The communication systems of moths and butterflies have been the focus of scientific interest for years. However, very little is known about the behaviour of larval Lepidoptera, particularly with regards to their acoustic capabilities. Sound production in caterpillars has been noted for over one hundred years, yet the functional significance of air or substrate borne acoustic signaling has yet to be investigated in most species. Late instar larvae of the silk moth, Antheraea polyphemus (Saturniidae), produce a clicking noise with their mandibles when physically disturbed. In addition, the acoustic signals typically precede or accompany regurgitation. We hypothesize that sound production in the polyphemus caterpillar is a warning signal to regurgitation, the acoustic equivalent of visual aposematism, which is directed towards hearing predators. Preliminary results suggest that the oral secretions are unpalatable and have emetic properties. Spectral analysis reveals that the acoustic signals are broadband, with most energy falling between 2 and 20 kHz. In addition, comparative evidence reveals that numerous other species from the superfamily Bombycoidea are producing acoustic signals with similar spectral and temporal qualities to that of the polyphemus caterpillar. Sound production is also coupled with regurgitation in these species. Our results indicate that the polyphemus caterpillar and several other larval Lepidoptera are utilizing sound to communicate with their predators, a behaviour we believe to be crucial to their survival.