Direct and indirect phenotypic selection on developmental trajectories in Manduca sexta

1. Adult size and development time are the outcomes of growth and differentiation throughout the life of an individual organism – its developmental trajectory. Correlations among ages and sizes across different life stages may influence both direct and indirect components of selection on body size and age during development. 2. We used two field studies in experimental gardens in fall and summer to evaluate phenotypic selection on size and age across larval, pupal and adult stages in the tobacco hornworm, Manduca sexta, and how this may vary across seasons. 3. Rapid larval development was positively associated with survival to pupation and adulthood, in part because it allowed escape from larval parasitoids. Larval mortality owing to parasitoids was greater in the fall than in the summer. Egg production was positively correlated with adult mass, but not with development time. 4. Principal component analyses of size and age throughout development showed that adult size and development time were not negatively correlated, contrary to life-history expectations. As a result, selection favouring larger adult size (via female reproduction) and selection favouring rapid larval development (via juvenile survival) do not act in opposition in this system. 5. We discuss the physiological mechanisms that may underlie the independence of adult size and early larval development for holometabolous insects, and the implications for selection on body size and developmental trajectories.