verse interval [ms] verse duration [ms]
FIGURE 3.27 Results of phonotactic experiments with females of Tettigonia viridissima by means of one sound source only. Percentage of females that walked towards loudspeaker shown in dependency of the stimulus parameter. Left: variation of the verse interval; right: variation of the verse duration. The vertical dotted lines represent the values of the corresponding parameter of the natural songs of T. cantans (T.c.) and T. viridissima (T.v.). Schematic drawings of some stimuli are shown. Number of females tested was 20 for each verse interval and 11 to 15 for each verse duration. (After Jatho, M., Untersuchungen zur Schallproduktion und zum phonotaktischen Verhalten von Laubheuschrecken [Orthoptera: Tettigoniidae], Cuvillier Verlag, Gottingen, 1995. With permission.)
not at all or only unspecifically to airborne-sound stimuli. By contrast, the responses to vibratory stimuli are similar if not identical. The functions of the subgenual organs in all six legs are basically identical, but the functions of the middle and distal parts of the CA remain to be characterised.
Summarizing the results of extensive investigations on the morphology, development, bioacoustics properties and physiology of the auditory-vibratory system of bushcrickets at the receptor and ventral cord level, basic similarities become evident when comparing the conditions in different species belonging to different subfamilies. The prothoracic spiracle and acoustic trachea in the foreleg represent the main input for airborne sound. Comparative morphometric measurements and theoretical calculations revealed that the acoustic trachea acts as a finite length exponential horn resulting in broadband transmission with superimposed resonances caused by internal reflections. Laser-vibrometry measurements of the vibrations of the tympana in the foreleg tibiae indicate that these structures play a crucial role in determining the overall acoustic impedance of the bushcricket ear, in particular the terminating properties of the acoustic trachea. This is most important for stimulus transmission to the receptor cells situated on the dorsal wall of the acoustic trachea.
In spite of differences of the dimensions, e.g. in leg size and resulting length of the acoustic trachea or of the morphology of the stimulus transmitting structures, like open and covered tympana, decisive structures inside the organs responsible for the acousto-mechanical stimulus transmission and transduction into bioelectrical receptor cell responses are very similar. The important function of these supporting structures for auditory stimulus transmission is also supported by studies on the postembryonic development of the sound conducting acoustic trachea and attachment structures inside the receptor organs. Similarities in the dimensions of these structures obviously cause very similar response types of auditory receptor cells, which can even be found in different species. Nevertheless, species-specific physiological adaptations were found in the different species investigated, especially when the frequency response properties of the entire auditory receptor neuron population was compared.
Basic similarities were also found in the structure and function of auditory-vibratory interneurons at the ventral cord level. Acoustic neurones of the ventral nerve cord ascending to the head ganglia are without exception bimodal auditory-vibratory in character. The convergence of the two sensory inputs at the ventral nerve cord level could be a fundamental element in the process of localising a stridulating partner at short distances (up to 1 to 2 m). Moreover, it could facilitate and improve the recognition of signals from conspecifics.
The most important function of acoustic communication in bushcrickets is the attraction of sexually receptive conspecifics. In most cases males produce sound and females locate the singing males. Sound (and vibration) signals are produced by elytro-elytral stridulation, and in most cases contain a broad range of frequencies starting in some species at frequencies as low as 3 to 4 kHz and ranging up to 80 kHz. Comparative studies show that there is a wide variation of time-amplitude patterns of song syllables in the song of different species of bushcrickets. Individual sound impulses, damped oscillations within song syllables, are caused by friction of the plectrum over a row of teeth on the pars stridens. These sound impulses are clearly separated in some species (nonresonant sound production) or can be superimposed in others (resonant sound production). In many species, a combination of resonant and nonresonant sound production is evident and was shown to affect the power spectra of the song, especially in amplifying low frequency components. In addition, the subtegminal air volume was shown to affect both the amplitude of the low and high frequency components of the song as shown by sound recordings in a helium-oxygen mixture. Sound is broadcast during species-specific daily activity periods, and sound transmission can be substantially influenced by vegetation, interference with songs produced by other species and, for example, climatic conditions. Therefore, song recognition and discrimination represent crucial features of acoustic behaviour. A series of detailed choice experiments in two closely related species, T. cantans and T. viridissima, using computer generated song models revealed that differences in the species-specific syllable pattern represent a most important cue for the discrimination of females between conspecific and heterospecific males.
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