The unique sound production of the Death's-head hawkmoth (Acherontia atropos (Linnaeus, 1758)) revisited.

Abstract

When disturbed, adults of the Death's-head hawkmoth (Lepidoptera, Sphingidae: Acherontia atropos) produce short squeaks by drawing in and deflating air into and out of the pharynx as a defence mechanism. We took a new look at Prell's hypothesis of a two-phase mechanism by providing new insights into the functional morphology behind the pharyngeal sound production of this species. First, we compared the head anatomy of A. atropos with another sphingid species, Manduca sexta, by using micro-computed tomography (CT) and 3D reconstruction methods. Despite differences in feeding behaviour and capability of sound production in the two species, the musculature in the head is surprisingly similar. However, A. atropos has a much shorter proboscis and a modified epipharynx with a distinct sclerotised lobe projecting into the opening of the pharynx. Second, we observed the sound production in vivo with X-ray videography, mammography CT and high-speed videography. Third, we analysed acoustic pressure over time and spectral frequency composition of six A. atropos specimens, both intact and with a removed proboscis. Single squeaks of A. atropos last for ca. 200 ms and consist of an inflation phase, a short pause and a deflation phase. The inflation phase is characterised by a burst of ca. 50 pulses with decreasing pulse frequency and a major frequency peak at ca. 8 kHz, followed by harmonics ranging up to more than 60 kHz. The deflation phase is characterised by a less clear acoustic pattern, a lower amplitude and more pronounced peaks in the same frequency range. The removal of the proboscis resulted in a significantly shortened squeak, a lower acoustic pressure level and a slightly more limited frequency spectrum. We hypothesise that the uptake of viscous honey facilitated the evolution of an efficient valve at the opening of the pharynx (i.e. a modified epipharynx), and that sound production could relatively easily have evolved based on this morphological pre-adaptation.