Defensive Sound Production in the Tobacco Hornworm, Manduca sexta (Bombycoidea: Sphingidae)

Abstract

The tobacco hornworm (Manduca sexta) is a model organism extensively studied for many aspects of its biology, including its anti-predator strategies. We report on a novel component of this caterpillar’s defence repertoire: sound production. Late instar caterpillars produce discrete clicking sounds in response to disturbance. Click trains range in duration from 0.3–20.0 s (mean 3.3 ± 4.8 s) and contain 2–41 clicks (mean 7.1 ± 9.5). Sounds are broadband with a dominant frequency of 29.8 ± 4.9 kHz. We investigated the mechanism of sound production by selectively ablating three identified sets of ridges on the mandibles, and determined that ridges on the inner face strike the outer and incisor ridges on the opposing mandible to produce multi-component clicks. We tested the hypothesis that clicks function in defence using simulated attacks with blunt forceps. In single attack trials 77% of larvae produced sound and this increased to 100% in sequential attacks. Clicks preceded or accompanied regurgitation in 93% of multiple attack trials, indicating that sound production may function in acoustic aposematism. Sound production is also accompanied by other behaviours including directed thrashing, head curling, and biting, suggesting that sounds may also function as a general warning of unprofitability.