Acoustic behaviour as a predator defence strategy in the females of a beetle species

Many animals produce sounds for intra- and interspecific communication. In insects, acoustic signalling of beetles has not been explored as extensively as those of other well-studied acoustic insects, such as cicadas, crickets, and katydids. In this study, we explored the acoustic behaviour of the females of the scarab beetle Polyphylla davidis for the first time. Detailed morphological features of the ‘alary–elytral’ type of stridulatory organ and acoustic structures of the sounds of the female beetles are presented. We found that sound-producing structures of P. davidis and another beetle species of the same genus are not homologous, which indicates that acoustic behaviour evolved independently in these two beetle species. The attack experiments supported that the female beetles use acoustic behaviour as an anti-predator strategy. The spectral features, especially lacking ultrasonic components, suggested that the mechanism of the sounds used in deterring predators is not sonar jamming. We found that in attack experiments, females exhibited both sound production and defensive regurgitation in response to attacks, which suggested that the sounds may function as aposematic signals to inform the predators that the beetles are chemically defended. Future studies using live predators are needed to experimentally confirm this acoustic aposematism hypothesis.