Ecology and Evolution of Physiological Time in Insects

Abstract

The fact that development rate of insects changes with temperature implies that the rate at which time passes for a developing insect depends upon the temperature regime it experiences. Understanding this so-called physiological time is of the utmost importance in explaining adaptations of insect life histories to resources that vary seasonally. This paper summarizes data on development rate for 54 species of insects and data on annual temperature regimes on an elevational and a latitudinal gradient. It then treats the curve representing temperature-dependent development as a filter of the temperature regimes during various seasons. The analysis points out several new evolutionary consequences of the position and shape of the development rate curve in relation to season and location. (1) If the criterion for maximizing fitness is fastest development accumulation, the optimal development curve can be specified. (2) In locations with high midsummer temperatures selection could favor different development curves in spring and summer and lead to polymorphism or species replacement in these seasons. (3) If the criterion is appropriate timing, some interesting possibilities emerge. (a) The importance of host-plant choice by a phytophageous insect may depend on season in locations with hot summers. (b) Development rate curves that appear to give equivalent timing at one season may actually lead to opposite responses to year-to-year changes in the temperature regime and to very different timings in different seasons. (c) Interactions between species which have temperature-dependent development rates may lead to coevolutionary changes in their respective development curves. (4) Selection along elevational and latitudinal gradients may lead to clinal variation in the development rate curve or to species replacement.