Summary
At 2 kHz, 3.5 kHz and 5 kHz the locust ear functions as a mixed pressure and pressure-gradient receiver. The ear is inherently directional at these frequencies. The directional characteristics are independent of the amount of body tissue (Figs. 6 and 7). At 15 kHz the locust ear functions mostly as a pressure receiver, and is inherently non-directional (Fig. 6d). Hearing is, however, directional at 15 kHz owing to diffraction caused by the body (Fig. 1). Auditory thresholds are influenced by the amount of body tissue at frequencies from 2 to 15 kHz (Fig. 8). At frequencies less than 6 kHz the sound conducted through the body is attenuated by 1 to 8 dB depending on the amount of body tissue. At frequencies greater than 12 kHz the sound conducted through the body is attenuated by up to 18 dB, and the attenuation is only slightly influenced by body tissue (Fig. 2). The attenuation of sound conducted through the body is independent of the direction of sound, but may be affected by the amount of tissue between the ears (Fig. 4). The tissue in the body appears to act as a ‘resistive’ element, which introduces a phase shift in the sound conducted through the body relative to that striking the front side of the tympanum. Body tissue can set the level of sensitivity, but does not influence the receiver characteristics of the ear.
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I thank Professor Ulrik Lassen, Zoophysiological Laboratory B, University of Copenhagen, for providing the necessary facilities for the experimental work. Some of the statistical analyses were completed at the University of Kentucky Computing Center and others at the Computer Sciences Department, Odense University. I wish to thank Andrzej Ormicke and Henning Noer for their assistance with some of the statistical methods, as well as Vibeke Eriksen and Aase Jørgensen for help with the illustrations. I especially thank Professor Axel Michelsen for helpful advice and discussions during all stages of this work, as well as for comments on the manuscript. The studies were supported in part by a University of Kentucky Faculty Summer Research Fellowship and a National Instituts of Health, U.S.A. grant (5 505 RR07114-04). The author held a NATO Postdoctoral Fellowship in Science during the experimental stage of this study.
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Miller, L.A. Directional hearing in the locustSchistocerca gregaria Forskål (Acrididae, Orthophera). J. Comp. Physiol. 119, 85–98 (1977). https://doi.org/10.1007/BF00655874
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DOI: https://doi.org/10.1007/BF00655874