Sexual size dimorphism in the two spot ladybird beetle Adalia bipunctata: developmental mechanism and its consequences for mating

Abstract  1. The literature on ladybirds indicates that males are consistently smaller than females but take the same length of time to complete their development. Rearing Adalia bipunctata at 20 and 25 °C confirmed that protandry cannot account for sexual size dimorphism in this species, nor can a difference in egg size.
2. Female larvae consumed more food and had a higher relative growth rate in the fourth instar than did male larvae.
3. When food is limited, small males appear to be more successful at mating than are large males.
4. To account for these results, it is hypothesised that the gonads of male larvae compete more strongly with the soma for resources and that this reduces the growth potential of the soma of male larvae relative to that of female larvae. The greater mating success of small males when food is limited supports the eat or mate hypothesis, which predicts that when food is limited small males will spend less time feeding and more time mating than will large males.     

Cannibalism and early instar survival in a larval damselfly

Cannibalism by larval damselflies late in larval development on larvae a few instars smaller has been widely documented. I examine here the survival of eggs oviposited near the end of the flight season of adult Enallagma boreale in the presence and absence of potential cannibals, individuals that hatched from eggs earlier in the season, over an extended part of the life-cycle. The role of competition as a modifier of cannibalism was examined by manipulating egg density, environmental productivity, and habitat complexity. Survival in the absence of potential cannibals ranged from 5% to nearly 50% but was only 0–3% in the presence of cannibals. Survival of small larvae was related to manipulations of habitat complexity but not initial density or resources. There were no significant interactions of the presence of large larvae with other experimental treatments on the survival of small larvae. The mean size of small larvae was greater in the presence of cannibals. This may be because the cannibalism treatment reduced the density of small larvae and reduced competition for resources, or that the cannibals preferentially fed on small larvae and only relatively large individuals remained. Fertilization of the habitat or manipulating the initial density of small larvae did not affect mass of small larvae at the end of the experiment, which would be expected if small larvae were affected by competition for resources. Potential cannibals, however, emerged at higher mass when small larvae were present at low density and when productivity of the habitat was increased. This suggests that the negative effect of competition by small larvae outweighs the positive effect of being potential prey for large larvae.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.     

The consequences of larval aggregation in the butterfly Chlosyne lacinia

1. Females of Chlosyne lacinia (Geyer) (Lepidoptera: Nymphalidae, Melitaenae), the bordered patch butterfly, clump eggs in a few large clusters on their host plant, Helianthus annuus. Resulting larvae form sibling aggregations to at least the third instar.
2. The effect of group size on survival and development of C. lacinia larvae was tested experimentally in the field. Larvae developed faster and survived better in larger groups.
3. The effects of various predator guilds (ground-dwelling arthropods, aerial arthropods and avian predators) on survival of larvae was then tested while controlling group size. Ground-dwelling arthropods, mainly fire ants Solenopsis xyloni , reduced larval survival greatly but other solitary invertebrate and avian predators did not alter survival. Group defences and aposematism of C. lacinia larvae are probably ineffective against predatory ants that attack en masse and recruit other colony members.
4. In laboratory experiments, two possible mechanisms underlying faster development of larvae in larger groups were tested: (i) overcoming the physical toughness of host plant leaves, and (ii) social stimulus to feed. Results support the physical toughness hypothesis but not the social stimulus hypothesis.
5. Feeding in large groups by C. lacinia larvae confers multiple advantages, including protection from solitary predators and increased feeding efficiency because grouped, early-instar larvae can initiate feeding wounds on tough sunflower leaves. These advantages of larval gregariousness, coupled with reduced desiccation at the egg stage, apparently outweigh disadvantages of aggregation, such as interference and exploitative competition among larvae.     

Host tree architecture mediates the effect of predators on herbivore survival

Abstract.  1. Vegetation structural complexity is an important factor influencing ecological interactions between different trophic levels. In order to investigate relationships between the architecture of trees, the presence of arthropod predators, and survival and parasitism of the autumnal moth Epirrita autumnata Borkhausen, two sets of experiments were conducted.
2. In one experiment, the architectural complexity of mountain birch was manipulated to separate the effects of plant structure and age. In the other experiment the trees were left intact, but chosen to represent varying degrees of natural complexity. Young autumnal moth larvae were placed on the trees and their survival was monitored during the larval period.
3. The larvae survived longer in more complex trees if predation by ants was prevented with a glue ring, whereas in control trees smaller canopy size improved survival times in one experiment. The density of ants observed in the trees was not affected by canopy size but spider density was higher on smaller trees. The effect of canopy structure on larval parasitism was weak; larger canopy size decreased parasitism only in one year. Until the fourth instar the larvae travelled shorter distances in trees with reduced branchiness than in trees with reduced foliage or control treatments. Canopy structure manipulation by pruning did not alter the quality of leaves as food for larvae.
4. The effect of canopy structure on herbivore survival may depend on natural enemy abundance and foraging strategy. In complex canopies herbivores are probably better able to escape predation by ambushing spiders but not by actively searching ants.     

Between-clutch interactions affect a benefit of group feeding for pipevine swallowtail larvae

Abstract.  1. In California, early instar larvae of the pipevine swallowtail ( Battus philenor ) develop at an accelerated rate when feeding in large groups compared with small groups due to a plant-mediated response to feeding group size. Larvae benefit from accelerated growth because the time larvae remain in early stages, where mortality is highest, is reduced. Occasionally, multiple clutches are laid on the same plant stem. Clutch size modification by females ovipositing on plant with previously laid clutches and the effect of kinship and group size on larval behaviour was examined. The direct and indirect interactions between clutches were investigated to determine if group size and time between clutch establishment affects the performance of early instar larvae.
2. Larger groups consume the young foliage more quickly and develop at an accelerated rate compared with smaller groups. Older foliage available to later clutches is an inferior food resource compared with younger foliage.
3. There was no evidence that females adjust clutch size in response to the presence of conspecific clutches.
4. Second groups of larvae readily joined previously established feeding groups. There were no observed behavioural differences between sibling and mixed-family groups.
5. The effect of a second group on the growth of the initial group was dependent on the size of both groups and the time interval between the arrival of the two groups.
6. Accelerated growth associated with larger feeding aggregations was absent when these groups were introduced to plants with previously established groups.
7. It is beneficial for ovipositing females to avoid plants with previously laid clutches because direct and indirect interactions with established clutches compromises larval performance.     

Intraspecific interference among larvae in a semivoltine dragonfly population

Summary This study focuses on ways that the size distribution of individuals influences the types and intensities of competitive interactions within a population of aquatic arthropod predators. Three field experiments and one laboratory experiment were designed to test for feeding interference, interference mortality, and dispersal effects within and between larval size classes of the primarily semivoltine dragonfly Tetragoneuria cynosura in Bays Mountain Lake. One field experiment documented the temporal pattern of colonization of large-mesh cylinders by the small, first-year-class larvae during a 30-day period; the results are consistent with passive (density-independent) colonization. A second field experiment examined the effect of large, second-year-class larvae at densities of 1 or 3 per cylinder (14 or 42 m^-2) on colonization by small larvae; this colonization was inhibited at the high density of large larvae. In the laboratory experiment, when larvae of the two size-classes were together in the same aquarium, small larvae moved around less than when by themselves (dispersal inhibition). Thus the inhibition of colonization observed in the field may result from interference mortality, rather than from a flight response to the presence of larger conspecifics.To evaluate this interpretation, the third field experiment measured the in-situ functional response of large larvae to each other and to their small conspecific prey. Results suggest a type 1 (linear) functional response, with feeding inteference among large larvae. Moreover, the interference mortality inflicted by larger larvae on smaller conspecifics was apparently more intense on larger individuals within the small size-class. Taken together, the three field experiments and a statistical power analysis show how colonization and interference interact to determine the local density of small larvae, and why such interference effects are difficult to detect experimentally in the field.     

Cohort studies on mosquito (Diptera: Culicidae) larvae living in the water-filled floral bracts of Heliconia aurea (Zingiberales: Musaceae)

Abstract. 1. An experimental and observational study of the three species of mosquito larvae living in the water-filled floral bracts of Heliconia aurea at Rancho Grande, Venezuela, was designed to determine if each mosquito species forms instar cohorts and excludes conspecifics of younger instars. This study was motivated by a statement of Maguire (1971) who suggested that mosquito larvae in Puerto Rican Heliconia inflorescences form instar cohorts and exclude younger mosquitoes from the floral bracts.
2. Field observations of naturally occurring populations of mosquito larvae from fifty-three H.aurea inflorescences give no indication that cohorts are formed for Wyeomyia felicia and Culex bihaicolus . However, there is strong evidence that cohorts of Trichoprosopon digitatum occur. These cohorts are not structured by larval competition but by female preference of the youngest bracts as oviposition sites and by large T.digitatum larvae feeding on smaller con-specifics.
3. Field experiments show that first instar W.felicia and C.bihaicolus survive as well over a 6 day period in the presence of third instar conspecifics as they do when all mosquito larvae in an experimental bract are first instars.     

Variations in the life-history parameters of Hemipyrellia ligurriens (Diptera: Calliphoridae) in response to larval competition for food

Abstract. 1. Larval rearing densities of Hemipyrellia ligurriens (Wiedemann) (Diptera: Calliphoridae) in standardized carrion were manipulated in order to investigate changes in life-history parameters in response to larval competition for food.
2. Competition was of the typical scramble type. Survivorship remained high at densities up to 32 larvae g liver^-1 but decreased rapidly as larval density increased further.
3. Emergent adults were undersized with reduced fecundity and longevity. Variations in adult body size apparently reduced the effects of competition on larval mortality.
4. Females of dry weight corresponding to only 10.4% of the potential maximum emerged at the highest rearing densities of 128 larvae g liver-1. However, these females had a nearly four-fold increase in reproductive investment (per unit weight) when compared to the largest individuals.
5. The duration of larval development declined when competition was intense (i.e. at high larval densities).
6. The short adult life of H.ligurriens, combined with the unpredictability of larval habitat availability, may reduce the value of long-range dispersal so that females 'do better' by maintaining reproductive investment despite a concomitant decline in dispersal ability.     

Interference competition among burying beetles (Silphidae, Nicrophorus)

Abstract. 1. This study investigated the impact of intraspecific and interspecific competition on the reproductive success of a biparental burying beetle, Nicrophorus defodiens Mannerheim.
2. Marked pairs or single females were placed on small and large mouse carcasses in the field in 1985 and 1986. Carcasses were exhumed after 9–10 days to determine the identity of the resident adult(s) and the production of young.
3. Competition was assessed by the prevalence of takeovers by intruders (unmarked adults). For N.defodiens , after the initial colonization of the carcass, interspecific competition from larger N.orbicollis Say and N.sayi Laporte was substantial and more intense than intraspecific competition. Competition was also greater in the middle of the breeding season and on large as opposed to small carcasses.
4. Successful takeovers resulted in the expulsion of the prior resident(s), killing of any offspring present on the carcass, and oviposition of a new clutch by the intruder.
5. Females aided by males were more likely than single females to avoid takeovers but did not produce larger broods or larvae of larger mass.
6. An additional laboratory experiment in 1985 and a field experiment in 1986 suggest that N.defodiens is able to reproduce on very small carcasses despite intense heterospecific activity.     

Selection of large host plants for oviposition by a monophagous leaf beetle: nutritional quality or enemy‐free space?

Abstract.  1. Oviposition site selection is crucial for the reproductive success of herbivorous insects. According to the preference–performance hypothesis, females should oviposit on host plants that enhance the performance of their offspring. More specifically, the plant vigour hypothesis predicts that females should prefer large and vigorously growing host plants for oviposition and that larvae should perform best on these plants.
2. The present study examined whether females of the monophagous leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) prefer to oviposit on large host plant individuals of the meadow clary and whether large host plants are of higher nutritional quality than small host plants. Subsequently, it was tested whether the female preference correlates with offspring performance and survival.
3. In the field, females preferred large host plant individuals for oviposition and host plant quality, i.e. leaf nitrogen content, was significantly higher in leaves of large than of small host plants.
4. In the laboratory, larval development time was shorter on leaves of large host plant individuals than on small host plant individuals, but this could not be shown in the field.
5. However, a predator-exclusion experiment in the field resulted in a higher survival of larvae on large host plants than on small host plants when all predators had free access to the plants. On caged host plants there was no difference in survival of larvae between plant size categories.
6. It is concluded that females of C. canaliculata select oviposition sites that enhance both performance and survival of their offspring, which meets the predictions of the plant vigour hypothesis.     

Intraspecific competition amongst larval Aedes aegypti: food exploitation or chemical interference?

Abstract. 1. Competition within and between the larval instars of the yellow fever mosquito, Aedes aegypti , can be measured by its effect on stage duration. In a series of laboratory experiments the relative importance of chemical interference and food exploitation in mediating competition between first and fourth instar larvae was investigated.
2. In contrast to the results of three previous studies, I found no evidence that a chemical growth retardant played any part in greatly prolonging the stage durations of larvae in both age classes.
3. When competition between the two age classes became important, the relative increase in stage duration wasgreater for first instar larvae than for fourth instar larvae. This result can be reproduced with Gilpin & McClelland's (1979) model of competition by food exploitation alone, providing the range of available food particle size is assumed to be an increasing function of age. An additional, but less important, refinement makes the conversion efficiency of food into larval biomass a decreasing function of age.     

A minimalist approach to the effects of density-dependent competition on insect life-history traits

Abstract  1. Due to its effects on the phenotypic and genotypic expression of life-history traits, density-dependent competition is an important factor regulating the growth of populations. Specifically for insects, density-dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size.
2. The aim of the work reported here was to test whether the established phenotypic effects of density-dependent competition on life-history traits could be reproduced in an experimental design requiring a minimal number of individuals. Larvae of the mosquito Aedes aegypti were reared at densities of one, two, or three individuals per standard Drosophila vial and in six different conditions of larval food availability. This design required relatively few individuals per independent replicate and included a control treatment where individuals reared at a density of one larva per vial experienced no density-dependent interactions with other larvae.
3. Increased larval densities or reduced food availability led to increased larval mortality, delayed pupation, and the emergence of smaller adults that starved to death in a shorter time (indicating emergence with fewer nutritional reserves).
4. Female mosquitoes were relatively larger than males (as measured by wing length) but males tended to survive for longer. These differences increased as larval food availability increased, indicating the relative importance of these two traits for the fitness of each sex. The role of nutritional reserves for the reproductive success of males was highlighted in particular.
5. This minimalist approach may provide a useful model for investigating the effects of density-dependent competition on insect life-history traits.     

Population density, phenotype and reproductive output in the grasshopper Chorthippus brunneus

ABSTRACT. 1. The effects of population density on the reproductive output of individual female grasshoppers ( Chorthippus brunneus Thunberg) and the relationships between reproductive output and component elements of each female's phenotype, were investigated.
2. Reproductive output was primarily determined by the rate of egg-pod production. Increased density led to significant reductions in the rate of egg-pod production and reproductive output.
3. Exoskeleton size (reflecting nymphal experience) was positively correlated with clutch size at both high and low densities, but condition (reflecting adult experience) showed no such correlation. At high density, exoskeleton size was more strongly correlated with the rate of egg-pod production and reproductive output than condition. It is concluded that the nymphal aspects of size are more important than the adult aspects.
4. At low density, females with small exoskeletons compensated for their smaller clutches by producing egg-pods at a faster rate. Thus, there was no overall relationship between reproductive output and any of the phenotypic characteristics.
5. The effects of competition were weakly asymmetric in high density populations. The weakness of the asymmetry suggests scramble-like interactions for resources. It is concluded that not only are the effects of competition influenced by individual differences, but also that competition may reveal differences that would not otherwise be apparent.     

To produce many small or few large eggs: a size-independent reproductive tactic of fish

Summary We demonstrate here the existence of a range of size-independent reproductive tactics in teleostean fish involving the allocation of a size-dependent reproductive effort between fecundity and egg size. Despite considerable evidence that larger eggs and the larvae hatching from them are more likely to survive than smaller ones, we found no evidence of evolutionary trends towards greater egg sizes. Fish with pelagic eggs tend to spawn many, and therefore small, eggs, whereas demersal spawners tend to produce large, and therefore few, eggs. Maximizing egg number should increase the number of eggs hatching in suitable locations in the patchy pelagic environment and, hence, increase the reproductive success of pelagic spawners. On the other hand, the reproductive success of demersal spawners, which reduce the variance in growing conditions experienced by the off spring, should be more dependent on the survival of the individual larvae, which increases as egg size increases.     

Effect of spatial pattern and microhabitat on pit construction and relocation in Myrmeleon hyalinus (Neuroptera: Myrmeleontidae) larvae

Abstract.  1. In two sets of enclosure experiments, we studied the spatial pattern, relocation rates, pit construction rates, and microhabitat preference of Myrmeleon hyalinus larvae.
2. We showed that M. hyalinus larvae actively prefer shady sites and often relocate to shady areas when exposed to the sun. This behaviour may constitute a life-saving strategy in desert environments.
3. The initial spatial pattern in a cluster of antlion larvae did not affect the final pattern, relocation rate, or the pathway moved while relocating. We interpret this finding to mean that the spatial pattern of M. hyalinus larvae is mainly influenced by factors operating at large time-scales, such as exploitation competition and physical-microclimatological constraints, rather than those operating at shorter time-scales, such as interference competition.
4. In contrast, the likelihood of pit construction was positively correlated with nearest neighbour distance, possibly as a result of interference competition.
5. Pit construction rates were constant throughout the experiment, while relocation rates decreased with time.
6. Contrary to previous studies, we found no correlation between body mass and spatial position inside the cluster, suggesting that these differences are the products of slower processes (e.g. exploitation and body condition).     

Effects of prey density, prey instar, and patch size on the development of the predatory mosquito, Toxorhynchites towadensis

Effects of prey density, prey instar, and patch size on the development of the predatory mosquito larva, Toxorhynchites towadensis, were investigated in the laboratory. Survivors of T. towadensis showed different developmental patterns in relation to prey age structure. All predatory larvae in containers with only second instar prey developed into the third instar. However, in several containers with fourth instar prey, mortality of predators was observed. During the third instar, no predatory larva died, but both prey density and prey instar significantly affected the survival of predators during their fourth instar. Large prey size promoted large predator adults, and predatory larvae which grew up in small surface containers responded by developing to large sizes than those in large containers. Larval developmental time of the predators differed in each treatment. During first and second instars, faster predator development was observed in containers with small surface areas and containing young prey individuals. However, when development was enhanced by the presence of old prey individuals, no surface effect was observed. The fastest predator development was observed with prey of mixed instars and high density. This study suggests that a small surface container containing prey of mixed instars and high density is suitable for development of predators.     

Prey behaviour and size-selective predation by fish

SUMMARY 1. We investigate the role of differential activity of chiro-nomids as an explanation for the size selection of small larvae by fish.
2. In the laboratory, pumpkinseed sunfish (Lepomis gibbosus L.) selectively consumed large larvae of Chironomus tentans Fab. when no tube-building materials were available. Small larvae were selectively consumed when a mud substrate was provided but there was no difference in predation rates on large and small larvae in sand.
3. Small larvae spent more time out of their tubes than large larvae in the presence of fish, which may explain the selection for small larvae in mud. Large larvae apparently compensated for decreased foraging activity in the presence of fish by increasing activity when fish were absent. Visibility of large larvae inside tubes may account for their increased mortality in sand.
4. Our results suggest that differential activity is important in explaining the size-selective mortality observed in the field.     

Determination of adult size in a folivorous moth: constraints at instar level?

1. Reaction norms for size and age at maturity were studied in Epirrita autumnata (Lepidoptera, Geometridae). Growth rates were manipulated by rearing larvae on different levels of food quality and quantity, and instar-specific final weights and development times were recorded.
2. Food level and initial weight of an instar accounted for most of the variance in final weights. Sexual dimorphism in pupal weights could be entirely ascribed to sex differences in initial weights of the last instar.
3. There were problems with considering the reaction norms optimal within the conventional demographic explanatory framework. Because fecundity increases linearly with body size and no costs of large adult size are known, one should expect female larvae to grow larger to increase their individual fitness. It is therefore likely that constraints play a major role in the determination, and evolution, of size in this species.
4. A focus on individual instars may be the best way to reveal the constraints on, and space for, adaptive evolution of insect growth. Some limit to the initial:final weight ratio of an instar, and the fixed number of instars, may represent important constraints.
5. Reaction norms like the ones described in this study lead to strong environmental determination rather than canalization of body size. Food quality throughout larval development may thus be very important for individual fitness and population dynamics in E. autumnata.     

Egg size affects larval performance in a coleopteran parasitoid

Abstract 1. Optimal progeny size models assume that the more eggs a female produces, the lower the amount of resource allocated per egg. As egg size generally correlates with the fitness of the emerging immature, this trade‐off can be expressed as a choice between the production of numerous low quality or fewer high quality progeny. 2. The first‐instar larvae of the coleopteran parasitoid Aleochara bilineata have to search for and parasitise dipteran pupae. The present study found a positive correlation between egg size and larval weight, but not between egg size and development time. Larger first‐instar larvae survived longer, were more active, and found and parasitised their host more rapidly. 3. Female A. bilineata may invest smaller larvae in conditions of high host density and low intraspecific competition, but investing fewer, larger larvae would bring them more fitness when hosts are scarce and competition high.