A PERIPHERAL MECHANISM FOR AUDITORY DIRECTIONALITY IN THE BUSH-CRICKET GAMPSOCLEIS-GRATIOSA - ACOUSTIC TRACHEAL SYSTEM

The hypothesis that the acoustic tracheal system is a peripheral mecha nism for auditory directionality was tested in the bushcricket Gampsoc leis gratiosa. The acoustic tracheal system comprises the large spirac le, vesicle, and long tapering trachea. Based on its anatomy and cross -sectional areas against axial distance, an acoustic model was propose d; this model is analogous to an exponential horn with a flaring const ant of about 0.075 mm-1. The effects of the acoustic tracheal system o n auditory threshold and directionality were measured by using an elec trophysiological recording from single auditory neurons. The intact tr acheal system was found to be essential for directional hearing. Block age of the spiracle markedly influenced the auditory threshold and dir ectionality of the ipsilateral auditory neurons, but had very little e ffect on contralateral auditory neurons. The effects of blockage of th e ipsilateral spiracle on auditory thresholds were frequency dependent with a gain of about 15-40 dB over a frequency range of 7-30 kHz. The directional threshold differences were measured for single auditory n eurons. Blockage of the ipsilateral spiracle abolished directional sen sitivity of these neurons. But blockage of the tympanal slits had less effect on directional sensitivity of this tettigoniid species, with a decrement of about 1 to 3 dB.