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 K-Iz 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. (C) 1996 Wiley-Liss, Inc.

The causes of the differential sensitivity of auditory receptor cells of two tettigoniid species to low-frequency airborne sound were investigated with neurophysiological, neuroanatomical, and bioacoustic methods. The experiments employed adult males of the Chinese species Gampsocleis gratiosa (Decticinae) and the Australian species Mygalopsis marki (Conocephalinae). G. gratiosa emits very broad-band songs with a fundamental frequency component at 3.8 kHz, extremely low for a bushcricket; in contrast, the songs of M. marki have a narrow-band frequency spectrum, from 9 to 25 kHz. Accordingly, the threshold of the auditory organ as a whole is low in the low-frequency range in the case of G. gratiosa, whereas M. marki is similarly sensitive only at frequencies above 5 kHz.The experiments were designed to reveal the origin of this sensitivity difference. One possibility considered was that low-frequency receptors of a particularly sensitive type are present in G. gratiosa and not in M. marki; alternatively, with a similar complement of receptor cells the whole-organ threshold curve might be expanded or restricted as a result of differences in the lower cutoff frequency of the sound-conducting system.The results confirm the latter interpretation. The proximal crista acustica and the distal intermediate organ comprise the same numbers and types of cells in the two species, and the difference in threshold of the low-frequency receptors is produced by a shift of the frequency range in which the acoustic trachea has an amplifying effect. (C) 1995 Wiley-Liss, Inc.

The morphology and acoustic characteristics of the acoustic tracheal system were examined in certain tettigoniid species. Morphological measurements and statistical analysis reveal that in all species of bushcrickets investigated the shape of the acoustic trachea can be approximated by the equation of an exponential horn. Based on this approximation the transmission functions of the different tracheae were calculated theoretically. Because of its small size, the acoustic trachea must not be treated as an infinite exponential horn but its transmission function must be calculated by means of the equations for a finite-length horn. The finite horn amplifies sound from a certain frequency on (cutoff frequency) in a broad range of frequencies as the infinite horn does. But the broadbanded transmission is superposed by a few resonances which are caused by reflections inside the horn. Bioacoustical measurements were made with a probe microphone. As expected from the theoretical considerations the course of the measured transmission function corresponds much better with the one calculated for the finite exponential horn than with the one calculated for the infinite horn. The present results are discussed referring to the consequences of the transmission properties of the acoustic trachea for the hearing ability of Tettigoniids. Additionally, they are compared with other investigations concerning the function of the acoustic trachea in tettigoniid hearing.

Variability in the chirp rate of the male song of the ephippigerine speciesEphippigerida taeniata during intraspecific communication was investigated in the laboratory. Conspecific chirps were used as auditory stimuli. The stimulus rate was controlled by computer. Experiments were carried out at 19, 27, and 35°C. Acoustically isolated males ofE. taeniata sang with a relatively constant chirp rate, which depended on the ambient temperature. Chirp rate significantly increased with rising temperature from 19 to 27°C, whereas at 35°C the chirp rate did not differ significantly from that at 27°C. Male chirp rates were affected by stimulus rates. Males significantly increased their chirp rate in response to increasing stimulus rates at temperatures of 19 and 27°C. At 35°C the increase in the chirp rate was not significant. At 27 and 35°C males sang with a higher chirp rate than the stimulus rate within a certain range. Evaluating stimulus and response chirp pattern when the males increased their chirp rate in response to the stimulus rate showed that an alternation pattern was established. More than 50% of the male chirps occurred at a characteristic time range at around 40% of the interstimulus interval, which was slightly affected by temperature.