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Intraspecific color polymorphism is widespread in insects, and various mechanisms have been proposed to explain its maintenance. Some explanations rely on the effect of body color on the organism’s thermal physiology. Darker individuals accumulate solar energy more efficiently, and therefore, dark body coloration in insects is frequently presumed to be an adaptation to low temperature conditions. However, it is largely unclear what is the importance of the thermal biology in comparison to other potential selective forces on body coloration. In this study, we evaluated the role of temperature as a potential selective factor maintaining color polymorphism in aposematic larvae of the moth Orgyia antiqua L. It was found that darker, and thus less aposematic, larvae accumulated solar energy more efficiently. However, in a set of laboratory and outdoor experiments, we found no evidence of temperature-dependent performance of different color morphs or in development of different morphs induced by rearing temperature. We conclude that the effects related to thermal physiology are not likely important determinants of optimal coloration in O. antiqua. The reasons may lie in high mobility of the larvae, which allows for effective behavioral thermoregulation, which is also shown in this study. Our results caution against an uncritical extrapolation of results obtained for model organisms and indicate the need for giving more attention to the species-specific ecological background in ecophysiological studies.

The number of larval instars varies widely across insect species. Although instar number is frequently considered to be invariable within species, intraspecific variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies, we show that intraspecific variability in the number of larval instars is widespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, food quality and quantity, humidity, rearing density, physical condition, inheritance, and sex are the most common factors influencing the number of instars. We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according to which instar number increases in adverse conditions when larvae fail to reach a species-specific threshold size for metamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more specific cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional generations in multivoltine species, or increasing the probability of surviving in long-lasting adverse conditions.

For three species of geometrid moths with noneruptive population dynamics, physiological and behavioral responses of ovipositing females to larval host deprivation were recorded. Single substrate trials were used. Availability of adult food was also varied to manipulate physiological condition of the moths. Egg production rate decreased only weakly in the absence of host plant, whereas it was strongly influenced by adult feeding. The time from the beginning of the experiment to initiation of oviposition was largely determined by presence of suitable host. This variable, oviposition latency, was only weakly affected by nutritional status of the females. Oviposition rates also remained lower on unsuitable hosts after oviposition was initiated. This indicates that the effect of inferior oviposition substrate was not limited to postponing oviposition. Host deprivation and adult feeding both lead to accumulation of chorionated eggs in abdomens. However, high egg loads of the fed females did not alter their behavior in the direction predicted by optimality models. We conclude that oviposition latency was the only variable in which female responses to unsuitable hosts were sufficiently strong to have potential significance for population dynamics. Latency presumably corresponds to dispersing behavior in nature. Such a response, not having been observed in an outbreaking geometrid studied earlier, may reflect a behavioral mechanism capable of stabilizing local population dynamics. The results are discussed in the light of the gradation from capital to income breeding. In herbivorous insects, oviposition latency may serve as an index of overall sensitivity of ovipositing females to adverse environmental conditions.

We studied which site characteristics were associated with susceptibility to outbreaks of Epirrita autumnata Borkhausen in a mixed coniferous forest area in Finnish Lapland during a high density phase of the population cycle from 1992 to 1995. This information is important in understanding population dynamics of the species and as a background in studies of biotic interactions between E. autumnata and its host plants and natural enemies. We determined relative densities of larvae living on birch, Betula pubescens Ehrhart, at 45 sites covering an area of ≈3,900 km2. Association of larval density with 5 site characteristics was evaluated: altitude-related winter minimum temperature, birch age, birch density, overall tree density, and forest type. Of these variables, local winter minimum temperature and birch age together accounted for nearly half of the variance in larval density; the rest of the variables were not important. There is a clear mechanism for the effect of winter minimum temperature: E. autumnata eggs are killed at temperatures below -36°C. Because of inversion, cold air accumulates in topographically low sites, killing eggs there. We have, however, no simple explanation for why sites with old birches had high larval densities.