2379 INTRODUCTION The production and reception of sound plays an important role in the life history of many insect species. For example, pair formation is ruled by the production and reception of sound between partners, and the orientation towards a prey or away from a predator can be elicited by acoustic cues (Greenfield, 2002). The success of such behaviour depends on the activation of complex auditory organs, either antennae working as near-field particle velocity detectors, or tympanal ears acting as far field pressure receivers (Robert and Göpfert, 2002; Robert and Hoy, 2007). In the latter case, the receiving organ is typically organized around a thin tympanal membrane (TM) made of cuticle, backed with air-filled tracheal sacs, and a set of sensory neurons connected to glial and support cells (for reviews, see Yager, 1999b; Yack, 2004). Such hearing organs have independently evolved in seven insect orders showing different degrees of development and organization (Hoy and Robert, 1996). Whatever the complexity of the hearing system is – from the ‘cyclopean’ ear of the praying mantis (Yager and Hoy, 1986), to the highly innervated pair of conspicuous cicada ears (Fonseca et al., 2000) – differences between male and female have been very rarely reported in tympanal structure and mechanics (Hoy and Robert, 1996). Sexual dimorphism has been documented in the ear anatomy of a small number of praying mantises (Yager, 1999a), bushcrickets (Bailey and Römer, 1991), flies (Robert et al., 1994) and moths (Minet and Surlykke, 2003), and was found to be obvious in cicadas (Pringle, 1954). Divergences in frequency tuning between sexes have been recorded in the ascending neurons of some crickets, bushcrickets, grasshoppers and cicadas (Gerhardt and Huber, 2002). However, little information has been made available on potential mechanical differences between male and female tympana (Meyer and Elsner, 1997). The origin of sexual dimorphism at the anatomical, mechanical or neuronal level may be explained by selective forces and constraints acting differently on the sexes. Several sex-linked factors can indeed be put forward: (1) sexes are exposed to different predators (Cardone and Fullard, 1988; Yager, 1990; Rydell et al., 1997); (2) prey or host detection is devoted to one sex only (Lakes-Harlan and Heller, 1992; Robert et al., 1994); (3) intra-sexual communication has been reduced, or disappeared in a single sex (Bailey and Römer, 1991; Mason and Bailey, 1998); (4) there is production of sex-specific signals in duetting species (Bailey, 2003); (5) the acoustic role of the sexes in pair formation is unbalanced; (6) each sex inhabits a specific niche implying different environmental constraints on sound propagation. To understand how one or several of these factors work at shaping the structural basis and functional diversity of insect auditory sexual dimorphism, it is necessary to study a model that shows an obvious sexual dimorphism and for which acoustic communication is well known. In cicada, one of the noisiest animals in the world (Bennet-Clark and Young, 1992), there is extreme sexual dimorphism in the sound production system. Males possess a pair of abdominal tymbals fully dedicated to the generation of the calling song, a unique system that does not appear in the female (Pringle, 1954; Bennet-Clark and Young, 1992; Young and Bennet-Clark, 1995), hence the absence of any inter-female acoustic communication. Both male and female are nonetheless endowed with fully developed tympana whose differences in size and shape have been recognized since the middle of the nineteenth century (Dugès, 1838; Powell, 1873). These tympana are extended by a cuticular apodeme to which a set of sensory neurons (scolopidia; type I monodynal receptors) are attached. Tympana can therefore be considered as the first and necessary step of the mechanical chain that ensures audition in cicadas. The male tympanum is always larger, and is often coupled to a large air-filled abdomen. This dimorphism has been associated The Journal of Experimental Biology 211, 2379-2387 Published by The Company of Biologists 2008 doi:10.1242/jeb.018804 Sexual dimorphism in auditory mechanics: tympanal vibrations of Cicada orni Jérôme Sueur 1, *, James F. C. Windmill 2 and Daniel Robert 2 1 Muséum National dʼHistoire naturelle, Département Systématique et Evolution, UMR 5202 CNRS & USM 601 MNHN, 75005 Paris, France and 2 School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK *Author for correspondence (e-mail: sueur@mnhn.fr) Accepted 12 May 2008 SUMMARY 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 cicadaʼs acoustic behaviour. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/211/15/2379/DC1 Key words: hearing, tympanum, biomechanics, laser vibrometry, high-frequency reception, predation, selective forces, cicada. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY