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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-Z1,*, 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


Figure 1
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  		Fig. 1. Left tegmen of G. bimaculatus, showing the main areas involved in sound production. The plectrum (green) and the harp (red) were the main regions analysed with a microscan Doppler laser vibrometer during plectrum stimulation with an excised file used as a gear. Nomenclature of wing venation follows Desutter-Grandcolas (Desutter-Grandcolas, 2003Go). M, medial veins; Cu, cubital veins; A, anal veins; SF, stridulatory file. Broken yellow line crossing the plectrum and harp shows the tegmen region used in phase shift analysis. Scale bar, 1 mm.

 

Figure 2
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  		Fig. 2. Close view of the left plectrum area in a recently emerged adult G. bimaculatus (dorsal aspect). The anal vein 1 bears the nonfunctional stridulatory file (ventrally modified with a series of teeth). Scale bar, 0.5 mm.

 

Figure 3
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  		Fig. 3. Close views of the stridulatory file of a newly emerged adult of G. bimaculatus. (A)Mid-region of the file showing tooth shape and tooth distribution. Scale bar, 200 µm. (B)Lateral aspect of file teeth (file focused anteriorly in relation to body coordinates), illustrating measurement of inter-tooth distances and tooth depth. Scale bar, 50 µm.

 

Figure 4
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  		Fig. 4. Preparation used for plectrum stimulation. (A)The gear-file system. The dorsal surface of a file segment was glued to the curved rim; most of the rim was removed after this procedure, leaving only the part containing the file. (B)Setup of the preparation. The insect was mounted in a holder and fixed to a special platform with wax, while its plectrum wing was maintained extended by fixing the wing hinge (axillary sclerites and folded membranes) with wax. The motor speed was controlled by applying different voltages to obtain the desired impact rate. The rotating file teeth produced impacts on the plectrum. Vibrations were recorded with a laser Doppler vibrometer and sound monitored with a velocity (Reference 1) and a pressure microphone (Reference 2). Reference 1 was used as a trigger during recordings. The cross-hatched green area depicts the scanned region of the wing, although a close-up view of the plectrum region was also scanned in the same preparation. (C)Cross section of the `cog-cricket' system and left tegmen, showing the angle of tooth-plectrum engagement used during experiments. Broken blue line depicts the imaginary tangential line that touches the circumference, formed by the gear-file rotation, at the point of plectrum contact. The broken yellow line represents an imaginary radius perpendicular to the blue tangent.

 

Figure 5
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  		Fig. 5. Capture of the video image from the laser Doppler vibrometer illustrating the plectrum-bearing wing during measurements and the lattice of laser scanning points. (A)Entire tegmen (N,200 points; scanning mesh size, 170µm; dot-positioning accuracy, ca. 1µm), scale bar, 1 mm. (B)High-resolution scan of the plectrum region (N,320 points; scanning mesh size, 85µm; dot positioning accuracy, ca. 1µm), scale bar, 1 mm.

 

Figure 6
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  		Fig. 6. Acoustic features of Gryllus bimaculatus. (A)A single chirp made of four phonatomes. (B)Zero-crossing analysis of the song sequence shown in A, depicting instantaneous frequency. (C)Spectral analysis of the pulses in A.

 

Figure 7
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  		Fig. 7. Comparison between inter-tooth spacing in files of Gryllus bimaculatus and the files used as a gear in our driving system. (A)Mean inter-tooth spacing of five intact specimens; error bars indicate standard deviation. (B)Mean inter-tooth spacing of the two files excised from two teneral males used in our gear-motor system (after bending). Note that mean inter-tooth spacing significantly increases after bending. Broken lines show the excised area of the file (including ~70 teeth) used bent and glued to the plastic-ring's rim. (C)Segment of a bent file used in the `cog-cricket' system. Scale bar, 100 µm.

 

Figure 8
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  		Fig. 8. Microtome section of the left plectrum of G. bimaculatus cut approximately one-third along length of the plectrum active area. Inside picture shows the dorsal surface of the plectrum. Note that the phase-shifting area looks broad in the microtome section as the vein A3 was sectioned through the region crossed by the red broken line. Scale bar, 100 µm.

 

Figure 9
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  		Fig. 9. Pulses produced by the gear-file system on the plectrum of a single specimen of G. bimaculatus and those produced by an intact specimen. (A)A calling song pulse produced by an intact specimen. Note the moderate drop in instantaneous frequency after ~10–12 ms. (B–D) Recordings of sound and vibration produced by plectrum stimulation using a gear-file system (data obtained from a single point on the harp region). (B)Vibration, recorded with a laser Doppler vibrometer. (C)Particle velocity, recorded with a velocity microphone. (D)Sound pressure, recorded with a condenser microphone. Corresponding instantaneous frequencies are shown above each pulse. Note the moderate increase in frequency after ~3 ms. Q, quality factor.

 

Figure 10
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  		Fig. 10. Spectral analysis of the average wing frequency response to plectrum stimulation in 10 specimens (indicated by different colour traces) using the `cog-cricket' motor system. (A)Vibrational response (4.9±0.7 kHz). (B)Particle velocity response (5.2±0.4 kHz). (C)Sound pressure response (5.2±0.4 kHz).

 

Figure 11
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  		Fig. 11. Scanned area and deflection shapes of the tegmen dorsal surface (harp and plectrum) in G. bimaculatus. (A)Orientation image relating tegmen topography (left image) to the position of the scanning lattice (right image). (B)Area scans of tegmen-membrane deflections at ~4.8 kHz (4800 teeth s–1). The deflections are shown each time for four different phases along the oscillation cycle. Deflections are additionally shown as profiles, looking at the tegmen from its anterior aspect. Red indicates positive displacements (or outward membrane deflections) and green indicates negative displacements (or inward membrane deflections).

 

Figure 12
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  		Fig. 12. Phase changes in the plectrum-bearing wing of G. bimaculatus. (A)Mean vector (red trace) calculated by combining each of 14 individual vectors using established methods for circular data. Error bars (in blue) represent standard error or mean. Mean phase shift shown by the red line. (B)Capture of the video image of the left tegmen illustrating the phase angle difference between plectrum area (red) and harp (green). Extrapolated vertical blue broken line associates the critical region where the phase shift occurs between data points and morphology. Horizontal yellow broken line shows the region across which phase was measured. (C)Coherence across the frequency range of the plectrum-bearing wing after gear-file stimulation. Black trace shows a mean of 14 specimens (different colours). Note high coherence levels around fc (~4.5–5.0 kHz).

 

Figure 13
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  		Fig. 13. The mechanical phase shifter in the plectrum of G. bimaculatus. (A) High resolution scan showing the critical area where the phase shift occurs. Red indicates positive phases (outward deflections) and green indicates negative phases (inward deflections). (B)SEM of the plectrum area shown in A, highlighting the critical region: a rigid vein (possibly Anal vein 3 serves as a mechanical phase shifter of the vibrations travelling from the plectrum to the harp. Scale bars, 0.5 mm.

 

Figure 14
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  		Fig. 14. Envelopes of mechanical deflections across plectrum and harp membranes in a specimen of G. bimaculatus along indicated transect (broken red line). The plectrum was stimulated with the `cog-cricket' system at a rate of 4.8 teeth s–1. (A)The position along the transect lines is given with a calibrated scale (broken line, 6 mm). This scale corresponds to the x axis in B and C. (B)Deflection envelopes constructed by displaying the instantaneous deflection velocities along the transect for a series of phases (in 20 deg. increments) along the full oscillation; the laser displacement relates to sound velocity. (C)Same as B but the cycle reference used was the Brüel & Kjaer condenser microphone, laser displacement relating to sound pressure.

 

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© The Company of Biologists Ltd 2009