GRANT, Jacqualine B; Colorado State University: Defensive behaviors affect caterpillar anatomy

Regurgitation is a common defensive strategy in many insects � to the point of being assumed ubiquitous among lepidopterous larvae. I tested this hypothesis by assaying defensive responses of 35 species of larvae from 16 lepidopteran families. Regurgitation was not a universal defensive behavior among caterpillars, which were classed as primary-, secondary-, or non-regurgitating. As regurgitant is assumed to emanate from the digestive tract, I tested the hypothesis that digestive tract anatomy differed among the three classes of larval behavior. The crop of caterpillars has long been assumed to be an organ of food storage, but it is also uniquely situated to act as an organ of defensive regurgitation. Therefore, I tested the specific hypothesis that crop length to total gut length ratio is greater in primary-regurgitators than in secondary- or non-regurgitators. Primary regurgitators possessed larger crops relative to total gut length than secondary regurgitators, which possessed larger crops relative to total gut length than non-regurgitators. Midguts were smaller in primary regurgitators than in secondary- or non-regurgitators, the implication of which is that a trade-off may exist between the behavior and digestive capacity. I examined a set of morphological and behavioral traits specific to this set of caterpillars to determine which were linked to the evolution of defensive regurgitation. A logistic regression of characters with regurgitative ability yielded one significant predictor: the possession of glandular spines. No correlation existed between caterpillar family and regurgitative behavior, suggesting that regurgitation evolved numerous times throughout the clade. This research suggests that 1) defensive regurgitation is not as common a defense as previously thought; 2) behavioral characteristics are reflected in caterpillar internal anatomy; and 3) many characters have influenced the evolution of defensive regurgitation in caterpillars.