The contribution of tympanic transmission to fine temporal signal evaluation in an ultrasonic moth

Abstract

In lesser waxmoths Achroia grisella, pair formation and female mate choice involve very fine discrimination of male ultrasonic signals. Female A. grisella prefer male signals with longer pulses and longer 'asynchrony intervals', and evaluate differences in these characteristics in the range of 80-260 mu s. The first step in the evaluation of these characteristics is the tympanic transmission of stimuli. We used laser vibrometry to describe the mode of vibration, frequency tuning and stimulus transmission of the tympana of A. grisella. The tympanic response consisted of a rotational mode of vibration, in which the anterior and posterior sections moved out of phase; the posterior section of the tympanum vibrated with all points moving in phase and maximum displacement at the attachment point of the scoloparium that contains the receptor cells. The tympana of A. grisella were tuned to high ultrasonic frequencies and had an estimated time constant (i.e. the limit to their temporal acuity) of about 20-50 mu s. Pulse length and all but the shortest asynchrony interval were thus well resolved by the tympanum. We discuss implications for the evaluation of pulse length and asynchrony interval.