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In Polysarcus denticauda, a phaneropterine bushcricket with extremely thick uncovered tympana and an aberrant morphology of the cristae acusticae of the complex tibial organs, the electrophysiology of the auditory-vibratory ventral cord neurons ascending to the brain was investigated. Although the receptor organs in this species have some extraordinary response properties, the central auditory-vibratory neurons could be basically classified into the same functional types of S-, V-, and VS- neurons previously described for other bushcricket species. However, in some details the responses of most of the S- and VS- neurons are different. The S-neurons are generally more tonic and the VS-neurons give smaller responses to airborne sound stimulation than do the neurons belonging to the same functional types in most other bushcricket species. Adult males of P. denticauda use an extremely complicated and variable proclamation song for intraspecific acoustic communication. The song, which is often broadcast for a long time, can be divided into three phases on the basis of its time-amplitude structure. The syllable sequence exhibits some constant and other highly variable parameters. The basic, repeated element of the proclamation song is a pair of syllables; both syllables in the pair differ in duration and amplitude as well as in repetition rate between the three phases. When P. denticauda was stimulated with the conspecific male song, the responses of most S-neurons were found to copy the time patterns relatively well. Some of the V- and VS-neurons also responded relatively well to the syllables when vibrational signals were presented simultaneously. However, under these conditions the time structure was not copied, as it was by the S-neurons. In comparative investigations, the responses of central neurons of Decticus albifrons and Tettigonia viridissima, two species with more simply constructed songs having either low or high repetition rates, reflect the parameters of the song of P. denticauda less well than do the central neurons of P. denticauda. Therefore, some physiological adaptations to this complex male proclamation song seem to be inherent in the reactions of these central neurons in P. denticauda. J. Exp. Zool. 279:9–28, 1997. © 1997 Wiley-Liss, Inc.

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.