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First published online July 14, 2008
Journal of Experimental Biology 211, 2379-2387 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018804

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Sexual dimorphism in auditory mechanics: tympanal vibrations of Cicada orni

Jérôme Sueur^1,*, James F. C. Windmill² and Daniel Robert²

¹ Muséum National d'Histoire naturelle, Département Systématique et Evolution, UMR 5202 CNRS & USM 601 MNHN, 75005 Paris, France
² School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK

^* Author for correspondence (e-mail: sueur{at}mnhn.fr)

Accepted 12 May 2008

In cicadas, the tympanum is anatomically intricate and employs complex vibrations as a mechanism for auditory frequency analysis. Using microscanning laser Doppler vibrometry, the tympanal mechanics of Cicada orni can be characterized in controlled acoustical conditions. The tympanum of C. orni moves following a simple drum-like motion, rather than the travelling wave found in a previous study of Cicadatra atra. There is a clear sexual dimorphism in the tympanal mechanics. The large male tympanum is unexpectedly insensitive to the dominant frequency of its own calling song, possibly a reflection of its dual purpose as a sound emitter and receiver. The small female tympanum appears to be mechanically sensitive to the dominant frequency of the male calling song and to high-frequency sound, a capacity never suspected before in these insects. This sexual dimorphism probably results from a set of selective pressures acting in divergent directions, which are linked to the different role of the sexes in sound reception and production. These discoveries serve to indicate that there is far more to be learnt about the development of the cicada ear, its biomechanics and evolution, and the cicada's acoustic behaviour.

Key words: hearing, tympanum, biomechanics, laser vibrometry, high-frequency reception, predation, selective forces, cicada