2105
INTRODUCTION
Male field crickets (Orthoptera: Gryllidae) conventionally generate
mating calls by rubbing together specialized regions of the forewings
or tegmina (e.g. Ewing, 1989). A vein in both forewings is ventrally
modified with a series of hard pegs that form a stridulatory file,
while the anal wing region harbours a plectrum on its medial side.
As the plectrum of one wing is swept across the file of the opposite
wing, a series of impacts occur, generating vibrations of the
surrounding wing membranes (Pierce, 1948).
During singing, the two forewings open and close simultaneously,
yet most of the acoustic energy is produced during the closing stroke
(Walker et al., 1970; Elliott and Koch, 1985; Koch et al., 1988;
Bennet-Clark, 1989). Even though the file and the plectrum are
featured on both wings, in crickets it is usually the animal’s right
wing (RW) that lies on top of the left wing (LW). Thus, during
stridulation, the plectrum of the LW contacts teeth on the ventral
side of the RW (Elliott and Koch, 1985) (see also supplementary
material Movie 1). Sound production by tegminal stridulation is
therefore functionally asymmetrical, with the two wings having
different functions (Forrest, 1987).
The main sound radiator in the forewings of crickets is a
specialized region known as the ‘harp’ (Fig. 1) (Bennet-Clark, 1970;
Nocke, 1971; Michelsen and Nocke, 1974; Bennet-Clark, 1999a;
Bennet-Clark, 2003), although other wing cells (i.e. wing regions)
have also been attributed a role in the overall resonant behaviour
of the wing (Bennet-Clark, 2003).
The carrier frequency (f
c
) of the calling song of most cricket
species is highly tonal (Leroy, 1966; Otte, 1992; Walker and
Moore, 2002). This observed acoustic purity is explained by an
escapement-like mechanism, analogous to the escapement of
clocks (Elliott and Koch, 1985; Koch et al., 1988). In this model,
the vibration of the wing cells (at their resonant frequency, f
o
)
controls the catch and release of the plectrum from tooth to tooth
in the file (Elliott and Koch, 1985; Koch et al., 1988; Prestwich
et al., 2000; Bennet-Clark and Bailey, 2002). In crickets and mole
crickets, one file sweep creates a single sound pulse or syllable.
A song pulse is thus made of sequential tooth strikes sustained
at a nearly constant rate and the catch and release of the plectrum
from every file–tooth pair is produced at a more or less constant
rate by the up and down vibration of the right harp and file at
their f
o
(Bennet-Clark and Bailey, 2002).
It has been shown, however, that cricket wings do not operate
as perfect clocks, as most species exhibit frequency modulation (FM)
in the pulses of their calls, which is observed as a fall in frequency
or ‘glissando’ within the song pulse of some 10–15% of the main
carrier frequency (Leroy, 1966; Koch et al., 1988; Simmons and
Ritchie, 1996; Prestwich et al., 2000; Bennet-Clark and Bailey, 2002;
Bennet-Clark, 2003). This problem was first pointed out by Leroy
(Leroy, 1966), but was revived by Simmons and Ritchie (Simmons
and Ritchie, 1996), who were the first to try to find a morphological
explanation for the glissando. Bennet-Clark (Bennet-Clark, 2003),
working on Teleogryllus oceanicus, suggested that this drop in
The Journal of Experimental Biology 214, 2105-2117
© 2011. Published by The Company of Biologists Ltd
doi:10.1242/jeb.056283
RESEARCH ARTICLE
Sound radiation and wing mechanics in stridulating field crickets
(Orthoptera: Gryllidae)
Fernando Montealegre-Z*, Thorin Jonsson and Daniel Robert
School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK
*Author for correspondence (bzfmz@bristol.ac.uk)
Accepted 8 March 2011
SUMMARY
Male field crickets emit pure-tone mating calls by rubbing their wings together. Acoustic radiation is produced by rapid
oscillations of the wings, as the right wing (RW), bearing a file, is swept across the plectrum borne on the left wing (LW). Earlier
work found the natural resonant frequency (f
o
) of individual wings to be different, but there is no consensus on the origin of these
differences. Previous studies suggested that the frequency along the song pulse is controlled independently by each wing. It has
also been argued that the stridulatory file has a variable f
o
and that the frequency modulation observed in most species is
associated with this variability. To test these two hypotheses, a method was developed for the non-contact measurement of wing
vibrations during singing in actively stridulating Gryllus bimaculatus. Using focal microinjection of the neuroactivator eserine into
the cricket’s brain to elicit stridulation and micro-scanning laser Doppler vibrometry, we monitored wing vibration in actively
singing insects. The results show significantly lower f
o
in LWs compared with RWs, with the LW f
o
being identical to the sound
carrier frequency (N44). But during stridulation, the two wings resonate at one identical frequency, the song carrier frequency,
with the LW dominating in amplitude response. These measurements also demonstrate that the stridulatory file is a constant
resonator, as no variation was observed in f
o
along the file during sound radiation. Our findings show that, as they engage in
stridulation, cricket wings work as coupled oscillators that together control the mechanical oscillations generating the remarkably
pure species-specific song.
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/214/12/2105/DC1
Key words: neuroactive substances, microinjection, stridulation, resonance, acoustic radiation, laser vibrometry.
THEJOURNALOFEXPERIMENTALBIOLOGY