Comparison of song frequency and receptor tuning in two closely related bushcricket species

The songs and the structure and physiology of the auditory organs in the closely related bushcricket species Tettigonia viridissima and Tettigonia cantans were investigated comparatively using bioacoustical, histological and neurophysiological methods. The morphology of the crista acustica, the main auditory receptor organ, is very similar in the two species in respect to both the distribution of scolopidia along the length axis of the crista and the dimensions of corresponding scolopidia and attachment structures. The only obvious difference is that T. viridissima has one more scolopidium in the crista acustica and that the overall length of the crista is by about 50 microns larger than in T. cantans. In contrast, differences were found in the physiology of individual auditory receptor cells. Comparison of the threshold characteristics of all the receptor cells of the crista acustica in both species reveals a differential sensitivity of groups of auditory receptor cells at dominant frequencies of the song. In each species, the sensitivity of auditory receptor cells is method to the energy spectrum of the song. These differences in the physiology can partly be explained by differences in transmission characteristics of the acoustic trachea.