Mechanical time resolution in some insect ears - I. Impulse responses and time constants

1. The mechanical time resolution is estimated in the ears of noctuid moths (Noctuidae) and locusts (Acrididae). The vibration velocity of small areas on the tympanal membrane is measured by means of laser vibrometry. The impulse response (Figs. 2B and 5 A) and the transfer function (Fig. 3) are obtained directly by stimulation with very short impulse sounds and pure tones, respectively. The transfer function is also calculated from the experimentally determined impulse response, and vice versa. Finally, the impulse response is obtained by calculation from the measured vibrations caused by noise. The directly determined and the calculated transfer functions are rather similar (Fig. 3A-B). 2. The impulse response of the attachment area of the receptor cells in the noctuid ear is a short, damped vibration with a 'time constant' of about 60 μs (Table 1). The attachment area of the receptor cells can thus separate impulses arriving with time intervals larger than 150-200 μs (Fig. 4). 3. The 'time constant' of the attachment area of the d-cells in the locust tympanum is about 90 μs (Table 1). The 'time constant' for other parts of the locust tympanum varies between 50 μs and 200 μs (Fig. 6).