Mechanical time resolution in some insect ears - II. Impulse sound transmission in acoustic tracheal tubes

1. The impulse response of the tympanal membrane was determined in six species of crickets and bushcrickets by stimulating the ear with impulse sounds and recording the resulting tympanal vibration with a laser vibrometer. 2. In the bushcricket ears studied the composite impulse response consists of a short, low-amplitude 20-30 kHz vibration followed by a longer, high-amplitude vibration with a 'time constant' of 50-60 μs (Figs. 1 and 2). The former vibration is produced by the sound impinging on the exterior surface of the tympanum, while the latter is caused by the sound travelling through the acoustic trachea. The ratio between the first and the second vibration is about 1:5. A 'gain' of 5 in the acoustic trachea at 20-30 kHz makes it improbable that the directionality of the tettigoniid ear is determined by pressure difference properties of the ear near the dominant frequencies of the calling song. 3. The impulse response of the cricket tympanum depends on the state of the acoustic spiracles. If the ipsilateral spiracle is open, the composite impulse response is similar to that of the tettigoniid ear (Fig. 3). The ratio between the two 35-50 kHz vibrations is about 1:1.5 in two species and about 1:3.5 in two other species. If the ipsilateral spiracle is closed, the impulse response is of low amplitude, and it contains two superimposed frequencies (Fig. 4), a 50 kHz component and a 6 kHz component, the latter having a 'time constant' of about 185 μs. The state of the contralateral spiracle does not change the impulse response appreciably (Fig. 6). 4. Impulse sounds are distorted during their transmission through the acoustic tracheal tubes, presum ably because the propagation velocity in narrow tubes is a function of frequency.