Bioacoustics of Hecatesia exultans

One reasonably well-understood example of a percussive mechanism involving two-body parts is found in members of the Austrlian moth genus, Hecatesi (Bailey, 1978). In these moths an area of the costae of the fore wings is modified to form small hard knobs whichhave been called castanets. These are repeatedly struck together at the top of the wing stroke to produce sounds which have given these insects the popualr name of 'whsitling moths'. Two of the three known species have been looked at, and of these, one, H. exultans, produces the sounds while at rest, while the other. H. thyridio, does so only during a 'nuptial' flight. The sounds are produced by the males only and are presumed to have the function of attracting potential mates. Both species produce trains of pulses at about 200s-1 in H. exultans and 80s-1 in H. thyridion. Each pulse in the former species consists of an oscillation which is lightly damped, suggesting that the sound-radiating surfaces are uncoupled from the main mass of the moth. Thin crescentic areas of clear corrugated cuticle posterior to the castanets probably have this function, the wings distal to the castanets being set into resonance (Figure 2.1). A power spectrum of the pulses shows the maximum energy to be fairly sharply peaked at between 25 and 30kHz in H. exultans. The shape of the pulses with a gradual increase in amlitude over several cycles indicates that we are dealing witha reosnant structure which is being loaded. This could be the space between the wings, whcih apprximates a sphere of radius 5-6mm, that is, half the wavelength of teh carrier frequency of pulses. In H. thyridion the individual pulses are shorter than in H. exultans. This is due to the wings being opened after each collision which has the effect of dissipating the energy. The view that the wings radiate the sound energy is reinforced by the observation that the fundamental frequency of the pulses in H. thyridion is lower at between 17 and 20 kHz and the wings are correspondingly larger.