DIRECTIONAL RESPONSES TO SOUND IN THE CENTRAL NERVOUS-SYSTEM OF THE CRICKET TELEOGRYLLUS-COMMODUS (ORTHOPTERA, GRYLLIDAE) .1. ASCENDING INTERNEURONS

1. In the cricket, Teleogryllus commodus, two ascending auditory interneurons, the S and L neurons, are present in each cervical connective of the ventral nerve cord. The responses of these interneurons to sounds which varied in intensity and direction were simultaneously recorded from both connectives. 2. When the forelegs are positioned symmetrically at 90‡ to the body axis, an asymmetry exists between the responses of pairs of S and L neurons on each side to sounds incident along the body axis. This asymmetry is analogous to the biasing previously reported in the cricket's orientation behaviour. 3. Both S and L neurons show directional responses over a limited intensity range above their thresholds. These thresholds differ by about 15 dB so that the intensity ranges over which S and L neurons are directional partially overlap, to give a total range over which directional sensitivity occurs of more than 50 dB. 4. At very high intensities (e.g. TL+50 dB) the latency difference between L neurons may provide directional sensitivity. 5. The increase in directionality between homologous S and L neurons as a sound source moves laterally from the body midline is not a linear function of incident angle, and the maximum rate of increase occurs over the first 30‡. As a result, although both S and L neurons can detect a 5‡ shift in a sound source from 0‡, this frontal acuity is not maintained laterally. 6. Directivity patterns of S and L neurons are cardioid with maxima along the 90‡ axis. Because the shape and magnitude of the pattern are little different from those recorded from receptors, little integration of directional information takes place at the level of the first interneuron compared with other Orthoptera. 7. Both excitatory and inhibitory neural interactions occur across the prothoracic ganglion and affect spike numbers and latency of responses. Their role in directionality is not as significant as non-neural factors associated with the prothoracic tracheal system. 8. S and L neurons provide the cricket with potential directional information over a sufficient intensity range to mediate location of a singing male by a female. © 1979 Springer-Verlag.