Cheetahs of the caterpillar world: Are ground-feeding Speyeria faster than vine-feeding Agraulis?

DeRosa University Center, Ballroom

Movement of animals in nature is critical to their survival and reproduction, for example, to avoid predation and find food. The order Lepidoptera is one of the three most diverse orders of insects and their larvae (caterpillars) are almost exclusively herbivorous, relying on walking locomotion to feed upon their host plants. The wide variety of plants used as hosts by lepidopteran larvae vary in size and shape from tiny herbs to widespread grasses, to shrubs, vines, and trees. In this study, we sought to investigate the link between caterpillar movement and host plant form and habitat in two closely related butterfly genera: Agraulis and Speyeria. Agraulis primarily feed on passion vines (Passiflora spp.) along forest margins and fields. In contrast, Speyeria larvae feed on violets (Viola spp.) on the ground in forests and open grasslands. Lab observations have indicated that Speyeria larvae walk relatively fast, which represents a potential adaptation to two important aspects of their life history compared to Agraulis. First, Speyeria hatch on leaf debris, overwinter as young larvae and need to find their host in the spring. In contrast, Agraulis larvae hatch directly on their host plant. Second, Speyeria larvae are capable of eating their entire host in later instars, requiring them to seek additional food, whereas Agraulis larvae rarely do this and do not leave their host plant. This predicts that Speyeria larvae have increased velocity, due to natural selection, to help them find their host plant at multiple points in their life. Therefore, we examined both early instar locomotion as well as last instar locomotion to test whether Speyeria ground-feeding larvae have a higher relative velocity compared to Agraulis vine-feeding larvae.