Biophysics of the ensiferan ear

A method for determining the steady-state gain in the tracheal hearing trumpet in bushcrickets is described: The expected change in force, caused by moving the front leg from one position to another (Fig. 1) can be calculated (Figs. 2 and 3), assuming a certain gain. Laser vibrometry is used for determining the velocity of a tympanal membrane in the two leg positions. The observed change in velocity (Figs. 4 and 5) can then be compared with the calculated change. The approximate gain is determined as a ‘best fit’ between the two sets of data.The steady-state gain is about 1 at 1–3 kHz and about 2 at higher frequencies.The change in the threshold of the ear, caused by moving the leg, is measured by recording from the auditory nerve (Fig. 6). The steady-state gain thus determined is of the same order of magnitude as that determined by means of laser vibrometry.The scattering of sound waves around a cylindrical obstacle has been studied, in order to estimate the magnitude of the local variations in sound pressure at some distance from the body of an insect (Figs. 7 and 8).The experimental evidence supporting the Zeuner-Nocke theory for the bushcricket ear is discussed.