The physiology of the locust ear

Summary

• 1. The expected resonance frequencies of the tympanal membrane have been calculated from its dimensions, mass, and compliance. The thin part of the tympanal membrane may vibrate independently of the entire tympanum. Thus, there are at least two sets of resonances (Fig. 8).

• 2. The two sets of vibrations have been observed by means of laser holography (Figs. 13–15) and measured with a capacitance electrode (Figs. 16–18). The position and amplitude of the vibration patterns, the phase relationships, and the niteraction of the two sets of vibration have been studied. The results are compared with the frequency sensitivity of the four groups of receptor cells.

• 3. The groups of receptor cells are attached to four specialized areas on the tympanum (Fig. 6). The vibrations of these areas of attachment are a maximum at the frequencies of maximum sensitivity in the receptor cells (Figs. 16 and 17). Thus, the frequency discrimination seems to be a purely physical phenomenon, based partly on the presence of the tympanal resonances, and partly on the different positions of the receptor cells on the tympanal membrane.

• 4. The two sets of vibrations have different spatial positions on the tympanum. The centre of the entire-membrane-vibrations is situated in one end of the membrane (Fig. 15), whereas that of the thin-membrane-vibrations is almost at the centre of the tympanum (Fig. 14). The positions of the centers of vibration are, however, not constant (Figs. 13 and 14). Different modes may have somewhat different centre positions, and these positions may change with frequency because of interactions between the two sets of resonances. Therefore, receptor cells attached to different areas on the membrane may pick up different modes of vibration. Also, the receptor cells may almost fail to respond to some modes, if their area of attachment is at a nodal circle of these modes at resonance.