Sound-induced cuticular vibrations in the atympanate bladder grasshopper Bullacris membracioides

Abstract

In terrestrial animals, detection of sound waves over long distances conventionally requires the presence of tympanal ears. Although it lacks tympana, the South African bladder grasshopper Bullacris membracioides (Orthoptera, Pneumoridae) is endowed with an acute sense of hearing. Long-range acoustical communication is mediated by duets in which the males loud song (1.7 kHz, 98 dB SPL at 1 m) is answered by the females milder reply (at 4 kHz, 60 dB SPL), allowing the male to locate her. Males present a conspicuously inflated abdomen that constitutes a resonant sound radiator. Both males and females harbor six pairs of mechanosensory organs attached to the wall of abdominal segments A1–A6 that are involved in sound reception [M. van Staaden and H. Roemer, Nature 394, 773]. Using microscanning laser vibrometry to investigate the mechanics of atympanate hearing, sound-induced vibrations were shown to differ between male and female in their deflection shapes and amplitudes. Male cuticular vibrations culminate at 1.7 kHz, confirming the resonant nature of their abdominal wall. In both sexes, vibrations at the points of sensory insertions indicate that conventional tympana do not constitute a necessary condition for highly sensitive auditory function. [Work supported by the Austrian and the Swiss Science Foundations.]