No abstract

Temperature drives development in insects and other ectotherms because their metabolic rate and growth depends directly on thermal conditions. However, relative durations of successive ontogenetic stages often remain nearly constant across a substantial range of temperatures. This pattern, termed ‘developmental rate isomorphy’ (DRI) in insects, appears to be widespread and reported departures from DRI are generally very small. We show that these conclusions may be due to the caveats hidden in the statistical methods currently used to study DRI. Because the DRI concept is inherently based on proportional data, we propose that Dirichlet regression applied to individual-level data is an appropriate statistical method to critically assess DRI. As a case study we analyze data on five aquatic and four terrestrial insect species. We find that results obtained by Dirichlet regression are consistent with DRI violation in at least eight of the studied species, although standard analysis detects significant departure from DRI in only four of them. Moreover, the departures from DRI detected by Dirichlet regression are consistently much larger than previously reported. The proposed framework can also be used to infer whether observed departures from DRI reflect life history adaptations to size- or stage-dependent effects of varying temperature. Our results indicate that the concept of DRI in insects and other ectotherms should be critically re-evaluated and put in a wider context, including the concept of ‘equiproportional development’ developed for copepods.

Tortoise beetles ( Cassida and related genera) are a large cosmopolitan group that includes several pests of agricultural crops and natural enemies of weeds but their biology and ecology remain poorly known. Using a set of environmental chambers, we address simultaneous effects of temperature and photoperiod on immature development and adult body mass in two European species, C . rubiginosa and C . stigmatica . Consistent with its broader distribution range, the former species is less susceptible to low rearing temperatures, develops faster and has a larger body mass than the latter. However, C . rubiginosa seems to be less adapted to late-season conditions as a short-day photoperiod accelerates its immature development to a lesser extent than it does in C . stigmatica , which nevertheless results in greater larval mortality and slightly but significantly smaller adults. By contrast, in C . stigmatica , which is more likely to encounter late-season conditions due to its slower life cycle, short-day acceleration of development is achieved at no cost to survivorship and final body mass. The experiment with C . stigmatica was repeated during two consecutive years with different methods and the main results proved to be well reproducible. In addition, laboratory results for C . rubiginosa agree with field data from literature.

Temperature and nutrition are crucial environmental variables that determine rates of growth and development in insects. However, the simultaneous effect of these factors on life‐history traits is rarely addressed. In the present study, the influence of two diets (linden fruit and sunflower seeds) on the duration of immature stages and thermal reaction norms for development is tested in the bug Pyrrhocoris apterus L. (Heteroptera: Pyrrhocoridae). Eggs and larvae are reared at five constant temperatures (20, 22, 24, 26 and 28 °C) under an LD 20 : 4 h photocycle. Development rates deviate from linearity in the studied thermal range, especially in larvae; therefore, a nonlinear (power‐law) approximation is also attempted. Parental diet causes no change in thermal reaction norms for egg development. However, the progeny of sunflower‐fed bugs are more variable in terms of their development time, suggesting a transgenerational effect. Larval mortality rates increase in cooler conditions and are always higher on sunflower seeds. This is accompanied by more variable, less temperature‐dependent and generally slower larval development. A review of previously published case studies on temperature–diet interactions in the control of insect development leads to two general conclusions. First, there are two approaches for assessing the temperature‐dependent development in insects: one based on the concept of the sum of degree‐days and the other based on the concept of reaction norm. Despite an obvious non‐exclusiveness, the two approaches appear to have developed in isolation from each other. Second, three principal patterns of temperature–diet interactions can be recognized. The pattern found in P. apterus (the direct effects of diet are stronger at higher temperatures and much weaker or absent at lower temperatures) appears to be the most widespread.

One of the leitmotifs of the ecophysiological research on ectotherms is the variation and evolution of thermal reaction norms for biological rates. This long‐standing issue is crucial both for our understanding of life‐history diversification and for predicting the phenology of economically important species. A number of properties of the organism's thermal phenotype have been identified as potential constraints on the evolution of the rate–temperature relationship. This comparative study addresses several such constraints by testing whether the actual interspecific variation of thermal reaction norms across nearly hundred leaf beetle species agrees with the expected patterns. The results show that developmental rate and its temperature‐dependent parameters are similar in closely related species and that the variation pattern depends on the taxonomic scale, the thermal reaction norms being mostly parallel for the representatives of distant subclades but intersecting more often farther down the phylogenetic tree. The parallel shift disagrees with the putative ubiquity of a positive slope–threshold relationship, whereby thermal reaction norms should normally intersect, and even more contradicts with the common‐intersection hypothesis. The ability to develop in cooler conditions is not traded off at higher temperatures, which is an exception to the “warmer is better” principle. A comparison of high‐ and low‐quality data indicates that some of these discrepancies with earlier findings may stem from a likely presence of noise in previous analyses, which may have affected the variation patterns observed. Overall, the failure to support the universality of the predicted patterns suggests that the evolution of thermal reaction norms in leaf beetles has largely overcome the hypothesized constraints.