The Auditory System of Homorocoryphus (Tettigonioidea, Orthoptera)

Abstract

The responses of the auditory interneurones indicate that the tettigoniid ear discriminates frequencies. The T fibre receives strong ipsilateral and weak contralateral excitatory inputs and a strong contralateral inhibitory input, from the tympanic nerves. These inputs are frequency-sensitive, the response being greatest at 30 kcyc./sec. and above. Responsiveness is low in the region of 15 kcyc./sec., which is the dominant song frequency. At 30 kcyc./sec. the T fibre is most sensitive to amplitude increments, and conveys maximal directional information. Both the T fibre and the ear (as judged by the compound potential in the tympanic nerve) respond to steps of 2 dB. The directionality of the ear is enhanced by the contralateral inhibitory connexions of the T fibre. At 15 kcyc./sec. directionality is poor, but is present at 10 kcyc./sec. The T fibre is inhibited by continuous sounds, including the species song. The extent of the inhibitory effect varies with the amplitude of the continuous sound. This may assist in explaining the ‘phasic’ response of the T fibre. There is little habituation to repetitive stimuli. A small interneurone seen in split connectives gives a ‘tonic’ response to song and to continuous sound. It may inhibit the T fibre. Two other auditory fibres are occasionally recorded in the connectives. The T fibre has all the properties required of a warning system responding to pulsed high-frequency sound, and it responds well to bat cries. There is, however, no evidence that it mediates a behavioural response.