Chamber music – An unusual Helmholtz resonator for song amplification in a Neotropical bush-cricket (Orthoptera, Tettigoniidae)

Abstract

We use laser vibrometry, micro computed tomography and finite element modelling to show that an unusual pronotal inflation covering the wings of a bush-cricket acts as a Helmholtz resonator. AbstractAnimals use sound for communication, with high-amplitude signals being selected for attracting mates or deterring rivals. High amplitudes are attained by employing primary resonators in sound producing structures to amplify the signal (e.g., avian syrinx). Some species actively exploit acoustic properties of natural structures to enhance signal transmission by using these as secondary resonators (e.g., tree-hole frogs). Male bush-crickets produce sound by tegminal stridulation and often use specialised wing areas as primary resonators. Interestingly, Acanthacara acuta, a Neotropical bush-cricket, exhibits an unusual pronotal inflation, forming a chamber covering the wings. It has been suggested that such pronotal chambers enhance amplitude and tuning of the signal by constituting a (secondary) Helmholtz resonator. If true, the intact system -when stimulated sympathetically with broadband sound -should show clear resonance around the song carrier frequency which should be largely independent of pronotum material, and change when the system is destroyed. Using laser Doppler vibrometry on living and preserved specimens, micro computed tomography, 3D printed models, and finite element modelling, we show that the pronotal chamber not only functions as a Helmholtz resonator due to its intact morphology but also resonates at frequencies of the calling song on itself, making song production a three-resonator system.

Morphological determinants of signal carrier frequency in katydids (Orthoptera): a comparative analysis using biophysical evidence of wing vibration

“…For example, in several species not only does the right mirror radiate sound but also the adjacent cells (the neck and the harp, Fig. a,b) exhibit high levels of deformation during resonant vibration (Montealegre‐Z & Postles, ; Chivers et al ., ; Jonsson et al ., ). In other species (e.g.…”

“…Not surprisingly, males of these species tend to produce broadband calling songs (Morris & Pipher, , ; Pipher & Morris, ; Morris et al ., ; Heller, ; Jatho et al ., , ; Morris & Mason, ; Hemp et al ., ; Lemonnier‐Darcemont et al ., ). Even when the left stridulatory cell shows some levels of activity, high levels of mechanical asymmetry between both left and right wings are observed, with maximum amplitude exhibited by the right stridulatory area (Montealegre‐Z & Mason, ; Montealegre‐Z & Postles, ; Sarria‐S et al ., ; Chivers et al ., ; Jonsson et al ., ). This wing asymmetry seems to have been selected as a way to reduce acoustic interference between two wings and favoured the use of ultrasonic frequencies (Montealegre‐Z, ; Gu et al ., ).…”

3D Printing: Applications in evolution and ecology

“…As demonstrated by Policha et al (2016) (Morris & Mason, 1995). By changing the material properties of the chamber (photopolymer resin in the 3D model, instead of insect cuticle), Jonsson et al (2017) showed that the morphology of the structure alone is responsible for the amplification of sound. Surface texture can also be modeled well by 3D printing.…”

“…In the Neotropical buch‐cricket ( Acanthacara acuta ), the sound produced by the wings is amplified by an unusual extension of the pronotum (the dorsal covering of the thorax), which forms a chamber over the wings. To explore the function of this chamber, 3D printed models have been used to replicate the chamber (Jonsson et al ), which is hypothesized to work as a Helmholtz resonator (Morris & Mason, ). By changing the material properties of the chamber (photopolymer resin in the 3D model, instead of insect cuticle), Jonsson et al () showed that the morphology of the structure alone is responsible for the amplification of sound.…”

“…To explore the function of this chamber, 3D printed models have been used to replicate the chamber (Jonsson et al ), which is hypothesized to work as a Helmholtz resonator (Morris & Mason, ). By changing the material properties of the chamber (photopolymer resin in the 3D model, instead of insect cuticle), Jonsson et al () showed that the morphology of the structure alone is responsible for the amplification of sound. Surface texture can also be modeled well by 3D printing.…”