Acoustic defence in an insect: characteristics of defensive stridulation and differences between the sexes in the tettigoniid Poecilimon ornatus (Schmidt 1850).

Many insects exhibit secondary defence mechanisms upon contact with a predator, such as defensive sound production or regurgitation of gut contents. In the tettigoniid Poecilimon ornatus, both males and females are capable of sound production and of regurgitation. However, wing stridulatory structures for intraspecific acoustic communication evolved independently in males and females, and may result in different defence sounds. Here we investigate in P. ornatus whether secondary defence behaviours, in particular defence sounds, show sex-specific differences. The male defence sound differs significantly from the male calling song in that it has a longer syllable duration and a higher number of impulses per syllable. In females, the defence sound syllables are also significantly longer than the syllables of their response song to the male calling song. In addition, the acoustic disturbance stridulation differs notably between females and males as both sexes exhibit different temporal patterns of the defence sound. Furthermore, males use defence sounds more often than females. The higher proportion of male disturbance stridulation is consistent with a male-biased predation risk during calling and phonotactic behaviour. The temporal structures of the female and male defence sounds support a deimatic function of the startling sound in both females and males, rather than an adaptation for a particular temporal pattern. Independently of the clear differences in sound defence, no difference in regurgitation of gut content occurs between the sexes.