No abstract

The transmission characteristics of the acoustic tracheae in the forelegs of seven tettigoniid species were investigated by sinusoidal analysis. The species were selected to represent a range of body sizes and leg lengths. Four subfamilies were included, with two species each from three of them; the tracheae in such closely related pairs could be expected to be similar in shape despite their different dimensions. The tracheae were dissected out for morphometric analysis and compared with one another with respect to their overall dimensions and those of typical subsections. The amplitude-versus-frequency response of acoustic transmission in the tracheae was measured at various positions with a probe microphone. The stimuli were continuous sinusoidal signals at an intensity of 100 or 110 dB SPL. The tracheae of all the species studied here (in males and females) are distinguished by a bandpass-limited transmission characteristic. In the frequency range above 5 kHz (at least to 40 kHz) the sound signals are amplified by 10-15 dB during passage through the tracheae. These results are compared with the threshold curves of the auditory organs and the spectra of the conspecific songs. Although in some cases there are considerable differences in the dimensions of the tracheae, the transmission characteristics are very similar; no specific adaptations to the frequency composition of the conspecific song were found.

The morphology and the physiology of the auditory receptor organs in the forelegs of the tettigoniid species Gampsocleis gratiosa were investigated in detail. According to the tonotopic organization of the receptor cells of the crista acoustica in bushcrickets the threshold curves of all receptors found in the organs were correlated with the morphology of the auditory receptor cells. This was done by arranging them in relation to their frequency tuning with the receptors along the chain of cells in both organs starting proximally with the extreme low frequency cell up to the highest ultrasonic cells a t the distal end of the crista acoustica.This investigation reveals that the auditory receptor organs of Gampsocleis gratiosa possess a broad banded hearing threshold curve including very low frequency parts. Also the conspecific stridulatory song is very broad banded with extraordinarily low frequency components. Therefore the sender and the receiver of this species-specific communication system seem to be well adapted to each other. o 1993 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 trac'heae 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. ¸ 1995

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.