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Sex-specific mechanisms of the determination of insect body sizes are insufficiently understood. Here we use the common heath moth, Ematurga atomaria (Lepidoptera: Geometridae) to examine how larval growth trajectories differ between males and females. We monitored the development of 1379 larvae in controlled laboratory conditions. Sexually dimorphic development times during the first four instars were associated with sexual size dimorphism (SSD) in the beginning of the fifth (last) instar, when females were on average 15% heavier than males. Similarly, the duration of the last instar was about 13% longer in females. Further, we specifically focussed on the estimates of differential (instantaneous) growth rates of the larvae based on 24h mass increments of the 2nd, 3rd, 4th and 5th day in the beginning of the last instar. We calculated ‘allometric’ differential growth rates as the per-day increase in cube-root-transformed mass of the larvae. We found that allometric growth rates were slightly but significantly larger in females than in males. As this measure of growth rate (in contrast to the relative growth rate, based on the ratio of masses recorded at consecutive measurements) did not depend on body size, it allows an unambiguous separation of the effects of sex and size. We conclude that in accordance with an emerging general pattern, larger female body size in E. atomaria is achieved primarily by means of a longer growth period. Furthermore, our study shows that the differential growth rate can also be sexually dimorphic and contribute to SSD. This contribution, however, is lower than that of the development time by an order of magnitude. In addition to development periods and growth rates, other parameters of the non-linear growth curves of insect larvae also need to be considered in the context of SSD determination. In particular, weight loss prior to pupation was shown to be considerably larger in females than in males. [ABSTRACT FROM AUTHOR]

An evolutionary explanation should consider the balance between environmentally-based selective pressures, and the resistance of the organism's phenotype to adaptive evolution, with the latter being captured by the concept of constraint. The limited attention to non-adaptive explanations in evolutionary ecology is at least partly caused by methodological difficulties with respect to identifying and quantifying constraints. As an example of an experimental approach evaluating a constraint-based explanation, we present a cross-species comparison of the shape of reaction norms for size and age at maturity. Instar- and sex-specific development times and final sizes were recorded for two distantly-related species of insects ( Lepidoptera), with larval growth rates being manipulated by means of refined starvation treatments. We found that (1) the 'classical' L-shaped reaction norms for final size and development time are characteristic also of individual larval instars; (2) these responses show a high degree of quantitative similarity across the species, different larval instars, and sexes within species; and (3) the similarity among species and sexes is higher for the penultimate than for the final instar. The high degree of similarity suggests that some physiological mechanisms determining such reaction norms are evolutionarily conservative. An alternative explanation (i.e. quantitative similarity of ecologically based selective pressures) appears less likely. The results of a previous study on a third lepidopteran species not only support our general conclusions, but also provide a clear case of adaptive evolution in some aspects of such reaction norms. The present study shows one way how the data required to measure evolutionary conservatism in reaction norms for body size can be obtained empirically. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114, 296-307. [ABSTRACT FROM AUTHOR]

The theory of life history evolution generally predicts a negative across-environment correlation between development time and size at maturity in response to variations in environmental quality. Deviations from this pattern occur under specific circumstances. In particular, organisms may mature both early and at a small size when (1) some ultimate change (e.g. time constraint, resource exhaustion) in the environment precludes further growth, or (2) when there are predictable among-environment differences in mortality rates. The first scenario is frequently documented in insects but evidence for the second possibility is scarce. Here we report a crowding-induced plastic response resulting in a clear positive across-environment correlation between final weight and development time in a geometrid moth. The response was apparent during the entire larval period and in the last larval instar. Crowding also led to increased growth rates. As outbreaks have not been reported for this species it is unlikely that early pupation is a response to anticipated food shortage. Instead, we suggest that crowded larvae may perceive a higher risk of predation, perhaps because they are unable to distinguish conspecifics from potential predators. A possibility for a plastic increase in growth rate implies that the uncrowded larvae grow at submaximal rates, which indicates a cost of high growth rate. [ABSTRACT FROM AUTHOR]

Assumptions about mortality rates form a cornerstone for models of life-history evolution. When seeking adaptive explanations for body sizes, the size dependence of predation risk is of particular interest. Here, we review published studies that provide (1) estimates of the daily predation rates experienced by insect larvae feeding on tree leaves or (2) evidence concerning the relationship between predation risk and larval size. Larvae were found to experience an average avian predation rate of 3.1% per day and an average arthropod predation rate of 10.5% per day. In some studies, mortality rates were systematically dependent on parameters of the larvae (e.g. coloration) or of the environment (host plant, season), but not to the extent that would render generalizations meaningless. Nevertheless, mortality rates varied considerably more for arthropod than avian predators, making an estimate of avian predation rate more reliable for use in quantitative models. Moreover, birds tend to be a more important predator guild exploiting the larger larval stages, as indicated by the predominantly positive size dependence of avian predation risk. By contrast, predation by arthropods was generally negatively size dependent. Based on the available data, we estimate that avian predation rates increase approximately 3.6-fold, while arthropod predation rates decrease approximately 4.9-fold, in response to a 2-fold increase in the linear size of prey. A modelling exercise showed that realistic mortality rates - if assumed to be independent of size - cannot serve as a basis for adaptive explanations for observed body sizes. However, by assuming a positive size dependence of mortality risk within the limits observed for bird predation, it is possible to explain a wide range of body sizes within an optimality framework. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 1-18. [ABSTRACT FROM AUTHOR]