Auditory interneurones in the mesothoracic ganglion of crickets

Auditory interneurone responses in the mesothoracic ganglion of the cricket Gryllus bimaculatus were investigated with special regard to temporal features of the calling song. Units representing five response types were found. One type codes verse syllables and intensity. The second codes syllables of highfrequency verses. The third responds as a pulse marker. The fourth shows adaptation and the response pattern depends on the verse frequency. The fifth fires a burst at verse onset. Responses of mesothoracic units recorded in two other cricket species do not differ markedly from those of Gryllus bimaculatus. Particularly, no tuning is found to species-specific differences in their calling songs. The stimulus direction can affect the threshold in different ways: dependence at all frequencies, dependence only between 3 and 6 kHz, and independence are found. The dependence is mainly expressed by a higher threshold for contralateral sounds. The mesothoracic branching of a few neurones was demonstrated by extracellular CoS-staining. These cells pass through the ganglion as connective fibres giving off small branches into the ventro-medial and dorso-medial neuropiles.