Function of the ‘Mirror’ in Tettigonioid Orthoptera

Abstract

FOR, decades, this area of excessively thin, ‘Cellophane’-like cuticle of the tegmen of tettigonioids (Fig. 1) has been regarded as important acoustically—for example, Pierce¹, who went so far as to calculate a resonant frequency for it on the assumptions that it is: (a) circular, (b) clamped peripherally and (c) made of material the relevant physics of which is that of ‘Pyroxylin’ plastic; he obtained good agreement between calculated and experimental figures, which could be interpreted as supporting the hypothesis², but could equally well mean that the choice of ‘Pyroxylin’ was a fortunate one, since there is no evidence that the tegminal material resembles it in anything but appearance. Assumptions (a) and (b) are also of doubtful validity, and for some years I have doubted whether the mirror has any major acoustic importance; Dumortier³ expresses similar doubts. It could function acoustically in any of several ways, some of which are mentioned by Dumortier: (1) as a narrow-band resonator, tuned like a drum; (2) as a general sounding-board or acoustic coupler (belly of violin, loudspeaker diaphragm); (3) as a tymbal, actively buckling to and fro like that of a cicada. On the other hand, considering its high gloss, it could give a visual signal, especially perhaps in darkness, enabling the searching female, once she reaches close quarters, to locate the dorsum of the male on which she has to climb for copulation.