times for adult females (�) and

adult males (o) are shown separately. I Il III

110

90

70

H

60 C.:) H

50 ::c:

30

10

3 4 5 6 3 4 5 6 o:r 3 4 5 6 ('Jr � 9 (fr � GRASSHOPPER INSTAR c± Figure 17 The ene_rgy return in kJ/minute standard

errors). for each o;f three magpies ;feedi_ng

on particular grasshopper instars.

• '- -- •male grasshoppers

o--ofemale grasshoppers ====c::--===c::-======�======---::�-----_-,:--;;:"_--�- -

,-...

lO

LO

t---::::,,-0---I ,-... � -- i:r: --- E-1 U) � --- (0 z - H

-- lO � P-1 � �- U) --- U) M � c_!)

------__ l-0-i

H LO

M

0 0 0 0 0 0 tO LO '¢ (") N ,-

(NIW/.C)I)

! ------:: -· -.:.:.aCI=---;=:=-:=::--:::C--:C::--:C::--:C:C--:C:C- ::C:::--:::::2--:C::-:::::2--:C:C--:C::--::C:C--::C:C--r=�����

96

of importance in other contexts where the behaviour of

the predators at least will be different. Within this constraint, the results indicate clearly the importance

of backround length, grasshopper density and the ability

of grasshoppers to match their background as factors in

predator avoidance. Long backgrounds resulted in longer

catching times because the shape of the grasshoppers is

concealed on lengthy grasses, a feature noted in some

desert acridids by Otte and Joern (1977). Whellan (1973)

notes that the general morphology of the genus Acrida provides excellent camouflage on long grass which the grasshoppers accentuate by flattening the body and hind limbs and placing the antennae together when disturbed.

Further, they tend to slide around a grass stem so that it is always between them and their predator. Although he suggests that crypsis can be maintained on short grass, these results show that it is not as successful. Long backgrounds also provide a greater diversity of resting sites, contrasts of shadow and light, and the chance to burrow if attacked. These factors contribute to the longer times required to catch even contrasting grasshoppers on long backgrounds, where they exploit the advantages of long grass.

While the increase in density from one to five grasshoppers presented caused a drop in capture times, there was no similar reduction when the numbers were increased to 10. Possibly the extra grasshoppers confused the birds, and they were slower to concentrate 97

on and seize one individual. Groups of grasshoppers on

long backgrounds were less vulnerable than those on short

backgrounds, because the long backgrounds gave greater

opportunities to leap and vanish or to disconcert by a

sudden, unforseen jump. Such "confusion effects" have

been documented by Gillett et al. (1979). However,

Gillett and Genta (1978) in a brief review of aggregation

in locusts, refer to studies which demonstrated that

grouping was only useful against the smaller vertebrate

predators, as well as parasitoids. While useful against

birds as large as magpies, the aggregated distributions

of A. conica observed in the field would be of little use

against storks or similar large birds. The observation

that grasshoppers which matched their backgrounds took

significantly longer to catch agrees with the major function

of crypsis. Further, it shows that the cryptic colouration

is aimed at predators with similar visual acuity to birds.

When dead grasshoppers were presented the influence of

grasshopper behaviour was removed, and there were differences

in the capture efficiencies of the birds. These may reflect

differences in search patterns between i�dividu�ls that showed when the grasshoppers could not redistribute themselves.

Background length and grasshopper density remained significant main effects for all birds, reflecting the concealment value of long grass irrespective of grasshopper behaviour and the increased vulnerability of aggregations of grasshoppers.

Capture times for bird C fell with each increase in density, suggesting that appropriate leaping behaviour was needed

_ _ ----- I:: __ 1.-::· · - = ---��---======-=--=--=- -=--=--=-�------=�==·-=--=--=--=- :=::-- :=--::c::c-====-ST,==-,::C::c-=-=:c==-=--=-=--=-=--=:c-:c::=- =

98

for the confusion effect at higher densities. However, the

increase in density from five to lO grasshoppers did not

reduce the capture times of the other birds, presumably

because they were confused briefly in deciding which of

the available_ grasshoppers to grab.

The variations in interaction effects between

individuals- are caused by chance factors of grasshopper

placement which are absent when live animals can move.

They make small adjustments in position to maximize their

crypsis on suitable backgrounds, and of course this is

los:t with corpses.

The preference of magpies for the larger prey presented

in pairs a,grees- with the basic tenet of optimal foraging

theory that predators choose the la,rgest prey available

fJia,t they can handle readily (Emlen l966) . It supports

the data of Vestjens and Carrick (_l974) who found exactly

the same results when feeding phasmids to the Eastern

Magpie (race dorsalis). on the basis of the handling

time data, magpies should prefer fourth and fifth instar

females because they can gain more energy/unit time feeding

on these grasshoppers. This agrees wi:th the field data,

which, whi·le scanty, show grasshoppers in these size classes

being eaten. The individual variation between the birds

is important, showing that overall size selection is the

sum of the. preferences of the individual predators.

Palmer (l98ll a_rgued that prey which vary in handling

time are not functionally equivalent to those that vary in 99

energy return, because the cost of an error in prey selection is greater with handling time than with energy.

Consequently, predators are likely to reevaluate their prey options under varying handling times. If predators are

themselves more at risk when feeding, this will be encouraged. Further, in a study of differential mortality between male and female cicadas (Magicicada cassini), Karban

(1983) noted that females had more than twice the lipid and protein content per unit prey dry weight than males. He attributed this to the females' ovarian development, and suggested that this explained the preference of birds for female cicadas. Similar factors could operate in grasshopper predators, and underlie the behaviour of the magpies.

Field data on the diets of magpies (.Vestjens and Carrick

1974) show that acridids rank as the third most important prey in terms of the number of birds taking them, coming after ants and weevils. However, during summer and autumn over

75% of magpies studied had been feeding on acridids,. showing that grasshoppers are major prey items when abundant. Although no data on the proportions of prey type by volume were given, it seems plausible that the biomass of acridids eaten would be greater than that of ants, and would have a greater ratio of body tissue to chitin that the birds could digest. Further, although magpies ate gryllids, tettigoniids, mantids and cockroaches they took them in far smaller proportions than acridids. It appears that magpies are suited to preying on the orthopteroid orders, and that the behaviour and relative abundance of acridids makes them more common as prey. 100

Overall, both direct and indirect evidence indicate that predation is disproportionately heavier o� females.

In the case of invertebrate predators this is largely the result of different microhabitat preferences of the sexes, while birds appear to select larger females deliberately.

These results lead to several inter,esting questions :

what selective advantage could large females have

to override the heavier predation they suffer?

why do juvenile males and females have different

microhabitat preferences?

what are the consequences for sexual selection

of the unequal sex ratio caused by predation?

They are addressed in the following chapters. 101

7. CONDITION, FECUNDITY AND DIET

7.1 Introduction

The association between sex and colour in juvenile

grasshoppers and in adults at two sites suggests that

the sexes prefer different microhabitats, with the

females preferring lusher areas. This would lead to

differences in diet which could influence the development

rates, survival rates, mature size and fecundity of the

grasshoppers. Grasshoppers are well known to be selective

feeders (Sheldon and Rogers 1978, Joern 1979). Food plants

are located largely by visual and hydroscopic cues rather

than by olfaction (Barton-Browne 1975), and the acceptance

or rejection of a particular plant as food is determined

by palpation and biting (Abushama and Elkhider 1976,

Mordue 1979). Food preferences in the field may result

from phylqgene:tic

1981), the predictability of host plants (Mitchell 1975)

or to a variety of plant chemical deterrents (Bernays

e.t al. 1977} . Competition has been claimed as a cause

of feeding preference by Uekert and Hansen (1971), but

Sale (1974) argued against this on the grounds that grass­

hoppers tend to converge on a few food sources rather than

diverge. The more recent field studies of Joern (1979)

have failed to resolve this area. The physiology of

digestive enzymes is unlikely to be a major factor, as

they are very similar throughout the Acrididae (Morgan

1976). However, differences in the morphology of the 102

alimentary canal may predispose species to special diets.

Although distinct food preferences may exist, these broaden

under conditions of reduced food availability and starvation

(Barton-Browne 1975).

There are numerous laboratory studies of the influence

of restricted diets on the life histbry of a range of

acridids. Generally, the same foods were favoured by all

stages of the life cycle (Bernays et al. 1977), although

they noted that diet changes were often forced during a

season by shifts in plant species availability. Hoekstra

and Beenakkers (1976) have claimed greater digestive capacity

for juveniles, but this clashes with the observation of

Bernays et al. (1977) that diet broadens in mature grasshoppers.

Diet has been found to influence growth rates (Bailey and

Mukerji 1976, Bajoi and Knutson 1977), survival to maturity

(Bernays et al. 1977), the colour of some species with

environmentally influenced colour change (Ibrahim 1974),

ingestion rates (Freeland 1975, Hoekstra and Beenakkers

1976), the duration of instars (Bailey and Mukerji 1976)

and the fecundity of adults (McCafferey 1975, Lee and Wong

1979) . In one species, a restricted diet was found to bias

the sex ratio in favour of females (Pfadt and Smith 1972).

On the whole broader diets led to improved survival, faster

rates of. development and greater adult fecundity, but this

could be an artifact of grasshoppers consuming more food

when f�ced with a diverse diet.

Dadd (1963) noted that the most important requirements

in the diets of acridids and tettigoniids are proteins, \------=---=------j

103

carbohydates and a range of vitamins. Lipids and minerals

have an uneven influence on grasshoppers of different ages

and species. However, specific nutrient classes sel·dom

have been associated with responses to specific diets,

except for the implication of high protein foods in improved

fecundity (McCafferey 1975, Lee and Wong 1979).

My first concern with A. conica was to test for possible

responses to diet by rearing grasshoppers on diets selected

to give extreme responses. Having demonstrated this capacity,

the likely influence of diet on grasshoppers in the field was

checked by comparing the condition factors (a ratio of weight

to pronotal length) of subadult grasshoppers collected from

different vegetation zones at site I.

7.2 Methods

Laboratory experiments

Grasshoppers were collected as second instar individuals

in early December, 1981. The possible influence of previous

diversity in diet and environment was minimized by collecting

grasshoppers from a limited area, and taking only green ones.

They were maintained in cages similar to those described in

chapter 4.3, and kept in groups of 10 under room conditions.

Groups were fed exclusively on one of the following diets

A. A mixture in approximately equal proportions of

young seedlings of maize, wheat, and Avena sp.

(wild oats) .

B. Wheat alone, with a supplement of Avena every five days.

C. Avena. alone, with a supplement of wheat every five days.

-- --- I - ---_-_- c _-___ - -===-=-·=-=--=--=--=--cc::.::=:= -==:r;-.,..,cc-_.-�-'z:-===�-?--�-=--=-==- ::r:::- -= ------_ ___ -=--�-c---J--_- -J-- :.:-- �-�-c=i______:______- ---_--_-__ -______---:--�J�_i::--:-:_.,-.;

104

These were chosen deliberately to accentuate dietary

influences on growth and development. Grasses were

provided in excess, using whole plants with their roots

immersed in water-fil.led, sealed polythene bags. Fresh

plants were given every second day, and the cages were

cleared of fragmented plant material and grasshopper

droppings every week. The number of days taken for each

grasshopper to mature were noted.

Adult females were transferred to cylindrical breeding

cages (30 ems tall by 15 ems diameter) and kept on the same

diets they had been reared on. Three adult males caught

in the field were placed with each female. The excess of

males was to allow for possible sexual selection, in case

the breedi.ng performance of females was related to the quality

of their mates. Females oviposited on the sides of the cages

or at the base of the grasses, and the .egg pods were removed,

dissected in physiological saline, and the eggs counted.

For each female the number of eggs laid, the number of eggs/

pod, and the total number of eggs laid was recorded. Females

remained in the breeding cages until death.

Field studies

Final instar grasshoppers were collected at site I

during December 1981 and January 1982 to compare the condition

factors of subadult grasshoppers caught near the lake's edge

with those living in the drier grass further back. A total

of 78 male fifth instar grasshoppers were collected over a

three day period during the last week of December 1981, and 105

90 sixth instar females were collected during the first week

of January 1982. The last juvenile instar was chosen to

allow maximum development of zone related influences without

the complications of increased mobility and reproductive

condition that would occur if adults were used. Collecting

was restricted to three days for each sex to reduce any

possible changes in condition with time, and was kept at

least 200 m clear of the areas used for MRR work. The colour

of each grasshopper was noted together with its weight to

the nearest milligram, and pronotal length to the nearest

0.1 mm. Condition was calculated as a ratio of weight/

pronotal length. The grasshoppers were returned to the

areas where they were caught.

7.3 Results and discussion

Mortality on diet A was nearly 30% in the juvenile stages, while the other diets had mortalities of more than 50%. Adult females which died before laying were excluded from fecundity data analysis. Grasshoppers responded to an improved diet with faster. development in both sexes (Table 251 although the differences were greater in females than in males. Females on better diets laid more eggs per pod and more eggs overall (Table 26) although these factors are related. The number of plant species avai·lable was probably the important factor, together with the use of maize which is documented as a nutritious food for Acrida spp. (Ibrahim 1974, Ram 1978). Similar differences should occur under field conditions, because females from areas of lusher vegetation showed significantly greater condition factors than = C::C:- Ci ======-=- =-=--=-=-=- ===--=--=--=--=--=--=--=-·=····-=-�== =---=--=--=--=--=--=· ·======·=·=-:=- ===--�---�-�-�- '"'"'--:".'--�.,-.,.-�.V.:.:.:..::"...::O

Table 25. Number of days to maturity for grasshoppers

raised from the second instar on three

different diets. Sample sizes are in parantheses.

Details of diets are given in the text. All

values are given ± st�ndard error.

x days to maturity x days to maturity

(male) (female)

+ + A 40.6 - 1.0 49.3 - 1.0

( 11) ( 21)

Diet B 45.8 ± 1.0 58. 3 ± 1. 3

(10) ( 13)

C 44.5 ± 0.8 58.0 ± 1.0

(13) (18)

F 7.059 23;067

2 2 ANOVA

31 49

Significance 0.01 0.01 ======<=---=-=--=--=--=--=--=-=--�---=====---=--=-=--=--=--=-- =-- ======--:::::C--::C:::-:=- -=::---�--=--=---0::::-::::C-I::I ===- ;z---�-�--�--z::-:cz-:,:.:--�--z:c-3-1=:-·------_ _,-___ _,.:_-_,---

Table 26. Data on the number of eggs and egg pods laid by

females on three different diets. Sample sizes

are in parantheses. Details of diets are given

in the text. All values are given ± standard

error.

x eggs/pod x eggs/� x pods/�

A 23.2 :!:: 1.7 47.8 ± 5.1 2.1 ± 0.2 ( 31) ( 15) (15)

Diet B 16.7 ± 1.4 30.7 ! 4.6 1.8 ± 0.1 (10) ( 20) (10)

+ C 16.4 � 1.8 32. 8 :!:: 3. 7 2.0 ± 0.1

( 18) (10) (10)

F 5.348 3.781 0.665

2 2

ANOVA

66 32 32

Significance 0.01 0.05 NS

- - ! - -! -----==- -=-=-=-=-=- =--==7";., ------..------·------� •=-=--=-=- -CC-:- -CCC-======-=•-=•- -= - - =•=-- -===-======•-=--=- =--=--=-=--=-'-"C--�-�-c:c_:,·---cc::,-·=--·--· � ---f-----:::::-::-7

Table 27. Condition factors of female grasshoppers

collected at site I in 1981/1982.

Grasshoppers were taken from four areas 2 each of 100 m. Areas 1 and 4 were

within a belt of green vegetation extending

lO - 15 m from the lake's edge, while

areas 2 and 3 were 30 - 40 m from the

lake in a zone of drier vegetation.

Sample sizes are in parentheses, and

means are given with standard errors.

The ANOVA uses log-transformed data to

correct for correlation between the

means and variances of cells. GRASSHOPPER COLOUR

Green Brown

AREA 1. 8.0 + 0.9 (11) 5.4 + 0.6 (2)

( 7) AREA 2 5.0 + 0.9 (5) 4.2 + 0.6

AREA 3 5.8 :!: 0.3 (5) 5.4 + 0.1 (5)

AREA 4 9.2 � 1.6 (41) 7.3 + 0.5 (14)

Analysis of Variance

(log-transformed data)

ss Source of variation DF MS F Significance of F

Main effects

Morph 0.048 1 0.048 1.567 0.214

Area 0.510 3 0.170 5.582 0.002

2-way interaction

Morph x Area 0.024 3 0.008 0.258 0.855

1: r:------0 �------·------3==--=--=--=--=- =--=-- =- ======::r:r=--,;,- __ --=----:.,c.---�-.:..:-_·.-----=--=--=--==---:===

Table 28. Condition factors of male grasshoppers

collected at site I in 1981/1982.

Grasshoppers were taken from four areas 2 each of 100 m. Areas 1 and 4 were

within a belt of green vegetation extending

10 - 15 m from the lake's edge, while

areas 2 and 3 were 30 - 40 m from the

lake in a zone of drier vegetation.

Sample sizes are in parentheses, and

means are given with standard errors.

The ANOVA uses log-transformed data to

correct for correlation between the

means and variances of cells.

!::':------GRASSHOPPER COLOUR

Green Brown

+- + AREA 1 4.9 0.8 (7) 5.0 - 0.6 (11)

+ + AREA 2 3.9 - 0.2 (3) 4.7 - 0.4 ( 15)

+- AREA 3 (0) 4.4 0.4 (19)

+- +- AREA 4 5.6 0.6 (10) 5.4 0.3 (13)

Analysis of Variance

(log�transformed data)

Source of variation ss DF MS F Significance of F

Main effects

Morph 0.002 1 0.002 0.089 0.766

Area 0.123 3 0.041 1. 758 0.163

2-way interaction

Morph x Area 0.003 2 0.002 0.067 0.935 106 others (Table 27). However, there was no change in condition with grasshopper colour, although this associates strongly with location (chapter 4.2). Possibly brown grasshoppers which move into lusher areas gain weight rapidly and improve their condition factor markedly, while the reverse occurs for green grasshoppers moving into drier grass. This would prevent significant differences in condition between morphs.

There were no significant differences in condition

between males in the two areas (Table 28), which was surprising

because of the faster development they showed under favourable conditions in the laboratory. Possibly

significant differences could not be detected because of the smaller size of males, necessitating finer resolution in measurements of both weight and pronotal length.

Alternatively, field conditions .may be inadequate for

condition factor differences to develop between males in different microhabitats.

For females, there appears to be a strong return in terms of improved fecundity for favouring richer microhabitats.

However, the males appear to have a different.order of preferences. Part of the explanation may be that the time of maturation of males should be close to that of females to maximize chances of encounter and successful mating. If both sexes were on the richest diet the males would mature too early, since they have one less instar. Different diets would help to achieve synchrony, while allowing females the chance for optimal weight gain. This idea is supported by the life history data, which show an extended third or 107

fourth instar in the males. Males appear to make a deliberate reduction in diet quality with the aim of synchronized maturation.

Predation is another factor. Males which slowed their development relative to the females would emphasize the sexual dimorphism in size, and encourage predators to divert their attention to the females. Microhabitat segregation is a further inducement for predators to concentrate on areas away from the males, so a complex of forces selecting against males taking improved diets would be established. The extent to which microhabitat selection would provide such protection would vary with the vegetation of different locations, being greatest where there are clear dichotomies in vegetation blocks such as those at site I. In more homogeneous areas the relationship would probably break down, with males unable to regulate their development rates to the same extent.

.- !::·-:------:,--- ��~··�-c---=-=1�-�------

108

8. A NOTE ON SEXUAL SELECTION

8.1 Introduction

The data on predation and diet choice suggest that

males benefit by choosing foods that retard their

development and accentuate the sexual dimorphism. This

reduces their risk of predation, and helps to synchronize

their maturation with that of the females. However, this

ignores the possible conflicting variable of sexual selection.

The unbalanced sex ratio provides ample opportunity for

females to choose mates and for males to compete for access

to females. Male weight is likely to be an important factor

in both cases, with heavier males being favoured. This

causes a conflict in strategy because males maximizing their

weights would suffer a greater risk of predation and would

mature much earlier than the females.

In his major monograph on sexual selection O'Donald

(1980) suggested that it occurs " ... because the males'

high potential for fertilization is limited to the number

of ova which the females actually produce and in which they

invest much of their energy for reproduction". Males may

compete for females by fighting (Parker 1970, 1974)

or by territoriality and display (Selander 1972). The

females do not have to accept passively the victors of these

contests, but choose their mates from the pool of available

males. While the extent of female choice has been argued

and at times denied (Thornhill 1980), it is now accepted

(O'Donald 1980). These mating preferences may be complete 109

with females accepting only particular types of males, or they may depend on the encounter rates of females with the different types of males available in the population. In this case, the selection may be frequency-dependent with the proportions of different males encountered crucial in determining choice. In some cases, this may lead to rare males having an advantage in mating (Spiess and Ehrman 1978).

In a move towards developing strategies for mate choice

Trivers (1972) suggested that the main force in sexual selection is the relative investment of the sexes in their offspring. Male energies are devoted to competition for females because the larger female gamete is a greater investment than the male one. In the few situations where male_ gametic investment is higher than the female, role reversal occurs and females compete for access to males

(Gwynne 1984).

Because of the potential for sexual selection caused by A. conica's unequal sex ratio, I observed mating behaviour in the laboratory to collect data on the influence of male size, the duration of copulation and the incidence and duration of postcopulating mate guarding. Field studies were impractical because grasshoppers were caught in copula only rarely.

8.2 Methods

Mating behaviour was observed in five of the cages prepared for the fecundity experiments described in chapter 7, with three males and one female present in each cage. The llO

males were marked individually with a small spot of gloss enamel paint on the pronotum, and were observed for eight hours after introduction to the cage. The weight of each male was recorded, together with the additional information on mating behaviour. If mating was in progress at the end of the time period, observations were continued until it was over. The grasshoppers were then isolated, and the trials repeated three times at intervals of three days. All grasshoppers were the same colour.

8.3 Results and discussion

Males approached the females without audible stridulation or similar attempts at attraction. If only one male

encountered the female he would touch her several times with

his antennae and then attempt mounting. Females ignored

the males until between one and two minutes after mounting,

when they kicked off the males who had not established genital

union. Dislodged males attempted copulation again unless

the female moved away. When more than one male encountered

a female they would touch each other as well as her, move

around while doing this, and finally one would attempt

copulation. If he failed and the female did not move away,

the touching recommenced and eventually copulation was

attempted again. Invariably, the male which made the

first attempt also made the second attempt.

Table 29 summarizes the data on mating. The two most

striking results were the high proportion of unsuccessful

mating attempts, and the consistency with which the heaviest ======-=--=---=-=-=- -=------1 _,------=--===�===�

Table 29. Weights of males, numbers of copulatory attempts,

duration of copulation (� standard error) and + duration of mate guarding {- standard error). The

male initiating the first successful copulation is

shown*

Cage J' Weights (g) Attempts Duration Mate Guarding S US' (minutes) (minutes)

0.169 0 2

1 0.228 1 4 73 40 * 0.266 2 1 56.5 ± 11.4 44.0 ± 16.0

0.207 0 2

2 0.233 0 0 * 0.288 1 1 97 50

0.205 0 0

3 0.269 0 3 * + ll.5 0.362 2 2 53.5 + 26.4 51.5

0.197 1 3 17 5

4 0.245 1 1 46 31 + * 0.289 2 2 60. 5 ±: 11.5 43.0 - 7.0

0.173 0 1

5 0.202 0 2

* +,,_ ll.0 + 0.297 4 2 55.5 28.0 5 ..0

+ + X 60.0 - 7.0 39.9 4.6

! I ---- -�...:-'"'."

_111

male initiated the fir�t copulation. Although there was

no obvious intrasexual aggression, heavier males presumably

won their right during the touching encounters before

mating. Increased mating success for larger males

has been documented in a range of insects including

cantharid beetles (McCauley and Wade 1978), weevils

(Johnson 1982) dung flies (Borgia 1982) and cicadas

(Karban 1983). The behaviour of A. conica fits this

mould. However, increased weight is not always an

advantage, as both Mason (1964) and Scheiring (1977)

found that in cerarnbycid beetles it was males of

intermediate weight which were most successful in

mating.

The number of unsuccessful attempts reflected the

vigor with which females kicked males shortly after

mounting if they had not accomplished intromission.

This suggests a preference for males which begin mating

quickly, and might favour experienced males. Heavier

males might have a further advantage through being

harder to dislodge. This selection for greater weight

conflicts with the pressure to mature at the optimum

time, since weight gain would be concomitant with earlier

maturation.

Of course, results in the laboratory cannot serve as

an infallible guide to what can be expected in the field .

. The only field datum available on the weights of mating

males was collected in January 1984, when a male caught

in copula had a weight of 0.235g while the mean weight of

- 1_- =--=--=--cc::--':':-':':--=-==--�-CZ:--I•i======--=-=--=---= -=====, --�--�--""""--c'c'-""'-"'.''-::C--'.c".'-"-:".'-�"".'-�--l=E--CS:-CS:---�-CS:--'=- -',;---.- � �-c�:__--:_:_-_-:_-:_.J===:;-;.- ___:_------c------1 :------·--;;.;-;.. ... --,------__c____

112

27 other males collected that day was 0.253, with a

variance of 0.001. Clearly, the heaviest males are not

universally successful in mating.

In a study of the influence of weight on the mating

success of male cerambycids (Tetraopes tetraophthalmus),

McCauley (1982) pointed out that a range of factors absent

from laboratory cages could influence mating behaviour in

the field. These included the relative distribution of the

sexes, encounter rates, and maturation times. Further, he

suggested that heavier males might suffer a loss of agility

that could reduce their reproductive success. He believed

that these factors accounted for the disparity between his

laboratory experiments in which larger males were more

successful and his field data which showed that males of

intermediate weight were involved in the most copulations.

Any of these factors could operate in A. conica.

A final point is that seven of the lS males used in my

laboratory experiments never mated successfully, and two of

these did not try to mate at all. This suggests that other

factors apart from weight are involved in mating success.

However, the laboratory and field results combined with the

literature on insect mating do suggest that at least the

medium sized males have an advantage over smaller ones in

securing mates, and that this may extend to the larger males

as well. If this is true, then male grasshoppers must

balance the desirability of greater weight to improve the

chance of mating _against the possibility that heavier

males may mature at an unsuitable time to maximize encounters 113

with receptive females. This problem is discussed in the following chapter. 114

9. GENERAL DISCUSSION

There appear to be very different strategies used by

male and female A. conica to maximize fitness, and they are

explained most readily in the context of the sexual dimorphism

in size. Roff (1981) argued that although the body size of

a species could be influenced by a range of factors including

metabolic constraints, character displacement through competition

and predation, the optimal size could be expressed as that which

led to the largest rate of increase in the population. This

is essentially the definition of "classical fitness", given

by Dawkins (1982) as the reproductive success of an individual

based on its survival and fecundity. For females, there is

pressure to attain a large body size because this improves

fecundity. In turn, this influences both their microhabitat

selection and their antipredator defences, because growth and

ultimately fecundity are related to diet. Supporting evidence

comes from the association between sex and colour at some sites,

the segregation of the sexes at site I, habitat-related

differences in condition in field populations, and the relation-

ship between diet and fecundity shown in the laboratory.

Consequently, females tend to be green and to aggregate in

areas of lusher vegetation.

However, females suffer disproportionate predation as

juveniles because of their larger size. The larger birds

gain more prey mass per unit feeding time by attacking the

subadult females, whi_ch are handled easily and individually

provide a large reward. Similar cases of disparate predation 115

on male and female insects are described by Schulz (1981), Stamps

and Gon (1983), Karban (1983) and Ehrlich et al. (1984), and can

cause_ disparate sex ratios in adult populations (Glen et al. (1981).

Selective predation would be exacerbated by female microhabitat

choice, which allows predators to concentrate their search on

specific, recognizable areas (Lawrence and Allen 1983). This

situation involves active hunting by predators for prey, and

. is distinct from the sex-related mortality in Mecoptera in

which males are at greater risk because of their activity

patterns (Thornhill 1980).

A major consequence of the unequal sex ratio is the capacity

for intense sexual selection as an excess of males compete for

access to the females. The males attempting to mate have

already shown good resistance to predators and pathogens by

their very survival (Kodric-Brown and Brown 1984), and are

a worthwhile pool for females to choose from. Competition

between males selects a subset of potential mates. If the

impression of larger males attempting most copulations given

here is correct, better nourished and possibly more fertile

males are most likely to attempt to mate. Females would be

encouraged to make further selection, since it appears to

involve them in minimal cost in terms .of time, energy

expenditure, and increased exposure to predators {Parker

19831 They may continue feeding while mating, and can

fly in copula. Other female insects may also choose mates

(Thornhill 1980, Pyle and Gromk:o 1981, Gwynne 1984), so the

advantages appear to be widespread. The multiple mating of

females and their capacity to lay several egg-pods nevertheless 116

should increase the proportion of males mating, and so avoid the possibility of genetic drift in small populations where possibly only a very few grasshoppers might actually breed.

Overall, the improvement in fecundity and the increased opportunity for mate choice outweigh the greater mortality involved in increasing female size.

Male strategy accentuates the sexual dimorphism. As the larger juvenile females are attractive prey for birds, males benefit by being conspicuously smaller and avoiding the females. The resultant disproportionate predation would improve male survivorship. However, the adult males then face considerable problems in breeding because of the cbmpetition for mates. There is conflict between the need to be larger than rivals to improve mating chances, and the desirability to synchronize maturation with the females.

Given that the daily survival rate for adult males is approximately 0.9, the probability of a given male surviving

x x days until the availability of a female is 0.9 . Even a wait of seven days. would mean that a given indi vi_dual had less than an even chance of surviving to have an opportunity to mate. Against this is balanced the possible advantage of a faster development producing a heavier adult, with greater chance of mating successfully. The resolution of these pressures appe�rs to favour synchronization, because of the protracted third or fourth instar observed in developing males. Further, males opting for rapid development might lose some of the advantage of the enhanced sexual dimorphism, suffering both a reduced juvenile survivorship

! 117

and a long, hazardous wait ·for a mating chance.

Of course, this analysis concentrates on only some of the possible influences on grasshopper growth and development,

and through them on survivorship and fecundity. Others

include morphological variation in relation to habitat

(Monk 1983), the conflicting demands of crypsis and-body

temperature control (Joern 1981) and possible subtle influences of habitat variations in temperature and humidity

on development and fecundity (Ali 1982). In areas where

diseases and parasites are more prevalent, many of these

factors could influence the chance of infection. Nevertheless,

these predictions can be made

if an association exists between grasshopper sex and

colour, the females should be predominantly green;

the sex ratio should be close to 1:1 at hatching, but

biased to males in adult populations;

the unequal sex ratios should develop usually during

and after the fourth instar;

exclusion of birds should keep the sex ratio close to

1:1 in adult populations.

The inclusion of other factors could lead to refinements on

this basic scheme. Although group selection arguments have

been invoked to explain unequal sex ratios in invertebrates

(Colwell 1981, Wilson and Colwell 1981, Wilson 1983), they

apply to species with genetically determined unequal sex

ratios. This is not the case in A. ·conica, where the -;--�------.c_-=:����_--:.._-_-_�--_,---•,:_---.,;C:f- === ---:::: -::::--::::--::::-::::-::::--=-======--=---=--=--=-=--=-:::r::, :=:J:==C-:::,'----�--�:-c'."'.--�-�-----cz--cz----,z---c'.c'.-c'.c'.--:c,:r-[==-i._]:---�

118

group selection approach is inappropriate.

The interactions between A. conica and its predators reflect several tenets of optimal foraging theory. The preference of

·magpies for prey which provide the largest return of energy per unit feeding· time indicates that their foraging strategy is essentially energy-maximization. Consequently, the distribution of grasshoppers, their degree of crypsis, and the relative proportions of different sizes and colours should all influence the final prey choice (Pyke 1984). Models allowing for the action of crypsis are provided by Erichsen et al. (1980) and

Gendron and Stadden (1983, 1984). These differ in their understanding of crypsis, with Erichsen et al. (1980) taking it to imply prey that are· hidden completely, while the others regard cryptic prey as being exposed but difficult to see.

However, the basic predictions held in common are that crypsis decreases·the profitability of prey, and that the optimal search rate (defined as that which maximizes the rate of prey capture) should decrease as prey become more cryptic. Magpies may

overcome some of these constraints by foraging in flocks.

They move across open spaces in a rough line abreast formation,

so that walking birds may flush prey out in front of them.

This combines scanning search for cryptic prey which burrow

into the substrate, and pursuit of prey which fly to escape.

Since one bird may seize prey which were startled by its neighbour and large areas of ground are searched only once,

this is an efficient mode of searching likely to be successful

against grasshoppers which fly readily. Similar advantages

of flocking when feeding are documented by Charnov et al.(1976}.

-! 119

The interactions between grasshoppers and birds are significant for theories of frequency-dependent selection as well. To date, experimental work has emphasized a single attribute of prey such as colour (Cooper 1984) or striping

(Raymond 1984) and assessed predation patterns in relation to it. However, A. conica varies in both size and colour in juveniles, while striping and startle displays are added in adults. Consequently, the predation it suffers is a resolution of several conflicting forces. At one level, cryptic shape and colouration suggest that predation ought to be apostatic, given the results of Cooper (1984) that apostatic selection is intensified when prey match their backgrounds. If this were so, disproportionate mortality of female grasshoppers should be braked at_an early stage because both their greater tendency to be green and their larger size identify them as a rare form when the sex ratio becomes unbalanced. Instead, the aggregative behaviour appears to override considerations of crypsis by encouraging the disproportionate mortality of the rarer form in aggregations, a phenomenon noted by Allen (1976) and Allen and Anderson (1984}. As noted by Greenwood (1984), the rarer forms actually appear conspicuous against the background of their conspecifics in aggregations and predators can reduce or eliminate confusion by concentrating on them. Given the greater energy reward provided by subadult females, there are considerable pressures for their greater mortality. Neverthe- less, it is significant that the extreme sexual dimorphism is maintained despite this. The observauion of Greenwood (1984) that there is " general reduction in sexual dimorphism in - ·=_r;-;-;£-:=�=--=--�-- �---c:cc-- -cc:c·-cc-·:: -_::,=·--�-·C::C::C--�-1 ·::::C-::::C--::::C-::::C -:-::C:-:=- -c:c::- -:-::".--�-cZ---�---2---�-cZ.,--�--cZ.,..::c-cr:::-1 -.•- c , -======�-=- - - - - =�--�--:c:c-=--=--=--=--=-·=--======-=- · ,=�=�- - - - = ------..

120

species that live in aggregations in open places" was presumably

intended to apply to mammals only! The balance between mortality

and fecundity is a clear factor.

Not only magpies but other predators appear to concentrate

on different size classes of prey. Consequently, grasshoppers

pass through discrete danger periods in which they are attacked

by particular predators. In the early instars these are mainly

spiders, with birds attacking the larger grasshoppers. In the

case of predatory arthropods both Thompson (1975) and Polis

(1984) suggested that different predator instars were suited to

specific sizes of prey that minimized intraspecific competition

among age classes. Polis (1984) argued that this meant that

the instars functioned as ecological "species", and suggested

that the Dyar growth ratio observed in arthropods would help

in minimizing overlap in prey size. In the case of A. conica,

the species is not a predator but a resource available to a

range of enemies. By passing rapidly through the instars in

close synchrony juveniles may swamp each phase of predators in

turn, and reduce the losses that might occur if development was

irregular. The size increments at each moult would then be

important in moving grasshoppers out of danger of particular

groups of predators. In the case of females, growth eventually

reduces predation risk considerably as they become too large to

be economical prey for magpies, the largest predator noted. The

implication for predators is to exploit the brief transient

period of optimal prey availability, possibly in the case of

birds by synchronizing their breeding with the grasshopper

development. 121

These predator-prey interactions occur within the framework provided by the vegetation. The plants provide not only a background for the cryptic colour of the grasshoppers, but also regulate their colour and rate of development throu·gh their choice of food. Further, microhabitat preferences of grass- hoppers can lead to predators searching likely vegetation types; and so influence search success. This is a likely factor in the differential mortality of females, and illustrates the complexity of interactions between trophic levels that can occur in ecological systems (Price et al. 1980).

The different strategies of male and female A. conica and their patterns of interaction with both predators and food plants are not unique, and may be an example of a more general case applying to many animal taxa. Britton and Moser (1982) reported a similar framework in a study of gambusia fish living in swamps and ditches. In exposed marshes sex ratios were biased towards males, because the females suffered disproportionate predation. The females were larger than the males and had five to 25 times the energy content, while birds took only slightly longer to handle them. Further, the females were easier to catch when they were gravid and nutritionally richer, and their preference for wanner waters led them to expose themselves more to predators. In sum, these factors not only made females more desirable prey but increased the chances.of them being eaten by their distribution alone. Although no measures of fecundity were made for the gambusia, the larger females which survived would presumably have been more fecund, and they would have benefited from any resultant intensification of sexual selection. 122

The underlying mechanism of the sexual dimorphism and unequal sex ratio in both gambusia and A. conica may be widespread in other species in which females are larger than males.

Of course, there are other theories of sexual dimorphism in invertebrates that propose alternative mechanisms. For example, Wiklund and Fagerstrom (1977) suggested that it results from the advantage that males gain from maturing slightly before the females to bracket the period of maximum female availability. This should lead to smaller size to ensure that the males mature at the optimum early time. However, in A. conica's case the extra female instar accomplishes this without the need for other mechanisms. Boggs (1981) suggested the furthe.r variable of the storage of food as a reproductive reserve, arguing that this would in turn make females larger because of the greater size of their gametes. This could apply to grasshoppers.

Singer (_1982) pointed out that early male emergence was really only a viaole option with discrete generations, since if they' overlapped females would always be available and no advantage would accrue to males emerging at the right time.

He predicted that arthropods with discrete generations should have larger females because of this, while in species with continuous generations intrasexual selection would tend to increase male size. He collected supportive data on butterfly populations. Finally, Waldbauer and Sheldon (_1971) showed that in the special case of mimetic species, the optimal emergence time was after the model had emerged. 123

The range of explanations available for arthropod sexual dimorphism indicates clearly the multi-causal nature of ecological phenomena, and the fallacy of attempting to use a single mechanism to cover every possible case. Rather, the basic factors of survivorship and reproductive success common to all the suggestions emphasize that these features are the fundamental basis of life histories. ------__ �c,:� .-.---��c=__z•...... - .. . - -.--- ,,:.,:-�---:------1

124

REFERENCES

Abushama, F.J. and Elkhider, E.T.M. (1976}. Food preference

of the acridid grasshopper Truxalis grandis grandis

(Klug). Acrida 1, 245-255.

�ock, J. (1973). Cues used in searching for food by

Red-winged Blackbirds (Agelaius phoeniceus).

Behaviour Lt-6, 174-J.88.

Ali, S. (1982). Effect of temperature and humidity on the

development and fertility/fecundity of Acrida exaltata

(Walk.). Proc. Ind. Acad. ScL, Ann. Sci. 2_!_, 267-273.

Allen, J.A. (1972). Evidence for stabilising and apostatic

selection by wild blackbirds. �.-Nature 237, 348-349.

Allen, J.A. (1974). Further evidence for apostatic selection

by wild passerine birds: training experiments. Heredity

12, 361-372.

Allen, J.A. (1976). Further experiments for apostatic selection

by· wild passerine birds- ..... 9 :1 �xp�l:"����ts �-- Heredity }�-� 1. 7 3-180.

Allen, J.A. and Anderson, K.P. (1984), Selection by passerine

birds is anti-apostatic at high prey density. Biol. J.

Linn. Soc. 23, 237-246.

Anderson, D.R., Laake, J.L., Crain, B.R. and Burnham, K.P.

(1979). Guidelines for line transect sampling of 15iological populations. J. Wildl. Manage. 43, 70-78.

Anderson, M. (1980). Why are there so many threat displays?

J. theor. Biol.�' 773-781.

Anderson, R.V., Tracy,C.R. and Abramsky,Z. (1979). Habitat

selection in two species of short-horned grasshoppers:

the role of thermal and hydric stresses. Oecologia �, 359-374. 125

Ashford, J.R., Read, K.L. and Vickers, G.G. (1970). A system

of stochastic models applicable to ·studies in animal

population dynamics. J. Anim. Ecol. 12_, 29-50.

Baker, R�J. and Nelder, J.A. (1978). The GLIM System Release

3. Generalised Linear Interactive Modelling. Oxford:

Numerical Algorithms Group.

Bailey, C.G. and Mukerji, M.K. (1976). Feeding habits and

food preferences of Melanoplus bivittatus and

M. femurrubrurn (Orthoptera: Acrididae). Can. Ent.

108, 1207-1212.

Bajoi, A.H. and Knutson, H. (1977). Effects when restricting

an acridid species to a specific plant species. Acrida

i, 219-229.

Barnes, R.D. (1974). Invertebrate Zoology. Philadelphia:

W.B. Saunders Co.

Barton-Browne, L. (1975). Regulatory mechanisms in insect

feeding. Adv. Insect Physiol . .!.!_, 1-116.

Begon, M. (1979). Investigating .Animal· Abundance: Capture -

Recapture For Biologists. London: Edward Arnold.

Began, M. (1983). Abuses of mathematical methods in ecology:

applications of Jolly's capture-recapture method. Oikos

40, 155-158.

Q_nnack, D.E. (1981). The effects of mandible morphology and

photosynthetic pathway on selective herbivory in grass­

hoppers. Oecologia 51, 281-283.

Bernays, E.A. and Chapman, R.F. (.1975). The importance of chemical

inhibition of feeding in host-plant selection by Chorthippus

parallelus (Zetterstedt). Acrida !!_, 83�94. ------= 1 ------,J . 1- j r---�.;--.

126

G:nays, E.A., Chapman, R.F., Cook, A.G., McVeigh, L.J. and

Page, W.W. (1975). Food plants in the survival and

development of Zonocerus variegatus (L.). Acrida i,

33-46.

Bernays, E.A., Edgar, J.A. and Rothschild, M. (1977).

Pyrrolizidine alkaloids sequestered and stored by the

aposematic grasshopper, Zonocerus variegatus. J. Zool.,

�ond. 18i, 85-87.

Bernstein, R.A. (1982). Foraging-area size and food density:

some predictive models. Theor. Pop. Biol. �' 309-323.

Birley, M. (1977). The estimation of insect density and

survivorship functions from census data� J. Anim. Ecol.

�' 497-510.

Bishop, D.T. and Cannings, C. (1978). A generalized war of

attrition. J. theon· Biol. J..Q, 85-124.

Bishop, J.A. (1973). The proper study of populations. In

Insects: Studies In Population Management : Geir, P.W.,

Clark, L.R .. , Anderson, D.J. and Nix, H.A. (Eds).. Ecol.

Soc. Aust. (memoirs 1), Canberra.

Bishop, J.A., Cook, L.M., Muggleton, J. and Seaward, M.R.D.

(1975). Moths, lichens and air pollution along a transect

from Manchester to North Wales. J. appl. Ecol. 12, 83-96.

Blower, J.G., Cook, L.M. and Bishop, J.A. (1981). Estimating

the size of animal populations. London: George Allen

and Unwin Ltd.

Boggs, C.L. (1981). Nutritional and life-history determinants

of resource allocation in holometabolous insects. Am. Nat. 117, 692-709.

\_-_- "�- �I. , ------c _ _ ------::-:-=r:::=::: ------=- ,,______- -:_- ·-·-.:_l___ �--�---::---_---��------,1

127

Borgia, G. (1982). Experimental changes in resource structure

and mate density: Size-related differences in mating

success among male Scatophaga stercoraria.

Evolution�, 307-315.

Bradley, J.S. (1975). Population dynamics and natural selection

in a mixed colony of grasshoppers. Unpubl. Ph.D. thesis,

University of Liverpool.

Bradley, J.S. (1985). Comparative demography of four species

of grasshoppers on a common site. In Case Studies In

Population Biology Cook, L.M. (Ed.) Manchester:

Manchester Univ. Press.

Breder, C.M. (1967). On the survival value of fish schools.

Zoologica g, 25-40.

Britton, R.H. and Moser, M.E. (1982). Size specific predation

by herons and its effect on the sex-ratio of natural

populations of the mosquito fish Gambusia affinis

Baird and Girard. Oecologia �, 146-151.

Brock, V.E. and Riffenburgh, R.H. (1960). Fish schooling:

a possible factor in reducing predation.

J. due Conseil �, 307-317.

Brockmann, H.J. and Dawkins, R. (1979). Joint nesting in a

digger wasp as an evoluntionarily stable preadaptation

to social life. Behaviour 17, 203-245.

Brockman, H.J. Grafen, A. and Dawkins, R. (1979).

Evolutionarily stable strategy in a digger wasp.

J. theor. Biol.'!.}__, 473-496.

Brower, L.P. (1958). Bird predation and food plant specificity

in closely related procryptic insects. Am. Nat.�' 183-187. - - --�- --_.. ------_-,_ --- -�------_..,,_:r- ___, -:,-_._•--_,Cf=---C:::--:::C::--:::C::--:::C::--:::C::--�- -�-=--:-:0--:-:C-=-::c=�::::E:c:::�==---C:::::-C::::-=---=--=-�---

128

Brown, H.D. (1.974). Defensive behaviour of the wheat aphid

(Schizaphis graminum) (Rondani) (Hemiptera: Aphidididae)

against Coccinellidae. J. Entomol. 48, 157-165.

Bryant , D . M. ( 19 7 3 ). The factors influencing the selection

of �ood by the house martin (Delichon urbica (L.)).

J. Anim. Ecol. g, 539-564.

Buckland, S.T. (1980). A modified analysis of the Jolly-Seber

capture-recapture model. ·Biometrics 36, 419-435.

Cain, A.J. and Shepp�rd, P.M. (1950). Selection in the

polymorphic land snail Cepaea nemoralis.

Heredity_!, 275-294.

Calver, M.C. (1984). A review of ecological applications of

immunological techniques for diet analysis.

Aust. J. Ecol.�, 19-25.

�lberg, U. (1981). Defensive behaviour in females of the stick insect Sipyloides sipylus (Westwood) (Phasmida).

Zool. Ang. 207, 177-180.

Carothers, A.D. (1973a). The effects of unequal catchability on

Jolly-Seber estimates. Biometrics 29, 79-100.

Carothers, A.D. (1973b). Capture-recapture methods applied

to a population with known parameters.

J. Anim. Ecol. 42, 125-146.

Carothers, A.D. (1979). Quantifying unequal catchability and

its effect on survival estimates in an actual population.

J. Anim. Ecol. 48, 863-870.

�apman, R.F. and Page, W.W. (1979). Factors affecting the

mortality of the grasshopper Zonocerus variegatus, in

Southern Nigeria. J. Anim. Ecol. 48, 271-288. 129

�an, R.F., Page, W.W. and Cook,A.G. (19-79). A study- o;(

population changes in the grasshopper Zonocerus variegatus,

in Southern Nigeria. J. Anim. E.col. 48, 247--270�

Charnov, E.L. (1982). The Theory Of Sex AllocatJon

Princeton, N.J.: Princeton Univ. Press�

Charnov, E.L., Orians, G.H. and Hyatt, K.. (1976). Ecological

implications of resource depression. Am. Nat. 1.10, 2.47...-- J 259-.

Clarke, B. (1962). Balanced polymorphism and the diversity

of sympatric species. In: Taxonomy And Geograp�

Nichols, D. (Ed}.· Oxford: Systematics Association.

Clarke, B. (1972). Density..-dependent selection. Am. Nat.

106, 1-13.

Clutton..--Brock, T.H., Guiness, F,E. and Alban, S.D. (1982)_.

Red Deer: Behaviour And Ecology Of Two Sexes

Chicago: Univ. of Chicago Press.

Cohen, J.A. (1984). Sexual selection and the psychophysics of

female choice. Z. Tiepsychol. 64, 1-8.

Colwell, R.K. (1981) Group selection is implicated in the

evolution of female-biased sex ratios. Nature 29-0,

401..-404.

Cooper, J.M. (1984). Apostatic selection on prey that match

the background. Biol. J. Linn. Soc. 23, 221--228.

Coppinger, R. (1970}. The effect of experience and novelty on

avian feeding behaviour with refe;r-ence to the evoluti·on

of warnfng colouration in butterflies. II Reactions of

naive birds to novel insects. Am. Nat. 104, 323-327. 130

Cook, L.M. and Miller, P. (1977). Density-dependent selection

on polymorphic prey - some data. Am. Nat. 111, 594-598.

Cox, G.W. and Cox, D.G. (1974). Substrate colour matching in

the grasshopper Circotettix rabula (Orthoptera: Acrididae).

Gt. Basin Nat. �' 60-70.

Crowle, A.J. (1973). Immunodiffusion London: Academic Press.

Z. Tierpsychol. � H. (1970). Searching image in carrion crows. �, (suppl). 1-86.

Curio, E. (1975). The functional organization of anti-predator

behaviour in the Pied Flycatcher. A study in avian visual

perception. Anim. Behav. �' .l-ll5.

Cushing, D.H. and Harden-Jones, F.R. (1968}. Why do fish

school? Nature 218, 918-920.

Dadd, R.H. (1963). Feeding behaviour and,nutrition in-grass­

hoppers and locusts. Adv. Insect Physiol. _!_, 47-109.

Davies, N.B. (l976). Food, flocking and territorial behaviour

of the pied wagtail Motacilla alba yarrellii Gould, in

winter. J. Anim. Ecol. 46, 37-57.

- ns, M. (1971a). Shifts of "attention" in chicks during � feeding. Ani:m. Behav. 19, 575-582.

0--:wkins, M. (197.1:ti). )'erceptual changes in chicks: )lnother 1-:--1-�l�·t=;-� look at the ''search image" concept. Anim. Behav. 19,

566-574.

Dawki..ns, R. (1982)_ The Extended Phenotyoe: The Gene As The

·unit·of·selection. Oxford: W.H. Freeman and Co.

Dearn, J.M. (1984}. Colour pattern polymorphism in the

grasshopper Phaulacridum vittaturn. I Geographic variation

in Victoria and evidence of habitat associa,tion. Aust. J.

·zool� _E, 239-249.

-- I

I I 131

n • H.C., Wist, E.R., Dichgans, J. and Brant, T. (-1976!.. . :� The spatial effect on perceived velocity. Vis-. Res. 16,

169-176.

Dempster, J.P. (1960). A quantitative study of the p;redators

on the eggs and larvae of the broom beetle Phytodecta

olivacea Forster, using the precipitin test. J. Anim.

--Ecol. -2q, 149-167.

Dupont, W. D. (19-83). A stochastic catch-effort method for

estimating animal abundance. Biometrics�, 1021.,...l033.

Eaton, R.L. (l978). Why some felids copulate so much; a

model for the evolution of copulation frequency.

Carnivore!, 42�5l.

Eberhart, L.L. (.l978). Transect methods for population

studies. J. Wildl .. Manage. 42, 1=31. ends, M. (1972). Defensive behaviour in Ghana.ian mantids. Zool. J� Linn. Soc. 51, l-52.

Edmunds, M. (1974). Defence In Animals London: Longman.

Edwards, R.L. Cl961). Limited movement of individuals in a

population of the migratory grasshopper Melanoplus

bilituratus (Walker} (Acrididae}_ at Kamloops, Britsh

Columbia. Can. Ent. 93, 628-631.

Ehrli'ch, P.R. and Brown, I. L. (19801. Population ecology of

the checkerspot butterfly, Euphydras edi.tha. Structure

of the Jasper Reef colony. Oecologia 47, 239-251.

Ehrlich, P.R., Launer, A.E. and Murphy, D.D. (19_84)_. Can

sex ratio be defined or determined? The case of a

population of checkerspot butterflies. Am. Nat. l24,

527..--539. 132

Elton, R.A. and Greenwood, J.J.D. (1970}. Exploring apostatic

selection. Heredity�, 629-633.

Emlen, J.M. (1966}.. The role of time and energy in food

preferences. Am. Nat. 100, 6.11-617.

Emlen, S.T. and Oring, W. (1977). Ecology, sexual selection,

and the evolution of mating systems. Science�' 215..,..223.

Enders, F. (1975). The influence of hunting manner on prey

size, particularly in spiders with long attack distances

(Araneidae, Linyphiidae, and Sal ticidae) . Arn. Nat. 109_,

737..-.763. eler, J.A. (19781. A predator's vi:ew of animal colour

patterns. Evol. Biol. 11, 319-364.

Endler, J. (l980). Natural selection on colour patterns in

Poecilia reticulata. Evolution 2±, 76-91.

Endler, J. (1982}. Convergent and divergent effects of natural

selection on colour patterns in two fish faunas. Evolution

l§_, _178--188.

Ergene, S. (19-50}_. Untersuchungen uber Farbanpassung und · Farl5wechsel bei Acrida turrita. Z. vergl. Physiol. E,

530�551.

Ergene, S. (l9-51J. Farbanpassung entsprechend der jeuialegen Substratf�rbung bei Acrida turrita. z. vergl. Physiol. �, 69-74.

(\ Er.;i..,c .. h sen, J.T., Kreb -s, J.R. an d Hous t on,. A r I ..(1980) . __ . �/

Optimal foraging and cryptic prey. J. Anim. Ecol .. 49_,

27l-276.

Eshel, I. and ;Feldman, M.W. (1982}. On evolutionary stability

of the sex ratio. Theor. PopuL Biol. 2_1, 430-439. 133

Fairley, J.S. (1982). Short-term effects of ringing and toe­

clipping on the recapture of Wood Mice (Apodemus sylvaticus).

J. Zool. __ Land_. l97, 295-297.

Franzreb, K.E. (1981}. The determination of avian densities

using the variable-strip and fixed-width transect

surveying methods. In Estimating Numbers Of Terrestrial

Birds : Ralph, C.J. and Scott, M. (.Eds). Studies In Avian

Biology No. 6, l39-l45.

Fraser .....Rowell, J. (J.971}. The variable colouration of the

acridoid grasshoppers. Adv. Insect Physiol. _§_, 146-199.

Freeland, W.J. (19.75). Feeding behaviour of the Australian

acrfdid Valanga, irregularis. Entomol. Exp. Appl. 18,

28l-289 .

Fullick, T.G. and Greenwood, J.J.D. (.1979). Frequency-dependent

food selection in relation to two models. Am. Nat. 113,

762-765.

Furnell, D.J. and Schweinsburg, R.E. Cl984). Population

dynamics of central Canadian Arctic island polar bears.

J·� Wfldl. Mana,ge. 48, 722--728.

Gailes, C.E., Marshall, W.H. and Olson, D.P. (.1968). Line

transect method of estimating grouse population densities.

Biometrics 3j_, .135-145.

Gandar, M. V. (.19.82). The dynamics a,nd trophic ecology of

grasshoppers.lAcridoidea) in a South African savanna.

Oecologia_ 54, 370-378.

Gangwere, S.K., Chavin,W. and Evans, F.C. (1964) � Methods of

marking insects, with special reference to Orthoptera

(Sens. lat.). Ann.ent.Soc. Am.�, 662-669. - - -- - <.1===-r------_--- _-_cc __ -

134

Gendron, R�P. and Stadden, J.E.R. (1983). Searching for

cryptic prey: the effect of search rate. Arn. Nat. 121,

172-186.

Gendron, R.P. and Stadden, J.E.R. (1984). A laboratory

simulation of foraging behaviour: th� effect of search rate

on the probability of detecting prey. Arn. Nat. 124, 407-415.

Gill, P. (1981). The genetics of colour-patterns in the grass­

hopper Chorthippus brunneus. Biol. J. Linn. Soc. 16, 243-259.

Gill, P.E. and Murray, L. (1978). Algorithms for the solution

of non-linear least squares problems. SIAM·Journal on

Numerical Analysis 15, 977-992.

: �e t _,,,,-·s . D . and Gon ta, E . ( 19 7 8) . Locus ts as prey factors /� affecting their vulnerability to predation.· Artirn. Behav.

�, 282-289.

G!-J-lett, S.D., Hogarth, P.J. and Noble, F.E.J. (1979). The

response of predators to varying densities of gregaria

locust nymphs. Anim. Behav. 27, 592-596.

Gillis, J.E. (1982). Substrate colour-matching in the cryptic

grasshopper Circotettix rabula rabula (Rehn and Hebard).

Anim. Behav. i9_, 113-116.

Glen, D .M., Milsorn, N .F. and Wiltshire, C. W. (_1981). The effect

of predation by Blue Tits (Parus caeruleus) on the sex-ratio

of Codling Moth (.Cydia pomonella) . J. appL Ecol. 18, 133-140.

Galley, F.B. (1961). Energy values of ecological materials.

Ecology�, 581-?84.

Greathead, D.J. (1963). A review of the insect enemies of

Acridoidea (Orthoptera) . rrran.s. R. Ent� Soc� Lond. · 114,

437-517.

I -- - · \ _ -�- 135

Greathead, D.J. (1966). A brief survey of the effects of biotic

factors on populations of the Desert Locust. J. appl. Ecol._

i, 239-250.

Greenslade, P.J.M. (1964). The distribution, dispersal and

size of a population of Nebria brevicollis (p.), with

with comparative studies on three other Carabidae.

J. Anim. Ecol. �, 311-333.

Greenstone, M. (1977). A passive haemagglutination inhibition

assay for the identification of the stomach contents of

arthropod predators. J. appl. Ecol. 14, 457-464.

Greenwood, J.J.D. and Elton, R.A. (1979). Analysing experiments

on frequency-dependent selection by predators. J. Anim. Ecol.

48, 721-737.

Greenwood, J.J.D. (1984). The functional basis of frequency­

dependent food selection Biol. J. Linn� Soc.�, 177-199.

Gwynne, G.T. (1984). Sexual selection and sexual differences

in Mormon crickets (Orthoptera: Tettigoniidae, Anabrus

simplex). Evolution 38, 1011-1022.

Hamilton, W.D. (1971). Geometry for the selfish herd. J. theor.

Biol. 31, 295-311.

Harley, C.B. (1981). Learning the evolutionarily stable

strategy� J. theor. Biol. 89, 611-633. Harper, G. and Whittaker, J.B. (1976). The role of natural

enemies in the colour polymorphism of Philaenus spumaris.

J. Anim. Ecol. 45, 91-104.

Hartley, P.H.T. (1948). The assessment of the food of birds.

Ibis 90, 361-381.

Harvey, P.H., Jordan, C.A. and Allen, J.A. (1974). Selection

behaviour by wild blackbirds at high prey densities.

Heredity 32, 400-404. 136

Hassell, M.P. and May, R.M. (1974). Aggregation of predators

and insect parasitoids and its effect on stability.

J. Anim. Ecol. �, 567-587.

Hespenheide, H.A. (1973). Ecological inferences from

morphological data. Ann. Rev. Ecol. Syst. _!, 213-229.

Hilden, 0. (1981). Sources of error involved in the Finnish

line-transect method. In Estimating Numbers of Terrestrial

Birds : Ralph, C.J. and Scott, M. (Eds). Studies in

Avian Biology No. 6, 152-159. \------Hinton, H .E. (1976) . Possible significance of the red patches of the female crab spider Misumena vatis.

J. Zool., Land. 180, 35-39.

Hoddap, D. and Frey, D. (1982). Optimal foraging by firemouth

cichlids, Cichalosoma meeki, in a social context.

Anim. Behav. 30, 983-989.

Hoekstra, A. and Beenakkers, A.M. (1976). Consumption,

digestion and ubilization of food by fifth instar

larvae and adults of the migratory locust.

Entomol. Exp. Appl. 19, 130-138.

Holling, C.S., Dunbrack, R.L. and Dill, L.M. (1976). Predator

size a;1d prey size� presumed relationship in the mantid

Hierodula coarctata. Can. J. Zool. �, 1760-1765.

Hubbard, S.F., Cook, R.M., Glover, J.G. and Greenwood, J.J.D. (1982).

Apostatic selection as an optimal foraging strategy.

J. Anim. Ecol. 51, 625-634.

Ibrahim, M.M. (1974). Environmental effects on colour variation

in Acrida pellucida Klug. Z. fur Ang. Ent. 77, 133-136. /\------Jackson, J.F., Ingram, W., and Campbell, H.W. (1976). The dorsal pigmentation of snakes as an anti-predator strategy

a multivariate approach. Am. NaL 110, 1029-1053.

·. ! I I 137

Janzen, D.H. (1973). Embryonic field of ecology. Science 182,

1125-1126.

Joern, A. (1979). Feeding patterns in grasshoppers (Orthoptera:

Acrididae). Factors influencing diet specialization.

Oecologia�' 325-347.

Joern, A. (1981). Importance of behaviour and colouration in

the control of body temperature by Brachystola magna

Girard. (Orthoptera: Acrididae). Acrida 10, 117-130.

Joern, A. and Lawlor, L.R. (1980). Food and microhabitat

utilization by grasshoppers from arid grasslands :

comparisons with neutral models. Ecology 61, 591-599.

Joern, A. and Rudd, N.T. (1982). Impact of predation by· the

Robber Fly Proctacanthus milbertii (Diptera: Asilidae)

on grasshopper (Orthoptera: Acrididae) populations.

Oecologia �' 42-46.

Johnson, L.K. (1982). Sexual selection in a Brentid weevil.

Evolution�, 251-262.

Jolly, G.M. (1981). Mark-recapture - what next? In Estimating

Numbers of Terrestrial Birds : Ralph, C.J. and Scott, M.

(Eds). Studies in Avian Biology No. 6, 137-138.

Jolly, G.M. (1982). Mark-recapture models with parameters

constant in time. Biometrics�, 301-321.

Jolly, G.M. and Dickson, J.M. (1983). The problem of unequal

catchability in mark-recapture estimation of small mammal

populations. Can. J. Zool. 61, 922-927.

Jones, J.S. Leith, B.H. and Rawlings, P. (1977). Polymorphism

in Cepaea A problem with too many solutions?

Ann. Rev. Ecol. Syst. 8, 109-143.

Kamil, A. and Sargent, T.D. (1981). Foraging Behaviour :

Ecological, Ethological and Psychological Approaches.

N.Y. : Garland STPM Press .

. ! 138

Karban, R. (1983). Sexual selection, body size and sex-

related mortality in the cicada Magicicada cassini.

Am. Midl. Nat. 109, 324-330. "-- ; n, D.W. ( 19 7 4) • Adaptive colouration in Peromyscus e__::: polionotus : experimental selection by owls.

J. Mammal. 55, 271-283.

Kelly-Stebbings, A. and Hewitt, G.M. (1972). The laboratory

breeding of British grasshoppers. Acrida 1, 233-245.

Kenward, R.E. (1978). Hawks and doves: attack success and

selection in goshawk flights at wood pigeons.

J. Anim. Ecol.:[!__, 449-460.

Kettlewell, H.B.D. (1955). Selection experiments on artificial

melanism in the Lepidoptera. Heredity�, 323-342.

Kettl ewe11 , H . B .D . (19 5 6 ). Further selection experiments on

industrial melanism in the Lepidoptera. Heredity 10,

287-301.

6ewell, H.B.D. (1973). The Evolution of Melanism: A study Of A Recurring Necessity Oxford: Oxford Univ. Press.

Kettlewell, H.B.D. and Conn, D.L.T. (1977). Further background-

choice experiments on cryptic Lepidoptera. Nature 175,

943,-944_

�van, P.G., Cant, J.G.H. and Kevan, D.K. (1983). Predator

avoidance posturing of grasshoppers (Orthoptera: Acrididae)

from the Colorado alpine and plains. .Can. Ent. 115, 115-122.

Key, K.H.L. (1974). Orthoptera. In: CSIRO Insects of Australia

Carlton, Victoria: Melbourne Univ. Press.

Kodric-Brown, A. and Brown J.H. (1984). Truth in advertising:

the kinds of traits favoured by sexual selection.

Am. Nat. 124, 309-323.

Krebs, C.J. (1978). Ecology: The Experimental Analysis of

Distribution And Abundance NY: Harper and Row.

. -

I 139

Kumaraswamy, K.R. and Rajasekarasetty, M.R. (1976). Pattern

of C-banding in Acrida turrita (Acrididae - Orthoptera}.

Curr. Sci. 45, 762.

Kumaraswamy, K.R.. and Rajasekarasetty, M-R.� (1976) . Chromosomal

repatterning in Acrididae. Cytobios 19, 21-26.

w, R: (1979) , Optimal life histories under age-specific <::_ " predation. Am. Nat. 114, 399-417.

Lawrence, E.S. and Allen, J.A. (1983). On the term "search

image". Oikos 40, 313-314.

Lee, S.S. and Wong, I.M. (1979). The relationship between food

plant, haemolymph protein and ovarian development in Oxya

japonica Willemse (Orthoptera: Acrididae). Acrida 8, 1-8.

Leone, C.A. (1947). A serological study of some Orthoptera.

Ann. Ent. Soc. Arn. 40, 417-433 ..

Lorenz, K. (1968). On Aggression London: Methuen.

Lovejoy, T.E. (1974). Bird diversity and abundance in Amazon

forest communities. Living Bird 13, 127-191.

Gymbery, A. and Bailey, W. (1980). Regurgitation as a possible

anti-predator defensive mechanism in the grasshopper

Goniaea sp. (Acrididae, Orthoptera). J. Aust. ent. Soc.

19, 129-130.

MacArthur, R.H., MacArthur, J., MacArthur, D. and MacArthur, A.

(1973). The effect of island area on population densities.

Ecology 54, 657-658.

MacKinnon, J. (1970). Indicators of territoriality in mantids.

Z. Tierpsychol. 3]_, 150,-155.

Mccaffery, A.R. (1975}. Food quality and quantity in relation

to egg production in Locusta migratoria migratoides.

J. Insect Physiol. 21, 1551-1558.

I I 140

McCauley, D.E. (1982). The behavioural components of sexual

selection in the milkweed beetle Tetraopes tetraophthalmus.

Anim. Behav. �' 23-28.

McCauley, D.E. and Wade, M.J. (1978). Female choice and the mating

structure of a natural population of the soldier beetle,

Chauliognathus pennsylvannicus. Evolution E, 771-775. Manly, B.F.J. (1977). A simulation experiment on the application

of the jackknife with Jelly's method for the analysis of

capture-recapture data. Acta Theriologica _22, 215-223.

Manly, B.F.J. (1984). Obtaining confidence limits on parameters

of the Jolly-Seber model for capture-recapture data.

Biometrics 40, 749-758.

Manly, B.F.J., Miller, P. and Cook, L.M. (1972). Analysis of a

selective predation experiment. Am. Nat. 106, 719-736.

Martin, J.D. and Mackay, R.J. (1982). Interpreting the diet of

Rhyacophila larvae (Trichoptera) from gut analysis: an

evaluation of techniques. Can. J. Zool. 60, 783-789.

Mason, L.G. (1964). Stabilizing selection for mating fitness in

natural populations of Tetraopes. Evolution 18, 492-497.

Matthews, E.G. (1976). Insect Ecology St. Lucia, Qld.: Univ.

of Qld. Press.

Maynard -Smith, J. (1974) . The theory of games and the evolution

of animal conflicts. J. theor. Biol.�, 209-221.

Maynard-Smith, J. (_l979). Game theory and the evolution of

behaviour. Proceedings of the Royal Society of London, B

205, 475-488.

Maynard -Smith, J. (1982). Evolution And The Theory Of Games

Cambridge: Ca,mbridge_ Univ.. Press. - ___ ,-__ ·C• -·==--=--=--=--=--=--=- =·=·=·-=· =- :z::=:-==::E=-=·=-.:7.:-.::C::,- -====·====-=• :::,:::-======�a=--=-•=•·=··=·=••=·=·::C.:S··· =·-=···=· ·a.•---==•-=-=-=·======

141

Maynard Smith, J. and Parker, G.A. (1976). The logic of

asymmetric contest. Anim. "Behav. �, 159-175.

Maynard• ·Smith, J. and Price, G.R. (1973). The logic of animal

conflict. Nature 246, 15-18.

Milinski, M. (1977). Do all members of a swarm suffer the same

predation? Z. Tierpsychol. �, 373-388.

Milinski, M. (1984). Competitive resource sharing: an experimental

test of· a learning rule _for ESS's. Anim. Behav. �' 233-242.

Milner, R,J. (1978). On the occurrence of Entornophthora grylli, a

fungal pathogen of grasshoppers in Australia. J. Aust. ent.

Soc. l 7, 293-296.

Mitchell, J.E. (1975) Variation in food preferences of three

grasshopper species (Acrididae: Orthoptera) as a function

of food availability. Arn. Midl. Nat. 94, 267-283.

Monk, K.A. (1983). Morphological variation in relation to

habitat in some British grasshoppers. J. Nat. Hist.

g, 75-85.

Montgomery, W. I. (1985) . The effect of marking on recapture

and. the estimation of populations of Apodemus spp.

J. _ Zool., Lond. · (Al �' 267 .....269.

Mordue, A.J. (1979). The role of the maxillary and labial palps

in the feeding behaviour of Schistocerca gregaria. Ent. exp.

appl. �, 279-288.

Morgan, M.R.J. (1976) Gut carbohydrases in locusts and grasshoppers.

Acrida ..2,, 45-58.

Mueller, H.C. (1971). Oddity and specific searching image more

- important than conspicuousness in prey selection. ·Nature

233, 345-346. ------:;=[=-=--=---=--= --=--=--=-=--=--="=--,-=---====--=-======- =--=--====-=--=--=---=-=--

142

Murphy, D.D., Launer, A.E. and Ehrlich, P.R. (1983). The role of

adult feeding in egg production and population dynamics

of the checkerspot butterfly, Euphydras editha. Oecologia

56, 257-263.

Murton, R.H. (1971). The significance of a specific search

image in the feeding behaviour of the wood-pigeon.

Behaviour 40, 10-42.

NAG (1975). NAG Manual Oxford: Numerical Alogorithms Group.

Neill, S.R.St.J., and Cullen, J.M. (1974). Experiments on whether

schooling by their prey affects the hunting behaviour of

cephalopods and fish predators. J. Zool. , Lond. 172,

549--569.

Nentwig, W. (1982}. Why do only certain insects escape from a

spider's web? Oecologia �, 412-417.

Nentwig, W. (1983). The prey of web-building spiders. (Araneae:

Araneidae, Linphyiidae, Pholicidae, Agelenidae). Oecologia

56, 132-139.

Newbould, B.B. (1965). Production of allergic encephalomyelitis

in rats by injection of spinal cord adjuvant into the

inguinal lymph nodes. Immunology�' 613-614.

Nichols, J.D., Stokes, S.L., Hines, J.E. and Conroy, M.J. (1982).

Additional comments on the assumption of homgeneous survival

rates in modern bird banding estimation models. J. Wildl.

Manage. 46, 953-962.

Nie, N.H., Bent, D.H. and Hadlai-Hull, C. {_1975). SPSS: Statistical

Package For The social Sciences. NY: McGraw-Hill.

Norris, K.R. (1974). General biology. In: CSIRO Insects Of

Australia Carlton, Victoria: Melbourne Univ. Press. 143

Novak, V.J.A. (1975). Insect Hormones. London: Chapman and Hall.

O'Donald, P. (1980). Genetic Models Of Sexual Selection.

Cambridge: Cambridge Univ. Press.

Okay, S. (1953). Formation of green pigment and colour changes

in Orthoptera. Bull. ent. Res.!!, 299-315.

Olson, F.C.W. (1964). The survival value of fish schooling.

J. du Conseil �, 115-116.

Onsager, J.A. (1977). Comparison of methods for estimating the

density of rangeland grasshoppers. J. Econ. Entomol. �'

187-190.

Onsager, J.A. and Henry, J.E. (1977). A method of estimating the

density of rangeland grasshoppers (Orthoptera: Acrididae)

in experimental plots. Acrida .§_, 231-238.

�• D. an.a Joern, A. (1977). On feeding pa,tterns in desert

grasshoppers and the evolution of specialized diets. Proc.

Acad. Nat. Sci. Phila. 128, 89-126.

Palmer, A.R. (1981). Predator errors, foraging in unpredictable

environments and risk: the consequences of prey variation

in handling time versus net energy. Am. Nat. ll8, 908-915.

( Paloheimo, J.E. (1971a). On a theory of search. Biometrika �'

,,.. 61-75.

( Paloheimo, J.E. (1971b). A stochastic theory of search: \ ______implications for predator-prey situations. Math. Biosciences

l2, 105-132.

Parker, G.A. (1970). Sperm competition and its evolutionary

consequences in the insects. Biol. Rev� 45, 525-568. ,_ ------�- --�------,·-- - . - - -- I •_____ -- -:------. ----

144

Parker, G.A. (1974). Assessment strategy and the evolution of

fighting behaviour. J. theor. Biol. 65, 571-578.

Parker, G.A. (1978). Searching for mates. In Behavioural

Ecology: An Evolutionary Approach : Krebs, J.R and

Davies, N.B. (Eds). London: Blackwell Scientific

Publications.

Parker, G.A. (1983). Mate quality and mating decisions. In

Mate Choice Bateson, P. (Ed). Cambridge: Cambridge

University Press.

Pfadt, R.E. and Smith, D.S. (1972). Net reproductive rate and

capacity for increase of the migratory grasshopper,

Melanoplus ·sanguinipes sanguinipes (F). Acrida 1,

149-165.

Pickavance, J.R. (1970). A new approach to the immunological

analysis of invertebrate diets. J. Anim. Ecol. �,

715-724.

Polis, G.A. (_1984). Age structure component of niche width

and intraspecific resource partitioning: can age groups

function as ecological species? Am. Nat. 123, 541-564.

Pollock, K.H. (1981). Capture-recapture models: a review

of current methods, assumptions and experimental

design. In Estimating Numbers Of Terrestrial Birds

Ralph, C.J. and Scott, M. (Eds). Studies in Avian

Biology No. 6, 426-435.

Pollock, K.H. and Otto, M.C. (1983). Robust estimation of

population size in closed animal populations from

capture-recapture experiments. Biometrics 39,

1035-1049. 145

F.H. (1976). Multiple cryptic effects of crossbanded

and ringed patterns of snakes. Copeia 1976, 834-836. ,,,,.....---.... /' Price, P.W., Boulton,C.E., Gross, P., McPherson, B.A.

Thompson,J.N. and Weis, A.E. (1980). Interactions

among three trophic levels: influence of plants on

interactions between insect herbivores and natural

enemies. Ann�. Rev. Ecol. Syst. 11, 41-65.

Pulliam, H.R. (1973). On the advantages of flocking. J.

theor. Biol.�, 419-422.

Pyke, G.H. (1984). Optimal foraging theory: a critical

review. Ann. Rev. Ecol. Syst. 15, 523-576.

Pyle, D.W. and Gromko. M.H. (1981). Genetic basis for repeated

mating in Drosophila melanogaster �- Am� NaL 117,

133-146.

Ram, R. (1978). Consumption, digestion and utilization of

food by Acrida exaltata Walk. and Acrida gigantia Hbst.

Indian J. Entomol. 40, 277-279.

Raymond, D.L. (_1984). Wild birds prefer the familiar of striped

and unstriped artificial prey.· Biol� J. Linn. Soc��,

229-235.

Read, K.L.Q. and Ashford, J.R. (1968). A system of models for

the life-cycle of a biological organism. Biometrika �, 211-221.

Readshaw, J.L. (_1982). A method of estimating population size

of the sheep blowfly, Lucilia cuprina, using mark-

recapture data corrected for variation in catch and

loss of marked individuals. J. Aust. ent. Soc. 21,

297-299. 146

Reznick, D. and Endler, J. (1982). The impact of predation

on life history evolution in Trinidadian guppies

(Poecilia reticulata). Evolution�, 160-177.

Ricklefs, R.E. (1974). Ecological aspects of avian energetics.

In Avian Energetics Paynter, R.A. (Ed), Cambridge,

Mass.: Nuttall Ornithological Club.

Rilling, S., Mittelstaedt, H. and Roeder K.D. (1959). Prey

· recognition in the mantis. Behaviour 14, 164-184.

Robertson, D.R. and Hoffman, S.G. (1977). The roles of

------female mate choice and predation in the mating systems

of some tropical Labroid fishes. Z. Tierpsychol, �, 298-320.

Roeder, K.D. (1965). Moths and ultrasound. Sci. Am. 212(4),

94-102.

Roff, D.A. (1973). On the accuracy of some mark-recapture

estimators. Oecologia g, 15-34.

Roff, D.A. (1981). On being the right size. Am. Nat. 118,

405-422.

Roff, D .A. (1983). Analysis of catch/effort data: a comparison

of three methods. Can. J. Fish. Aquat. Sci. 40,

1496-1506.

Rothschild, G.H.L. (1966). A study of a natural population

of Conornelus anceps . (Gelrnar) (Hornoptera: Delphacidae)

including observations on predation using the precipitin

test. J. Anirn. . Ecol. �, 413-434.

Rothschild, G.H.L. (1970). Observations on the ecology of the

rice ear bug Leptocoris oratorius (F). (Herniptera:

Alyidae) in Sarawak (Malaysian Borneo}. J. appl. Ecol.

J_, 147-167. 147

Rothschild, G.H.L. (1971) The biology and ecology of rice­

stem borers in Sarawak (Malaysian Borneo). J. appl.

Ecol. �, 287-322.

�thschild, M. (1973) Secondary plant substances and warning

colouration in insects. Syrop. R. ent. Soc. Land. 6,

59-83.

-�schild, M., Rowan, M.G., and Fairbairn, J.W. (1977).

Storage of cannabinoids by Arctia caja and zonocerus

elegans fed on chemically distinct strains of Cannabis

sativa. Nature 266,· 650-651.

Sale, P.F. (1974}. Overlap in resource use, and interspecific

competition. Oecologia 17, 245-256.

Schaller, G. (1972). The Serengeti Lion Chicago: Univ. of

Chicago Press.

Scheiring, J.E. (l977). Stabilizing selection for size as

related to mating fitness in Tetraopes. Evolution 31,

447-449.

Schlee, M.A. (1983). An experimental study of prey-attack

behaviour in the European Blackbird Turdus m. merula.L.

z� Tierpsychol. 61, 203-224.

Schulz, J.C. (_1981). Adaptive changes in antipredator

behaviour of a grasshopper during development.

Evolution�, 175-179.

Scott, J.A. (1977). Competitive exclusion due to mate

searching behaviour, male-female emergence lags, and

fluctuation in number of progeny in model invertebrate

populations. J. Anim. Ecol. i.§_, 909-924. 148

Selander, R.K. (1972). Sexual selection and dimorphism in birds. In Sexual Selection And The Descent Of Man

Campbell, P. (Ed). Chicago: Aldine.

Serpell, J.A. (1982). Factors influencing fighting and threat

in the parrot genus Trichoglossus. Anim. Behav. �'

1244-1251.

Service, M.W. (1973). Mortalities of the larvae of the

Anopheles garnbiae complex and detection of predators by

the precipitin test. Bull. ent. Res. g, 359-369.

Service, M.W. (1976). Mosquito Ecology: Field Sampling Methods

Essex: Applied Science Publishers.

Sheldon, J.K. and Rogers, L.E. (1978). Grasshopper food habits

within a shrub-steppe community. ·oecologia E, 85-92. y, T.W. and McDade, L.A. (1982). Prey selection and

handli_ng in two neotropical hover-gleaning birds.

Ecology 63, 1016-1028.

Siegfried, W.R. and Underhill, L.G. (1975}. Flocking as an

anti-predator strategy in doves. Anim� Behav. �,

504-508.

Sih, A. (1984). Optimal foraging and density-dependent

predation. Am. Nat. 123, 314-326.

Sinclair, A.R.E. (1975). The resource limitation of trophic

levels in tropical grassland ecosystems. J� ·Anim� Ecol.

44, 497 ...520.

Singer, M.C. (1972). Complex components of habitat suitability

within a butterfly colony. Science 176, 75-77.

Singer, M.C. (1982). Sexual selection ;for p;i:-otandry. · Am. Na.t.

, 440-443. 149

Slatkin, M. (1979). The evolutionary response to frequency-

and density- dependent interactions. Am. Nat. 114, 384-398.

Smith, J.M.N. and Dawkins, R. (1971). The hunting behaviour of

individual Great Tits in relation to spatial variation in

th�:Lr food density. Anim. Behav. 19, 695-706.

Smith, S. and Silverman, P.H. (1966). Metamorphic antigens of

mosquitoes.. Mosquito News�, 544-551.

Smythe, N. (1970). On the existence of "pursuit'invitationll

signs in animals. Am. Nat. 104, 491-504.

Snedecor, G.W. and Cochran, W.G. (1976). Statistical Methods

Iowa: Iowa State Univ. Press.

Sokal, R.R. and Rohlf, T.J. (1969). Biometry San Francisco:

W.H. Freeman and Co.

Southwood, T.R.E. (1978). Ecological Methods - With Particular

Reference To The Study Of Insect Populations London:

Methuen.

Spiess, E.B. and Ehrman, L. (1978). Rare male mating advantage.

Nature·272, 188-189.

Stamps, J.A. and Gon, S.M. III (1983). Sex-biased pattern

variation in the prey of birds. Ann. Rev. Ecol. Syst. 14,

231-253.

Stewart-Oaten, A. (1982). Minimax strategies for a predator-prey

game. Theor� Pop. Biol. E, 410-424.

Sunderland, K.D. and Sutton, S.L. (1980}. A serological study

of arthropod predation on woodlice in a dune grassland

ecosystem. J. Anim. Ecol. 49, 987-1005.

Sutton, $�L. (19701� P�edation on woodlice: an investigation

using. the p;r-ecipitin test. Ent. Exp� Appl� l3, 279-285 ..

- . � ! 150

Tanner, J.T. (1978). Guide To The Study Of Animal Populations

Knoxville, Tennessee: Uni� of Tennessee Press.

Taylor, R.J. (1976). Value of clumping to prey and the

evolutionary response of ambush predators. Arn. Nat. 110,

13-29.

Thompson, D.J. (1975). Towards a predator-prey model

incorporating age structure: the effects of predator and

prey size on the predation of Daphnia magna by Ischnura

elegans. J. Anim. Ecol. 44, 907-916.

Thornhill, R. (1976a) Sexual selection and nuptial feeding

behaviour in Bittacus apicalis (Insecta: Mecoptera).

Arn. Nat. 110, 529-548.

Thornhill, R. (1976b). Sexual selection��nd paternal invest­

ment in insects. Arn. Nat. 110, 153-163.

Thornhill, R. (_1980). Mate choice in Hylobittacus apicalis

(Insecta: Mecoptera) and its relation to some models of

female choice. Evolution 34, 519-538.

Thresher, R.E. (1977). Eye ornamentation of Caribbean reef fishes. z. Tierpsychol. �, 152-158. Tinbergen, N., Impekoven, M. and Franck, D. (1967). An

experiment on spacing out as a defence against predation.

Behaviour 28, 307-32l.

Titmus, G. (1983). Are animal populations really aggregated?

Oikos 40, 64-68.

Tostawaryk, W. (_1972}. The effect of prey defence on the

functional response of Podius modestus (Hemiptera:

Pentatomidae) to densities of the sawflies Neodiprion

swainei and N. pratti banksianae (Hymenoptera:

Neodiprionidae). Can. Entomol. 104, 6l-69.

·. ! 151

Treherne, J.E. and Foster, W.A. (1980). The effects of group

size on predator avoidance in a marine insect. Anim.

Behav. �' 1119-1122.

Treherne, J.E. and Foster, W.A. (1981). Group transmission of

predator avoidance behaviour in a marine insect: the

Trafalgar Effect. Anim. Behav. �, 911-917.

Treherne, J.E. and Foster,W.A. (1982). Group size and anti­

predator strategies in a marine insect. Anim. Behav.

�, 536-542.

Treisman, M. (1975a). Predation and the evolution of gregarious­

ness. I Models for concealment and evasion. Anim. Behav.

�' 779-800.

Treisman, M. (1975b). Predation and the evolution of greagrious­

ness. II An economic model for predator-prey interaction.

Anim Behav. �' 801-825.

Trivers, R.L. (1972). Parental investment and sexual selection.

In Sexual Selection And The Descent Of Man B. Campbell (Ed).

Chicago: Aldine.

Trivers, R.L. (1974). Parent-offspring conflict. Amer. Zool. 14,

249-265.

Trivers, R.L. and Willard, D.E. (1973}. Natural selection of

parental ability to vary the sex ratio of off-spring.

Science !22., 90-92.

Ueckert, D.N. and Hansen, R.M. (1971). Dietary overlap of

grass}:loppers on sandhill rangeland in northwestern

Colorado. oecologia _ _§_, 276-295. Uvarov, B.P. (1966). Grasshoppers ·And·Locusts ·vol I

Cambri_dge: Cambridge Univ. Press.

! 152

Vestjens, W.J.M. and Carrick, R. (1974). Food of the Black-backed

Magpie, Gymnorhina t. tibicen, at Canberra. Aust. Wildl.

Res. !_, 71-83.

Waldbauer, G.P. (1968). The consumption and utilization of

food by insects. Adv. Insect Physiol. �, 229-288.

Waldbauer, G.P. and Sheldon, J.K. (1971). Phenological

relationships of some aculeate hymenoptera, their dipteran

mimics, and insectivorous birds. Evolution 32_, 371-382.

Westaby, M. (1978). What are the biological bases of varied

diets? Aro. Nat. 112, 627-631.

Central African acridoids and associated ph�nomena. Acrida

l_, 1-7.

White, E.G. (1975) Identifying population units that comply

with capture-recapture assumptions in an open community

of alpine grasshoppers. Res. Popul� Ecol. 16, 153-187.

�an, D.W. (1982). Grasshopper sexual pheromone, a component

of the defensive secretion of Taeniopoda agues. Physiol. Ent.

]_, 111-115. /� / Wiklund, C. (1975). Pupal colour polymorphism in Papilio

ma,chaon L. and the survival in the field of cryptic versus

non-cryptic pupae. Trans. R. Entomol. Soc. London 127,

173-184.

Wiklund, C. and Fagerstrom, T. (1977). Why do males emerge

before females? Oecologia �, 153-158.·

Willis, A.J., McEwan, J.W.T., Greenwood, J.D. and Elton, R.A.

U.980). Food selection by chicks: effects of colour,

density and frequency of food types. Anim. Behav. 28,

874-879. 153

Wilson D.S. (1983). The group selection controversy: history

and current status.· Ann. Rev. Ecol. Syst. 14, 159-187.

Wilson, D.S. and Colwell, R.K. (1981). Evolution of sex ratio

in structured demes. Evolution 2, 882-897.

Wright, C.A. (1966). Experimental taxonomy: a review of some

techniques and their applications. Int. Rev. Gen. and Exp.

Zool. �, 1-43.

Wooller, R.D. (1984). Bill shape and size in honeyeaters and

other small insectivorous birds in Western Australia.

Aust. J. Zool. E, 657-661.

Zaman, v. and Chellapah, W.T. (1963). Gel diffusion studies with

mosquitoes (Diptera: Culicidae) antigens. I Antigenic

analysis during metamorphosis. Exp. Parasit. Q, 108-112.

Zaman, V. and Chellapah, W.T. (1965). The agar gel-diffusion

technique as a means of differentiating mosquito larvae.

Experientia 21, 297-298. 154

APPENDICES

APPENDIX 1

Total numbers of grasshoppers handled during population studies at site I.

1980/1981

Instar Males Females

1 5 5

2 35 24

3 61 122

4 74 33

5 70 104

6 51

Adults 152 28

198l/1982

Instar Males Females

1 7 14

2 28 25

3 52 40

4 50 35

5 36 24

6 11

Adults 209 41

.·- I · !. I'·· 155

APPENDIX 2

Location of study sites

Site Number Local Government Area UBD Reference

1 City of Melville Map 35 BS

2 City of Canning Map 29 M4

3 City of Melville Map 26 L7

4 City of Canning Map 38 G4

APPENDIX 3

Sampling techniques for different grasshopper instars

Instar Sampling technique

1-5 1-6 Hand capture

6 7 Sweep netting

I··: __._-- .-_-