AN AUDITORY INTERNEURON TUNED TO THE MALE SONG FREQUENCY IN THE DUETTING BUSH-CRICKET ANCISTRURA-NIGROVITTATA (ORTHOPTERA, PHANEROPTERIDAE)

An auditory interneurone (AN1) of the bushcricket Ancistrura nigrovitt ata is described; it has a soma and dendrites in the prothoracic gangl ion, an ascending axon and axon collaterals in the protocerebrum. As j udged from morphological and physiological similarity, it is probably homologous to AN1 described in Tettigonia viridissima and to AN1 descr ibed in Gryllus bimaculatus. The occurrence and physiology of AN1 are not sex-specific. It receives predominant excitation between 12 and 16 kHz (male song frequency) and inhibition at lower frequencies and mor e strongly at higher frequencies. It shows optimum-type intensity/resp onse curves. Frequency tuning and intensity-dependence compare well wi th female behaviour. Lesion experiments demonstrate that AN1 receives excitation and frequency-dependent inhibition from the soma-contralate ral ear and inhibition from the soma-ipsilateral ear. The latter contr ibutes to the clear left-right difference in its responses. AN1 does n ot obviously discriminate between temporal patterns of different behav ioural effectivity. Its spiking, however, is coupled to the temporal p attern. It is hypothesized that AN1 may be involved in frequency proce ssing by female A. nigrovittata.