CAUSES OF THE DIFFERENCES IN DETECTION OF LOW-FREQUENCIES IN THE AUDITORY RECEPTOR ORGANS OF 2 SPECIES OF BUSH-CRICKETS

The causes of the differential sensitivity of auditory receptor cells of two tettigoniid species to low-frequency airborne sound were invest igated with neurophysiological, neuroanatomical, and bioacoustic metho ds. The experiments employed adult males of the Chinese species Gampso cleis gratiosa (Decticinae) and the Australian species Mygalopsis mark i (Conocephalinae). G. gratiosa emits very broad-band songs with a fun damental frequency component at 3.8 kHz, extremely low for a bushcrick et; in contrast, the songs of M. marki have a narrow-band frequency sp ectrum, from 9 to 25 kHz. Accordingly, the threshold of the auditory o rgan as a whole is low in the low-frequency range in the case of G. gr atiosa, whereas M. marki is similarly sensitive only at frequencies ab ove 5 kHz.The experiments were designed to reveal the origin of this s ensitivity difference. One possibility considered was that low-frequen cy receptors of a particularly sensitive type are present in G. gratio sa and not in M. marki; alternatively, with a similar complement of re ceptor cells the whole-organ threshold curve might be expanded or rest ricted as a result of differences in the lower cutoff frequency of the sound-conducting system. The results confirm the latter interpretatio n. The proximal crista acustica and the distal intermediate organ comp rise the same numbers and types of cells in the two species, and the d ifference in threshold of the low-frequency receptors is produced by a shift of the frequency range in which the acoustic trachea has an amp lifying effect. (C) 1995 Wiley-Liss, Inc.