Caterpillar sonic defences: diversity of vocalisations in silk and hawk moth (Bombycoidea) larvae

Several Bombycoidea caterpillars, renowned for their large size and diverse appearances, possess an intriguing hidden talent – the ability to ‘vocalise’. Vocalisation is a rare form of sound production in insects, whereby sounds emanate from the oral cavity by air being forced through the foregut. Here, we report on vocalisation in 10 Bombycoidea species that occur across three families. Sounds in all 10 species are evoked in response to simulated predator attacks. Species were identified as vocalisers based primarily on video evidence of mouthparts being open during sound production. Vocalisations, when considered collectively across all 10 species studied, sound like a train of ‘hisses’ (sound units) that occur following an attack. Each sound unit comprises a series of pulses (4–104 on average) and is broadband with high dominant frequency (24–49 kHz on average). Given that vocalising species occur in different families across this large superfamily, we asked whether related species shared similar sound features. We found considerable overlap between sound characteristics of different vocalising species, suggesting a shared mechanism overall. However, distinct differences were also noted between families, suggesting that vocalisation may have evolved multiple times within Bombycoidea. The evolutionary origins and specific functions of vocalisation in caterpillars are discussed.