High-frequency sound transmission in natural habitats: implications for the evolution of insect acoustic communication

Summary Transmission and reception of high-frequency sound in the natural environment of bushcrickets (Tettigonia viridissima L.) was studied using the activity of an identified neuron in the insect's auditory pathway as a biological microphone. Different positions of the receiver within the habitat were simulated by systematic variation of the distance from a loudspeaker and the height above the ground. Attenuation and filtering properties of the habitat were investigated with pure-tone frequencies between 5 and 40 kHz. Sound attenuation in excess of the attenuation due to geometrical spreading alone increased with increasing frequency, distance between sender and receiver, and decreasing height within the vegetation (Figs. 2–4). The data also confirm the existence of two kinds of excess attenuation. The amount of amplitude fluctuations in the sound signals was investigated by analysing the variability of the neuronal responses at a given receiver position. Variability increased with decreasing bandwidth of a noise signal at some distance from the loadspeaker. The variability in the responses to pure tones increased with both increasing frequency and distance from the source (Fig. 7). In the selected habitat, the temporal pattern of the natural calling song of male T. viridissima was very reliably reflected in the activity of the recorded neuron up to a distance of 30 m at the top of the vegetation, and 15–20 m near ground level (Figs. 5, 8). The maximum hearing distance in response to the calling song was about 40 m. Environmental constraints on long-range acoustic communication in the habitat are discussed in relation to possible adaptations of both the signal structure and the behavior of the insects.Abbreviations excessive attenuation EA - sound pressure level SPL Offprint requests to: H. Romer