The syllables of the calling songs of many bushcrickets (Orthoptera, Tettigoniidae) are formed of a series of discrete sound impulses. Bioacoustic investigations of E. ephippiger revealed that in some males the number of impulses per main syllable fall from about 50 impulses down to 10 impulses with increasing age of the male after the final moult. An influence of the altered signal on the frequency content of the acoustic signal could not be proved as far as averaged spectra are concerned. The geriatric effect on the syllable pattern is caused by mechanical attrition of the peaks of the teeth forming the pars stridens.
Two-choice experiments revealed that females can discriminate between songs with modified impulse repetition rate and songs with the normal impulse repetition rate. The inter-impulse intervals in the modified song vary from 0.8 ms up to 8.7 ms. Experiments with stimuli formed of natural impulses with a constant repetition rate proved that females are able to discriminate between these signals over a certain range. A stimulus that contains an inter-impulse interval similar to the species-specific one (1.87 ms on average) is preferred to stimuli with altered (~> ± l ms) inter-impulse interval when broadcast alternately. When the impulse repetition rate is only slightly altered (~ < ± l ms) the females cannot discriminate between these signals behaviourally.