Diversification of gut morphology in caterpillars is associated with defensive behavior

The relationship between insect gut structure and foraging strategy has been studied for several hundred years; however, we know little about how, or even if, other common insect behaviors are linked to gut morphology. For example, many insects defend themselves by regurgitation, a behavior which is expected to be closely connected to gut structure. Caterpillars belong to an insect taxon, the Lepidoptera, with a particularly well-studied digestive tract and a known predilection for defensive regurgitation. I have explored whether defensive regurgitation is associated with specific gut structure by examining the relationship between defensive behavior and gut morphology in the larvae of 36 butterfly and moth species. My analysis shows that predilection to defensively regurgitate is closely associated with fore- and midgut morphology. Species that primarily regurgitate in defense have enlarged foreguts and shortened midguts, whereas the opposite relationship is seen among those that do not readily regurgitate. These results reveal that: (i) defensive regurgitation is not the primary defense of all caterpillars, and (ii) gut morphology can be associated with other factors than the chemical and mechanical properties of ingested foods. These findings challenge long held beliefs concerning the ubiquity of defensive regurgitation in caterpillars and the causes of diversification of gut morphology, and further reinforce the gut's standing as a complex and sophisticated organ.