DETERMINATION OF ADULT SIZE IN A FOLIVOROUS MOTH - CONSTRAINTS AT INSTAR LEVEL

1. Reaction norms for size and age at maturity were studied in Epirrit a autumnata (Lepidoptera, Geometridae). Growth rates were manipulated by rearing larvae on different levels of food quality and quantity, an d 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 b e entirely ascribed to sex differences in initial weights of the last instar. 3. There were problems with considering the reaction norms opt imal within the conventional demographic explanatory framework. Becaus e fecundity increases linearly with body size and no costs of large ad ult size are known, one should expect female larvae to grow larger to increase their individual fitness. It is therefore likely that constra ints 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 insec t growth. Some limit to the initial:final weight ratio of an instar, a nd 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. Foo d quality throughout larval development may thus be very important for individual fitness and population dynamics in E. autumnata.