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Wing resonances in the Australian field cricket Teleogryllus oceanicus

H. C. Bennet-Clark

Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, UK

View larger version (66K): [in a new window]   Fig. 1. Drawing of the right fore-wing of Teleogryllus oceanicus made from a photograph. The wing is viewed from below. The nomenclature of the venation and fields of the wing follow those given in Table 1. Areas implicated in sound production are the harp (green) and mirror (blue) The flexible regions separating the lateral field from the wing base and lateral field are shown as purple areas, and additional flexible regions are highlighted by purple lines. For free vibration tests, the wing was waxed to the vibration probe (see Fig. 2) at the base of the lateral field (blue spot). For tests in which vibration was applied to the file, plectrum or anal area, the wing was glued to a steel rod at the bases of the Sc, R and M veins (red spot). Vibration via the anal area was applied at the position of the circle at the end of the `anal area' label line.

View larger version (59K): [in a new window]   Fig. 2. Diagram of the piezo-electric vibrator used to excite wing resonances. Excitation was applied either by the probe rod or after waxing a left plectrum with a 2 mm length of the left file, as shown, to the tip of the probe rod.

View larger version (25K): [in a new window]   Fig. 3. (A) Graph showing the tooth pitch on the right files of five Teleogryllus oceanicus fore-wings, showing that the tooth spacing increases along the active length of the file: during sound production, the plectrum moves from the posterior to anterior on the file, as shown by the arrow. (B) Graph showing the cumulative number of teeth versus distance along the file for six right files of T. oceanicus fore-wings. Note that approximately half the teeth are situated on the distal third of the file.

View larger version (25K): [in a new window]   Fig. 4. (A,B) Drawings of three file teeth of Teleogryllus oceanicus near the middle of the right file. (A) Optical section near the middle of the teeth; the arrow shows the direction of movement of the plectrum during the wing-closing movement. (B) Plan view of the teeth from the underside of the wing showing the sclerotised edge and the angle of the teeth relative to the axis of the file. (C) Diagram showing the engagement of the plectrum with the file, based on a geometric model in which the fore-wings are separated by the 5 mm distance between the wing bases and travel through complementary arcs: the arrow shows the direction of movement of the left plectrum. The right file is drawn as if it were stationary but in the model both wings closed in opposite directions through the same angle, which is shown by the degree scale above the drawings of the left plectrum. Note that the centre of the plectrum engages with the centre of the file over most of the length of the file and that the angle made by the edge of the plectrum to the axis of the file is slightly greater than the angle of the teeth to the file axis shown in B.

View larger version (38K): [in a new window]   Fig. 5. Details of the left plectrum. (A) Drawing of the left plectrum from below. (B) Drawing based on a scanning electron micrograph of a transverse section of the left plectrum cut approximately halfway along its length. The arrow shows the direction of the push used to elicit clicks of the plectrum and also the approximate line of engagement with the teeth of the file.

View larger version (29K): [in a new window]   Fig. 6. The calling song of Teleogryllus oceanicus taken from a recording made in the field at Carnarvon, Western Australia. (A) Oscillogram of one song echeme, showing the chirp of four pulses and a trill of paired pulses. (B) Oscillograms of the third pulse of the chirp shown in A and the first and second of the first of the pair of pulses of the trill shown in A. (C) Graph of the cycle-by-cycle frequency of the four pulses of the chirp shown in A (black lines) and the first (blue line) and second (red line) pulse of the paired pulses of the trill shown in A. The frequency against time plots have been superimposed by eye and the starts (S1 and S2) and ends (E1 and E2) of the paired pulses in the trill are shown by vertical, broken lines. Note that these data suggest that the 1st and 2nd of the paired pulses of the trill are produced by different parts of the file.

View larger version (31K): [in a new window]   Fig. 7. Comparison of the waveform and frequency spectrum of a typical song pulse with those of free vibrations of the left and right wings of Teleogryllus oceanicus. (A) Oscillogram of a pulse from a chirp of the song. (B) Oscillograms of the sound recorded from free vibrations of the left (red) and right (green) wings driven by 12.5-ms duration tone-bursts at their resonant frequencies. In A and B, the quality factor (Q) of the exponential decay of the vibrations is shown. (C) Frequency–energy spectra of the song pulse shown in A (black) and of the free vibrations of the left (red) and right (green) wing.

View larger version (56K): [in a new window]   Fig. 8. Drawing showing the areas of maximum vibration of the harp when driven by loud sound at 4.5 kHz, visualised with lycopodium powder placed on the underside of the right harp. At lower sound intensities, the visible vibration is confined to the cross-veins of the harp (shown by orange lines).

View larger version (28K): [in a new window]   Fig. 9. Comparison of the resonances of a left fore-wing of Teleogryllus oceanicus when driven by the vibration probe applied either at the plectrum or on the anal area. (A) Waveforms of the sounds produced by 10-ms duration tone-bursts at the resonant frequency: red, when driven at the plectrum; blue, when driven at the same frequency on the anal area. The quality factor (Q) of the decays of the vibration are similar. (B) Detail of A showing the region in the black box on an expanded time-scale to show the relative phase of the vibration when driven at the two positions.

View larger version (17K): [in a new window]   Fig. 10. (A) Diagram of the plectrum preparation from which the clicks shown in B were excited by pushing a probe rod past the edge of the plectrum at the position shown by the arrow. (B) Plectrum clicks produced by two different preparations; the fast component of the click is far briefer than the period of one cycle of the insect's song.

View larger version (25K): [in a new window]   Fig. 11. (A) Diagram of the right file from below showing the points (red circles) at which resonances were excited by vibration applied via the left plectrum. (B) Graph of the amplitude of the sound measured at the centre of the harp against the distance from the anal node at which the vibration was applied to the file. The red circles on the graph correspond to the points shown on A above.

View larger version (22K): [in a new window]   Fig. 12. Click produced by pushing the left plectrum across the file on the underside of the right fore-wing. (A) Oscillogram of the click; the arrows show points at which the amplitude has increased, probably due to impact of the plectrum on an adjacent file tooth. (B) Plot of cycle-by-cycle frequency of the pulse shown in A.

View larger version (52K): [in a new window]   Fig. 13. Drawing of part of the left wing of Teleogryllus oceanicus to show the effect on the resonant frequency of placing droplets of water (blue areas) on either the distal end of the harp or the proximal end of the anal area (for terminology, see Fig. 1). Vibration of the harp was excited by probe rod vibration applied at the plectrum. Q, quality factor; fo, resonant frequency.

View larger version (58K): [in a new window]   Fig. 14. Drawing of part of the right wing of Teleogryllus oceanicus to show the effect on the loudness of the sound produced by applying damping with a sharply pointed rod at the points shown as red circles. The wing was driven by the vibration probe attached to the lateral field (blue circle) after excision of the harp at its resonant frequency (fo; 5.15 kHz). The sound output, made with the probe microphone at the anal area, is expressed in dB relative to the sound output of the undamped wing.

View larger version (21K): [in a new window]   Fig. 15. Diagrammatic representation of the shape of the envelope of the different types of song pulses produced by Teleogryllus oceanicus. The lower horizontal scale shows the number of teeth along the length of the file. Each tooth equates with one cycle of sound in each pulse. The upper scale shows the distance along the file corresponding to particular file teeth. The long pulses of the chirp consist of approximately 180 cycles of sound and are assumed to be produced by the middle 180 teeth of the file. The first of the paired short pulses of the trill (blue) appears from Fig. 6C to be produced from a more posterior part of the file than the second of the paired pulses (red). The vertical line at tooth number 170, 2.2 mm from the posterior end of the file, is the point at which excitation of the right file produces maximal sound radiation by the harp (see Fig. 11). The horizontal arrow shows the direction of the wing-closing movement.

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