A critical evaluation of the insect body size model and causes of metamorphosis in solitary bees

The insect body size model posits that adult size is determined by growth rate and the duration of growth during the larval stage of development. Within the model, growth rate is regulated by many mechanistic elements that are influenced by both internal and external factors. However, the duration of growth is regulated by the physiological processes underlying metamorphosis—attainment of a critical weight, a terminal growth period, and finally cessation of growth itself. While the hormonal dynamics that regulate the terminal growth period and cessation of growth are well-understood, the mechanistic basis of the critical weight has remained elusive. More importantly, the body size model is based almost entirely on a few insect species, and its applicability for other insects is still an open question. In this study, we aimed to characterize the critical weight in the solitary bee, Osmia lignaria. In doing so, we found that this species does not have a “critical weight” per se, but rather uses food provisioning and its absence as a strong cue for metamorphic commitment. Individuals that are provisioned with an excess of larval provisions still eventually undergo metamorphosis—although after considerable delay—suggesting that there may be multiple cues that can trigger a critical weight decision. Finally, our study showed that nearly all (~90%) of variation in adult size for O. lignaria was determined by the relative timing of growth cessation during larval development. Thus, the duration of larval growth had profound impacts on adult size.