1. Reaction norms for size and age at maturity were studied in Epirrita autumnata (Lepidoptera, Geometridae). Growth rates were manipulated by rearing larvae on different levels of food quality and quantity, and 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 be entirely ascribed to sex differences in initial weights of the last instar.

3. There were problems with considering the reaction norms optimal within the conventional demographic explanatory framework. Because fecundity increases linearly with body size and no costs of large adult size are known, one should expect female larvae to grow larger to increase their individual fitness. It is therefore likely that constraints 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 insect growth. Some limit to the initial:final weight ratio of an instar, and 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. Food quality throughout larval development may thus be very important for individual fitness and population dynamics in E. autumnata.