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First published online July 20, 2007
Journal of Experimental Biology 210, 2637-2648 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005025

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Mechanics of a `simple' ear: tympanal vibrations in noctuid moths

J. F. C. Windmill^1,*, J. H. Fullard² and D. Robert¹

¹ School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK
² Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario, L5L 1C6, Canada

View larger version (77K): [in this window] [in a new window]   Fig. 1. (A) Close-up of the tympanum of the ear of Noctua pronuba. An opaque membrane, the conjunctivum (Cj), is situated distally from the tympanic membrane (TM), both of which lie above the tympanic cavity at the posterior lateral margin of the metathoracic segment. The stigma (arrow) marks the attachment site of the auditory receptor organ. Scale bar, 0.25 mm. (B) Morphological features of the tympanal system, highlighted as they pertain to the vibrational characteristics of the tympanum. The TM, sensu stricto, is bordered distally by a nodular sclerite, the epaulette (Ep), which separates it from the Cj. The attachment site of the auditory chordotonal organ (arrow in A) appears as a dark spot at the centre of the opaque zone (OZ), which is surrounded by the transparent zone (TZ).

View larger version (42K): [in this window] [in a new window]   Fig. 2. Response of the moth tympanal membrane to sound. (A) The amplitude-phase gain response of the transparent zone (TZ) of N. pronuba. (B) The coherence signal for the measurement in A. (C) The amplitude gain response of the TZ of A. exclamationis, X. c-nigrum and X. triangulum. (D) Measurement of the auditory neural activity of X. c-nigrum combined with the laser vibrometric measurement of TZ displacement (A1 and A2: auditory mechanosensory neurones).

View larger version (38K): [in this window] [in a new window]   Fig. 3. Area scan and deflection shapes of the tympanic membrane (TM) in N. pronuba. (A) Orientation image relating tympanal topography (left image) to the position of the scanning lattice (right image). (B) Area scans of tympanal deflections for three different frequencies. The deflections are shown each time for four different phases along the oscillation cycle (see also Movies 1–3 in supplementary material). Deflections are additionally shown as profiles, looking at the tympanum from its side. Red indicates positive displacement (or outward tympanal deflections), and green indicates negative displacement (or inward tympanal deflections).

View larger version (32K): [in this window] [in a new window]   Fig. 4. Area scan and deflection shapes of the tympanic membrane (TM) in A. exclamationis. Area scans of tympanal deflections for three different frequencies. The deflections are shown each time for four different phases along the oscillation cycle. Deflections are additionally shown as profiles, as if looking at the tympanum from its side. Red indicates positive displacement (or outward tympanal deflections), and green indicates negative displacements (or inward tympanal deflections).

View larger version (33K): [in this window] [in a new window]   Fig. 5. Area scan and deflection shapes of the tympanic membrane (TM) in X. triangulum. Area scans of tympanal deflections for three different frequencies. The deflections are shown each time for four different phases along the oscillation cycle. Deflections are additionally shown as profiles, as if looking at the tympanum from its side. Red indicates positive displacement (or outward tympanal deflections), and green indicates negative displacement (or inward tympanal deflections).

View larger version (35K): [in this window] [in a new window]   Fig. 6. Envelopes of mechanical deflections (nm Pa–1) across the tympanic membrane (TM) of N. pronuba along transect lines for different driving frequencies. (A) The position along the transect lines is given with a calibrated scale. This scale constitutes the x-axis in B–F. Scale bar, 0.25 mm. (B–F) The deflection envelopes are constructed by displaying the instantaneous deflection velocities along the transect for a series of phases (in 90° increments) along the full oscillation cycle. Driving frequencies: (B) 20 kHz; (C) 45 kHz; (D) 70 kHz; (E) 20 kHz; (F) 45 kHz; (G) 70 kHz; (H) 58.76 kHz (resonance). SA, sensory attachment; Ep, epaulette.

View larger version (28K): [in this window] [in a new window]   Fig. 7. Mechanical deflection across the membrane of a Brüel & Kjaer type 4138 pressure-field microphone. (A) Envelopes of deflection (gain) for driving frequencies of 25 and 70 kHz. (B) Area scans of microphone membrane deflections for driving frequencies of 25 and 70 kHz. The deflections are shown each time for four different phases along the oscillation cycle. Deflections are additionally shown as profiles, looking at the membrane from its side. Red indicates positive gain (or outward membrane deflections), and green indicates negative gain (or inward membrane deflections).

View larger version (18K): [in this window] [in a new window]   Fig. 8. Comparison between moth and microphone membrane tension. (A) Normalised maximum positive deflection (displacement) of the N. pronuba tympanic membrane (TM) and the B&K 4138 microphone membrane at 70 kHz. Each displacement is shown with its corresponding `fit', a Lorentzian for the moth and a sinusoid for the microphone. (B) Tension across a B&K microphone, fitted to a sinusoidal model, and across the N. pronuba TM, and fitted to a Lorentzian, when both are driven at 70 kHz.