Caterpillar sonic defences: Mechanisms and diversity of mandible stridulation in silk and hawk moth (Bombycoidea) larvae

Silk and hawk moth caterpillars produce a variety of defence sounds described as clicking, chirping, whistling, and vocalising. Such diversity provides opportunity to test hypotheses on the functions and evolution of insect defence sounds using comparative analyses. A key step to conducting comparative analyses is to categorise different phenotypes. Here, we describe mandible sounds of 20 species of late instar Bombycoidea caterpillars and establish objective criteria for differentiating between sound-producing mechanisms. First, we assess how the two mandibular mechanisms – clicking and chirping – differ from one another using two reference species: Antheraea polyphemus (clicker) and Saturnia pyri (chirper). In these references, clicks are produced by ridged and serrated mandibles and have short duration units with few pulses, whereas chirps are produced by scalloped mandibles and have longer duration units with more pulses. Second, we characterise acoustic and morphological traits of 18 additional species. These are categorised as clickers (13 species), chirpers (4 species), or other (1 species) using diagnostic features identified from the reference species. Third, these categorisations are tested using a predictive logistic regression model. The results of this study contribute to our understanding of acoustically mediated defences in caterpillars and provide necessary criteria for conducting further comparative studies.