Changing resonator geometry to boost sound power decouples size and song frequency in a small insect

Abstract

Despite their small size, some insects, such as crickets, can produce high amplitude mating songs by rubbing their wings together. By exploiting structural resonance for sound radiation, crickets broadcast species-specific songs at a sharply tuned frequency. Such songs enhance the range of signal transmission, contain information about the signaler's quality, and allow mate choice. The production of pure tones requires elaborate structural mechanisms that control and sustain resonance at the species-specific frequency. Tree crickets differ sharply from this scheme. Although they use a resonant system to produce sound, tree crickets can produce high amplitude songs at different frequencies, varying by as much as an octave. Based on an investigation of the driving mechanism and the resonant system, using laser Doppler vibrometry and finite element modeling, we show that it is the distinctive geometry of the crickets' forewings (the resonant system) that is responsible for their capacity to vary frequency. The long, enlarged wings enable the production of high amplitude songs; however, as a mechanical consequence of the high aspect ratio, the resonant structures have multiple resonant modes that are similar in frequency. The drive produced by the singing apparatus cannot, therefore, be locked to a single frequency, and different resonant modes can easily be engaged, allowing individual males to vary the carrier frequency of their songs. Such flexibility in sound production, decoupling body size and song frequency, has important implications for conventional views of mate choice, and offers inspiration for the design of miniature, multifrequency, resonant acoustic radiators.bioacoustics | biological modelling | biomechanics | finite element analysis M ale crickets produce high amplitude calling songs to attract conspecific females (1, 2). The sounds are produced by stridulation, a rapid and controlled rubbing of forewings against each other. The plectrum, a sclerotized portion on the anal edge of one wing, is drawn across the file, a series of teeth on the underside of a vein, on the other wing (reviewed in ref.3). The stridulatory apparatus acts as a mechanical frequency-multiplying system, converting the slow wing-stroke rate (ca 30 Hz) of the insect into a sound of much higher frequency (e.g., 4.5 kHz in the field cricket Gryllus bimaculatus) (3, 4). Stridulation sets the wing into vibration, and if the frequency produced by the plectrum-file interaction (the tooth strike rate) matches the resonance frequency of the wings a higher amplitude pure-tone sound can be produced (2). The exact biophysical mechanisms enabling such sound radiation using soft structures many times smaller than the sound wavelength remain elusive.The mechanism that crickets use to match stridulatory frequency to resonant frequency is similar to a clockwork-like escapement system (5-7). In the field of horology, escapement mechanisms display different degrees of sophistication (8), one is even called the grasshopper escapement (9). The funda...

Systematics and Acoustics of North AmericanAnaxipha(Gryllidae: Trigonidiinae)

“…When they studied the effect of aspect ratio on the resonances of plates that resembled the shape and structure of the wings of O. henryi, they found that changing the aspect ratio beyond the range of natural wings of O. henryi changed the deflection modes from their more efficient states in the real wings. unlike Sismondo (1979), who concluded that different cells of the forewings became the dominant resonators at different temperatures, Mhatre et al (2012) showed that wing cells did not resonate independently from one another but formed a single wing-shaped plate with different modes of vibration at different temperatures.…”

“…Synchronization of forewing oscillations and file tooth impacts.-How insects as small as crickets can produce sounds that are nearly pure in frequency has long been of interest and talented researchers have studied the problem using increasingly sophisticated physical and electronic methods (e.g., nocke 1971, Sismondo 1979, koch et al 1988, Bennet-Clark 1999, Bennet-Clark & Bailey 2002, Montealegre-Z et al 2009, Mhatre et al 2012, robillard et al 2013. Most of these authors relied on crickets that are easily reared or otherwise conveniently available for laboratory studies and few were concerned with the effects of temperatures on the frequencies produced.…”

“…More than 30 years later, using state-of-the-art technology, Mhatre et al (2012) extensively studied the resonances of the wings of Oecanthus henryi. With laser doppler vibrometry they measured the resonances of the five major vibrating areas of the left and right forewings of five males at 18, 22, and 27°C and reported that they "vibrated maximally and most coherently in response to sound between 2.5 and 4.5 kHz at all temperatures."…”

“…We are grateful to an anonymous reviewer who improved our understanding of Mhatre et al (2012) causing us to revise the section on Synchronization of forewing oscillations and file tooth impacts. For help in accessing specimens at anSP, uMMZ, and nCSu, we thank Jason Weintraub, Mark O'Brien, and Bob Blinn.…”

Active auditory mechanics in female black-horned tree crickets (Oecanthus nigricornis)

“…As male tree crickets produce calls from a dipole that is highly directional (Toms 1984;Forrest 1991 Male O. nigricornis call frequencies are temperature dependent varying by over 1 kHz with changes in temperature across a range of 15 o C (Sismondo 1979). This change is known to be generated by their elongate wing geometry and its physical properties (Mhatre et al 2012). However, whether corresponding changes in the mechanics and neurophysiology of the ear maintain signal-receiver matching has yet to be established in Oecanthus.…”