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The Journal of Experimental Biology 205, 2935-2942 (2002)
© 2002 The Company of Biologists Limited

Hearing asymmetry and auditory acuity in the Australian bushcricket Requena verticalis (Listroscelidinae; Tettigoniidae; Orthoptera)

Winston J. Bailey* and Suanne Yang{dagger}

Department of Zoology, University of Western Australia, Nedlands, Western Australia 6009, Australia
{dagger} Present address: Department of Biology, Washington State University, Vancouver Campus, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686-9600, USA

* Author for correspondence (e-mail: wbailey{at}cyllene.uwa.edu.au)

Accepted 10 June 2002

Fluctuating asymmetry in bilaterally placed sensory structures of insects may be considered maladaptive; natural selection should favour a symmetrical system. Among bushcrickets or katydids, threshold hearing sensitivity is, in part, dependent on the size of the auditory spiracle in the prothorax. We tested the degree of natural asymmetry in the hearing system of the ensiferan orthopteran Requena verticalis by measuring the size of the auditory spiracle of females. Naturally occurring asymmetry approached 8%, which translates to a hearing bias at a threshold of only 0.8 dB. Auditory asymmetry of females was experimentally exaggerated by packing cotton wool into either the right or left auditory spiracle. We made neurophysiological recordings from the ascending auditory T-fibre in the neck connective and found that the left—right bias created by this operation approximated 5 dB; this is greater than that found in nature. For these experiments, sound was delivered to the operated side of the insect from a speaker placed at either 90° or 30° to the long-body axis. To test the influence of this induced auditory bias, free-moving females were allowed to orient towards a speaker emitting male calls at near-natural call intensities of 51 and 82 dB (SPL), 80 cm from the speaker on a flat arena. There was no variation in angle or vector between experimental and control insects, and there was no difference in acuity between intensity. We discuss the relevance of threshold measurements of bushcricket hearing systems in regard to sound localisation and, from these experiments question any role of asymmetry.

Key words: Fluctuating asymmetry, sound localisation, auditory spiracle, neurophysiology, bushcricket, Requena verticalis, Tettigoniidae, Orthoptera


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© The Company of Biologists Ltd 2002