Physiology of auditory receptors in two species of Tettigoniidae (Orthoptera: Ensifera)

Abstract

1. The gross anatomical features of the crista acustica in the tettigoniids Mygalopsis marki and Polichne sp. are compared (Fig. 1). The crista in Polichne sp. was longer and contained 12 more receptors than that in M. marki. Both organs, however, displayed a gradual and even taper from the proximal to the distal end. The size of the attachment cells and the length of the sensory dendrites displayed a similar gradual decrease in size from the proximal to the distal end of the array (Fig. 1). 2. The characeristic sound frequency and roll offs in sensitivity of ascending auditory interneurons in the oesophageal connectives of M. marki did not alter after the leg cuticle covering the crista acustica was removed (Fig. 2). Since such interneurons receive excitatory and possible inhibitory input from several receptors in the crista this result indicated that the tuning of these receptors was not affected by removing this portion of the leg cuticle. 3. Physiological recordings were obtained from the cell bodies of individual receptors in the crista acustica of M. marki and Polichne sp. (Fig. 3 A, B). Receptors in M. marki produced a maximum response to an 80 ms tone pulse of 16 spikes/stimulus, had a dynamic range of approximately 30 dB and a rate of increase in the spike response of 6 spikes/10 dB (Fig. 3C). The maximum response, dynamic range and slope of the intensity-response characteristics of auditory receptors in Polichne sp. were less than that for receptors in M. marki (Fig. 3 D). The slope of the intensity response characteristics, dynamic range and maximum spike response of individual auditory receptors in both M. marki and Polichne sp. did not alter for sound frequencies above and below the characteristic frequency of the individual receptor (Fig. 3 C, D). 4. The tonotopic organisation of the crista acustica in M. marki and Polichne sp. was determined by the injection of Lucifer Yellow into the cell bodies and dendrites (Fig. 4 A) of receptors for which the frequency-threshold characteristics had been obtained (Fig. 4B). The characteristic frequencies of adjacent receptors in the crista acustica of M. marki were seperated by irregular amounts with receptor 6 tuned to 7 kHz, receptor 7 to 14 kHz, receptor 8 to 17 kHz and receptors 10 and 11 tuned to 20 kHz (Fig. 6). Although adjacent receptors in Polichne sp. were seperated by approximately 1 kHz, the crista acustica of the species contains four receptors tuned to 20 kHz (Fig. 7). 5. The discontinuities in the tonotopic organisation of the crista acustica of M. marki and Polichne sp. are incompatable with the notion that mechanical resonances within the receptor array could be responsible for the frequency selectivity of these auditory receptors. It is therefore proposed that these receptors are tuned as a result of electrical and/or mechanical resonance intrinsic to the individual sensilla.