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First published online October 16, 2009
Journal of Experimental Biology 212, 3533-3541 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.032425

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Auditory mechanics and sensitivity in the tropical butterfly Morpho peleides (Papilionoidea, Nymphalidae)

Kathleen M. Lucas^1,*, James F. C. Windmill², Daniel Robert¹ and Jayne E. Yack³

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, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6

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

Accepted 25 July 2009

The ears of insects exhibit a broad functional diversity with the ability to detect sounds across a wide range of frequencies and intensities. In tympanal ears, the membrane is a crucial step in the transduction of the acoustic stimulus into a neural signal. The tropical butterfly Morpho peleides has an oval-shaped membrane at the base of the forewing with an unusual dome in the middle of the structure. We are testing the hypothesis that this unconventional anatomical arrangement determines the mechanical tuning properties of this butterfly ear. Using microscanning laser Doppler vibrometry to measure the vibrational characteristics of this novel tympanum, the membrane was found to vibrate in two distinct modes, depending on the frequency range: at lower frequencies (1–5 kHz) the vibration was focused at the proximal half of the posterior side of the outer membrane, while at higher frequencies (5–20 kHz) the entire membrane contributed to the vibration. The maximum deflection points of the two vibrational modes correspond to the locations of the associated chordotonal organs, suggesting that M. peleides has the capacity for frequency partitioning because of the different vibrational properties of the two membrane components. Extracellular nerve recordings confirm that the innervating chordotonal organs respond to the same frequency range of 1–20 kHz, and are most sensitive between 2 and 4 kHz, although distinct frequency discrimination was not observed. We suggest that this remarkable variation in structure is associated with function that provides a selective advantage, particularly in predator detection.

Key words: butterfly, tympanal hearing, scanning laser vibrometry, extracellular nerve recording, chordotonal organ

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