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Chamber music: an unusual Helmholtz resonator for song amplification in a Neotropical bush-cricket (Orthoptera, Tettigoniidae)

ABSTRACT Animals use sound for communication, with high-amplitude signals being selected for attracting mates or deterring rivals. High amplitudes are attained by employing primary resonators in sound-producing structures to amplify the signal (e.g. avian syrinx). Some species actively exploit acoustic properties of natural structures to enhance signal transmission by using these as secondary resonators (e.g. tree-hole frogs). Male bush-crickets produce sound by tegminal stridulation and often use specialised wing areas as primary resonators. Interestingly, Acanthacara acuta, a Neotropical bush-cricket, exhibits an unusual pronotal inflation, forming a chamber covering the wings. It has been suggested that such pronotal chambers enhance amplitude and tuning of the signal by constituting a (secondary) Helmholtz resonator. If true, the intact system – when stimulated sympathetically with broadband sound – should show clear resonance around the song carrier frequency which should be largely independent of pronotum material, and change when the system is destroyed. Using laser Doppler vibrometry on living and preserved specimens, microcomputed tomography, 3D-printed models and finite element modelling, we show that the pronotal chamber not only functions as a Helmholtz resonator owing to its intact morphology but also resonates at frequencies of the calling song on itself, making song production a three-resonator system. Summary: Laser vibrometry, microcomputed tomography and finite element modelling are used to show that an unusual pronotal inflation covering the wings of a bush-cricket acts as a Helmholtz resonator.