Causes of the differences in detection of low frequencies in the auditory receptor organs of two species of bushcrickets

Abstract

AbstractThe causes of the differential sensitivity of auditory receptor cells of two tettigoniid species to low‐frequency airborne sound were investigated with neurophysiological, neuroanatomical, and bioacoustic methods. The experiments employed adult males of the Chinese species Gampsocleis gratiosa (Decticinae) and the Australian species Mygalopsis marki (Conocephalinae). G. gratiosa emits very broad‐band songs with a fundamental frequency component at 3.8 kHz, extremely low for a bushcricket; in contrast, the songs of M. marki have a narrow‐band frequency spectrum, from 9 to 25 kHz. Accordingly, the threshold of the auditory organ as a whole is low in the low‐frequency range in the case of G. gratiosa, whereas M. marki is similarly sensitive only at frequencies above 5 kHz.The experiments were designed to reveal the origin of this sensitivity difference. One possibility considered was that low‐frequency receptors of a particularly sensitive type are present in G. gratiosa and not in M. marki; alternatively, with a similar complement of receptor cells the whole‐organ threshold curve might be expanded or restricted as a result of differences in the lower cutoff frequency of the sound‐conducting system.The results confirm the latter interpretation. The proximal crista acustica and the distal intermediate organ comprise the same numbers and types of cells in the two species, and the difference in threshold of the low‐frequency receptors is produced by a shift of the frequency range in which the acoustic trachea has an amplifying effect. © 1995 Wiley‐Liss, Inc.