Effects of larval dietary restriction on adult morphology, with implications for flight and life history

Many morphological traits of adult insects show a plastic response to larval environmental conditions. Past studies of Lepidoptera, other than on irruptive species, focused on species with specific ecologies or life histories. These studies must be interpreted in the context of past selection associated with those specializations. Here, we examined the effects on adult morphology of quantitative dietary restriction in the late last instar of Speyeria mormonia Edwards (Lepidoptera: Nymphalidae: Heliconiini). This univoltine species lives in open populations in montane meadows. Adults fed ad libitum as larvae on Viola sororia Willd. (Violaceae) were larger than those fed a restricted diet, for wet and dry body mass, dry mass of head, thorax, or abdomen, and forewing length. Females were larger than males for all but head mass. However, both males and individuals fed a restricted diet had a heavier head and thorax as a proportion of total mass, indicating conservation of visual or other sensory functions and flight. Females maintained a higher relative investment of mass in abdomens, indicating conservation of mass allocation to reproduction. Wing loading was greater in females and for individuals fed ad libitum. The sex*treatment interaction was also significant. The difference in wing loading between individuals fed ad libitum and those on a restricted diet were greater for females than for males. The allometric relationship between dry mass and wing area in both sexes was significantly less than isometric. Thus, allometry did not explain the effect of dietary restriction on wing loading. In contrast, wing aspect ratio was invariant. In combination with earlier studies, these results suggest that mass allocation in support of flight occurs generally under larval nutritional stress. Flight supports both reproduction and feeding. Our work thus highlights the need to include resource acquisition in studies of life history and fitness under nutritional stress.