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Hearing in hooktip moths (Drepanidae: Lepidoptera)

Annemarie Surlykke^1,*, Jayne E. Yack², Andrew J. Spence³ and Ivar Hasenfuss⁴

¹ Center for Sound Communication, Institute of Biology, Southern University of Denmark, Odense Denmark
² Department of Biology, College of Natural Sciences, Carleton University, Ottawa, Ontario, Canada
³ Department of Applied and Engineering Physics, Cornell University, Ithaca, New York, USA
⁴ Karlsbader Strasse 9, D-91083 Baiersdorf, Germany

View larger version (144K): [in a new window]   Fig. 1. Tympanal hearing organs in Drepana arcuata. (A) A resting female D. arcuata. Scale bar, 6 mm. (B) Right lateral view of the thorax and anterior abdomen, showing the general location of the abdominal ear. White arrow, location of the anterior external membrane; black arrow, location of the posterior external membrane. Scale bar, 0.75 mm. (C) Scanning electron micrograph of the anterior and ventral portion of the first abdominal segment in a male D. arcuata. Each ear comprises a dorsal (dc) and ventral (vc) air chamber, between which the tympanal membrane is located. Sound is thought to enter the dorsal chamber by means of the anterior external membrane (aem) and posterior external membrane (not shown). Scale bar, 0.25 mm. (D) The tympanal membrane of the left ear as seen following removal of the dorsal chamber. Median is on the right. The four scolopidia are suspended between the two tracheal walls forming the tympanal membrane. The black and white arrows mark the locations of scolopidia 4 and 3, respectively. Scale bar, 0.08 mm.

View larger version (36K): [in a new window]   Fig. 2. Innervation of the abdominal tympanal ear in D. arcuata. (A) Dorsal view of the thoracic ganglia and the thoracic–abdominal connective. The main nerve roots, identified according to the Nüesch (1957) nomenclature, are illustrated. I, first thoracic ganglion; II, second and III, third thoracic ganglia (= pterothoracic ganglion); the tympanal nerve arises from 1N1 (= anterior branch of first abdominal ganglion), which is the first nerve root from the connective posterior to the pterothoracic ganglion. The second branch of 1N1 innervates the tympanal membrane. is, intersegmental muscle. Scale bar, 0.4 mm. (B) Tympanal scolopidia of a male, viewed from the dorsal chamber. Median is on the right, as in (D). The tympanal nerve departs at the ventral and median edge of the tympanal frame. Scale bar, 0.050 mm. AC, attachment cell; E, enveloping cell (= scolopale cell); F, system of longitudinally oriented fibrils; P, perineurium cell; Sc, 'scolopale' region, with scolopale rods, cap and dendritic cilium; SC, sensory cell. (C) Diagrammatic representation of the curved tympanic membrane in its frame with scolopidia 1-4 viewed from the dorsal chamber. Median is upwards and to the left. Scolopale regions are represented as dark swellings. Scale bar, 0.1 mm. (D) Schematic illustrating D, the distance from frame to frame, arcMem, the distance along the membrane, and R, the radius of the circle that follows the membrane.

View larger version (78K): [in a new window]   Fig. 3. Methylene Blue preparations of the right side tympanal scolopidia viewed from the dorsal chamber in two species of Drepanidae. Median is on the left. (A) Drepana arcuata. Composite image created from two micrographs taken at different focal planes. Sensory cell bodies of scolopidia 1-4 are marked, as well as the axons of cells 2 and 3 (arrowheads), and the location of the dendrite (d) and scolopale cell (arrow) of scolopidium 4. Scale bar, 0.05 mm. Inset: Scolopale rods and cap (c) and distal dendrite (d) of scolopidium 4. Scale bar, 0.005 mm. (B) Watsonalla uncinula. Scolopidia 1-4, marking the attachment cell (AC) and the location of the scolopale rods (arrow) of scolopidium 3. Scale bar, 0.05 mm. Inset: Scolopale cap (c) and distal fibrils (F) of the attachment cell in scolopidium 4. Scale bar, 0.01 mm.

View larger version (30K): [in a new window]   Fig. 4. D. arcuata audiograms. Individual audiograms of 8 females (red) and 4 males (blue). Mean audiogram of all is shown by the thick black line. The inset shows threshold curves for both A1 and A2 for one female where A2 spikes were clearly higher than A1.

View larger version (12K): [in a new window]   Fig. 5. Spike-traces at different stimulus intensities illustrating response characteristics of the two functional auditory sensory cells A1 and A2. (A) The recruitment of A2 is revealed by spikes with double height or double peaks. In this moth, A2 threshold was +17 dB re A1 threshold. Stimulus pulses: 10 ms, 30 kHz. (B) The mean number of spikes per stimulus (10 stimulations) as a function of intensity relative to A1 threshold (0 dB) from the same moth as in A. The number of A1 spikes per stimulus increases steeply from threshold to approximately +10 dB, where A1 starts saturating before the A2 threshold is exceeded at approximately +15 dB in this preparation. A2 is saturating at intensities greater than +30 dB above threshold.

View larger version (113K): [in a new window]   Fig. 6. Color rasters of two moths (A and B) showing dynamics of spike trains as a function of intensity and post-stimulus time. Stimulus intensities were changed in 1 dB steps. In (A) the moth was stimulated 10 times at each intensity level, whereas in (B) only two presentations were possible. Spike amplitudes are represented in color, and the color bar scale is in mV. The moth in B was chosen to illustrate the maximum response duration at high stimulus intensities. In this moth there was a clear difference in spike amplitude between A1 and A2 (inset) and both cells contributed to the long response train.

View larger version (15K): [in a new window]   Fig. 7. The relation between pressure changes (D/2R) and the curvature of a cylindrical membrane (arcMem/D). Pressure changes can be expressed as D/2R according to Laplace's law modified for a cylindrical membrane: D/2R=(P–P0)x(D/2), where is the tension of the membrane. The curvature of the membrane is related to pressure changes by the inverse sine function: arcMem/D= (2R/D)xarcsine(D/2R), and varies between 1 (a flat membrane) and /2 (a half circle). For constant D the curvature of the membrane will change with arcMem, and the same change in pressure will induce larger arcMem changes with increasing curvature of the membrane. Curvature of the drepanid tympanic membrane is indicated at the level of scolopidium 4 (circles) and at the level of scolopidium 1 (diamonds). Changes in arcMem of the tympanic membrane translates to length changes of the embedded sensory cells.