Physiological and biophysical properties of the auditory system of the New Zealand wetaHemideina crassidens (Blanchard, 1851) (Ensifera: Stenopelmatidae)

Abstract

Investigation of the physiological and biophysical properties of the auditory system of the New Zealand weta,Hemideina crassidens has revealed the following: 1. The frequency/threshold curve for the massed response of primary auditory fibres in the tympanal nerve has a peak of sensitivity at 2.0–2.5 kHz. Absolute threshold is 20–35 dB SPL in individual preparations and the roll-off is about 15 dB/octave below the optimum and about 27 dB/octave above the optimum frequency (Fig. 1). 2. Occlusion of either the anterior or posterior tympanum causes a small loss of sensitivity (<8dB) only for frequencies above the hearing optimum. Occlusion of both auditory tympana reduces the sensitivity of the ear by 20–25 dB from 0.63 kHz to 5.0 kHz and by 7–15 dB up to 10 kHz (Fig. 2). 3. Blocking the leg tracheae in the femur causes no change in the sensitivity of the ear to sounds of 0.63–10 kHz (Fig. 3). Shielding the tympanic membranes from external sound, with the tracheal system intact, reduces the sensitivity of the ear by about 40 dB at the optimum frequency and by more than 10 dB for other frequencies in the range 0.63–10 kHz (Fig. 4). 4. Reducing the volume of the tibial air space behind the tympana by approximately 60% increases auditory thresholds for frequencies at and below the hearing optimum, whereas thresholds for higher frequencies are unchanged (Fig. 5). 5. For sound frequencies from 0.63 kHz to 8.0 kHz, the intact auditory system inH. crassidens has no directional sensitivity (Fig. 6). 6. Stridulatory sounds produced byH. crassidens are broad-band, having a peak in the power spectrum near 2.0 kHz and a roll-off of about 15 dB/octave towards higher frequencies (Fig. 7). 7. The weta auditory system functions as a one-input pressure receiver; its characteristics are compared with the auditory systems of Gryllidae and Tettigoniidae.