Insect hearing in the field

Summary

• 1. Distances between calling maleMygalopsis marki Bailey (Orthoptera: Tettigoniidae) were measured in two geographically distinct populations. Mean intermale distances differed significantly in the two habitats (6.2 m and 11.5 m), but the sound pressure level of the nearest neighbour, measured at each focal male, was close to 65 dB SPL in both populations (Figs. 4, 6).

• 2. Using the response of an identified auditory interneuron as a ‘biological microphone’, the maximum hearing distance of males was tested. It was found to be affected by the size of the calling male, its posture, perch height and the vegetation density. Perch height alone influenced the maximum broadcast area of males by some 100%. Nearest neighbour distances were less than half the maximum hearing distances, confirming by neurophysiological methods that males do not space on threshold sensitivity to the complete call (Fig. 5).

• 3. The ability of the bushcricket to perceive the temporal characteristic of the call was also tested with the auditory interneuron over increasing distances from a signaller. The temporal integrity of the song was found to be maintained over the whole broadcasting area (Fig. 9).

• 4. The call has two energy peaks at 10 kHz and 20 kHz. The intensity of each peak attenuated differentially with increasing distance from the signaller; the low frequency component had a low variance at the nearest neighbour distance (SD: +4 dB/−6 dB) compared to the high frequency component (SD: +5 dB/−12.5 dB) (Fig. 8). The importance of the total sound pressure level and the relative intensity of the major frequency components of the song are discussed in relation to likely cues available to the male for maintaining spacing.