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First published online December 26, 2008
Journal of Experimental Biology 212, 257-269 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.022731

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Mechanical phase shifters for coherent acoustic radiation in the stridulating wings of crickets: the plectrum mechanism

Fernando Montealegre-Z^1,*, James F. C. Windmill², Glenn K. Morris³ and Daniel Robert¹

¹ School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK
² Centre for Ultrasonic Engineering, Department of Electronic and Electrical Engineering, University of Strathclyde, Royal College Building, 204 George Street, Glasgow, G1 1XW, UK
³ Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, ON, Canada, L5L 1C6

^* Author for correspondence (bzfmz{at}bristol.ac.uk)

Accepted 30 September 2008

Male crickets produce stridulatory songs using engaged tegmina (forewings): a plectrum on the left sweeps along a tooth row on the right. During stridulation, the plectrum moves across the teeth and vibrations are amplified by the surrounding cells and veins, resonating at the frequency of tooth impacts. The advance of the plectrum on the file is controlled by an escapement mechanism so that passing each single tooth generates one wave of a highly tonal signal. Both tegmina must oscillate in phase to avoid destructive interference. But as each plectrum-tooth contact begins, the right and left tegmina react in opposite oscillatory directions. A mechanical phase shifter is part of the left tegmen and compensates to achieve wing oscillation synchrony. We use a new technique to simulate plectrum-on-file interactions: in combination with laser vibrometry, this technique assessed plectrum mechanics in the cricket Gryllus bimaculatus. Using an excised teneral file, shaped like a partial gear and moved by a motor, and a microscan Doppler laser vibrometer, plectrum and left-tegmen mechanics were explored. The results show that plectrum and harp oscillate with a phase difference of ca. 156 deg., a shift rather than a complete phase inversion (180 deg.). This phase shift occurs at the site of a large wing vein (possibly A3). Plectrum and harp vibrate with similar fundamental frequency, therefore, plectrum torsion resonant frequency is important for maintaining vibration coherence. The mechanical aspects involved in this partial phase inversion are discussed with respect to the escapement mechanism. The plectrum mechanics and its implications in katydid stridulation are also considered.

Key words: biomechanics, stridulation, phase shifter, bioacoustics, Gryllidae, Orthoptera

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