The auditory‐vibratory sensory system of the bushcricket Polysarcus denticauda (Phaneropterinae, Tettigoniidae) II. physiology of receptor cells

The complex tibial organs of the bushcricket Polysarcus denticauda (Phaneropterinae) have some exceptional morphological features. In the forelegs, these organs have an extremely thick uncovered tympanum and about 50 receptor cells in the crista acustica. In the mid‐ and hindlegs, the cristae are extraordinarily reduced, with only seven or eight receptor cells. Physiological investigation of the receptor organs reveals that, in spite of the thick tympana, the auditory receptor cells of the forelegs have surprisingly low threshold values; they are as sensitive as the receptor cells of other bushcrickets, with very thin tympana. The high sensitivity is valid for the frequency range from at least 3–4 kHz up to 20 kHz. However, receptor cells tuned to frequencies above 20 kHz are less sensitive, suggesting a lack of discriminatory ability. This may be caused by the crowded arrangement of receptors at the distal part of the crista acustica. The frequency range of the conspecific proclamation song is therefore picked up only in the lower part of its power spectrum (10–20 kHz). The sensitive, low‐frequency detection may originate from the broad‐band sound transmission of the elaborate acoustic trachea of the forelegs, with a cut‐off frequency at 4.5 kHz. The bimodal vibratory‐auditory receptor cells of the tibial organs in the mid‐ and hindlegs are very sensitive to vibration, especially in the midlegs. They have significantly lower thresholds than receptors in the forelegs which are tuned to the same frequencies. This response property seems to have its origin within the specific structure of the organs in the mid‐ and hindlegs. © 1996 Wiley‐Liss, Inc.