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

Abstract

SUMMARY 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.