The response characteristics of the tympanal organs of two species of bush cricket and some studies of the problem of sound transmission

Summary

• 1. Response/intensity curves for the 50 ms periods following the onset of stimulation are given, for the anterior membrane ofPlatycleis intermedia, in Fig. 2A and for the posterior membrane ofHomorocoryphus nitidulus vicinus in Fig. 2B. These curves are frequency-dependent, varying in threshold intensity, intensity band and maximum amplitude criteria.

• 2. Similar curves are given for the 50 ms following the termination of the stimulus, in Fig. 3A-B. Attention is drawn to the significance of these ‘off’ responses.

• 3. The response/intensity curves are critically analysed by three different methods; for threshold; for a constant intensity level (CI); and for a constant response level (CR). The differences in the values obtained by these methods are shown in Fig. 4A-H.

• 4. The CR (sensitivity) curves for the anterior membrane ‘on’ response ofP. intermedia are given in Fig. 5 and for the ‘off’ response in Fig. 6. The threshold curves show the typical ‘narrow-band’ spectrum, with peak sensitivity at the main carrier frequency of the song, and smaller peaks at 0.8 kHz and 30 kHz. At higher response amplitudes however, these peaks disappear, or are greatly diminished relative to other frequencies except for the low-frequency ‘off’ response.

• 5. The CI (efficiency) curves for the anterior membrane ‘on’ response are shown for seven intensity values in Fig. 7. Increasing the intensity of stimulation emphasizes the broadband nature of the receptor and demonstrates an important high-frequency peak (70 kHz), not apparent at lower intensities. The ‘off’ responses (Fig. 8) show similar effects.

• 6. For the posterior membrane, curves were drawn of the maximum response, and of the stimulus required to give the maximum response, against frequency. These are given for the ‘on’ response (Fig. 9) and for the ‘off’ response (Fig. 10). The posterior membrane is insensitive to the termination of the stimulus.

• 7. No responses were obtained from the central membrane for frequencies between 200 Hz and 250 kHz.

• 8. The response activity during each 5 ms period of the stimulus and stimulus interval is shown for the anterior and posterior membranes in Figs. 11, 12. This steady-state activity is membrane- and frequency-dependent, and clearly demonstrates the nature of the ‘off’ responses.

• 9. The effects of varying the stimulus duration and the stimulus-interval duration, on the amplitudes of response and latencies of response, are shown in Figs. 13A and B. Consideration of these figures indicates that neither the ‘on’ nor ‘off’ responses are due to any inhibitory release or ‘rebound’ phenomena.

• 10. The effect of environmental attenuation of sound frequencies (0.8 kHz-20 kHz) (Fig. 14) on the song spectrum (Fig. 15) suggests that distance is perceived as a variation in the intensity-response at low frequencies.

• 11. The excess sound pressure at the spiracle, due to the diffraction of sound by the body surface, as a function of the angle of incidence (Fig. 16A-B), shows that directionality becomes possible only at frequencies above 7 kHz. The effect of this excess pressure on the song spectrum is demonstrated (Fig. 17) and suggests that directionality is a feature of higher frequencies, and can be coded as an intensity function.

• 12. When the pronotal shield is removed, this high-frequency directionality is eliminated (Fig. 16B). Thus, the pronotum is important for the effectiveness of the sound stimulus entering the spiracle.

• 13. The implications of these results for the mode of functioning of the ear and for acoustic behaviour are discussed. The way is now clear for the critical determination of the role of the acoustic trachea.