Hearing in Notodontid Moths: a Tympanic Organ with a Single Auditory Neurone

1. Notodontid moths possess paired tympanic organs basically similar to the ears in other noctuoid families, but with a single auditory .A cell. The A cell and the non-auditory B cell were studied anatomically by infusion of C0CI2 and physiologically by recordings from the tympanic nerve. 2. The response of the A cell is determined by intensity parameters and temporal parameters of an ultrasonic stimulus. The notodontid ear is as sensitive as the ears of sympatric noctuids. The directional sensitivity is approximately the same as in noctuids of comparable size (maximal interaural intensity difference of 10-15 dB). The dynamic range of the A cell is about 20-25 dB. Sound levels exceeding the threshold by 30-40dB will saturate the A cells in both ears. Stimuli with different pulse lengths (from 5 to 200 ms, corresponding to pulse repetition rates (PRR) from 100—2-5 Hz), but equal duty cycles (50%) gave a maximum response for pulse lengths lying between 30 and 50 ms. The receptor cell followed the sound pulses in a one-to-one manner even at a PRR of 200 Hz. 3. Notodontid moths seem to show the same 'bimodal' evasive behaviour as noctuids. This behaviour can be explained on the basis of intensity parameters, since only low intensity stimuli will give the notodontid directional information. Hence, directional evasive behaviour is expected at low sound pressure levels (SPL), while high SPL (saturating both A cells) should elicit a non-directional evasive behaviour. However, the evasive behaviour could also be explained in terms of time parameters. Hunting bats increase the PRR of their cries when closing in on a prey and the moths may be able to use these time cues for changing their behaviour.