The results of a fascinating new study by researchers from the School of Life Sciences are published today (17th August 2017) in the Journal of Experimental Biology.
This Leverhulme-funded research, carried out by Dr Fernando Montealegre-Z, Dr Thorin Jonsson and Benedict Chivers, investigates a unique form of amplification in the sound generation system of a tropical bush cricket.
All bush crickets generate their acoustic mating calls through tegminal stridulation: the rubbing together of their specially modified forewings. A scraper (also known as the plectrum) is passed along a series of hardened teeth with the subsequent vibrations being amplified by specialised wing cells which act similarly to speaker membranes. However, the species in this study (the Neotropical bush cricket Acanthacara acuta) utilises an extraordinary bodily structure to further amplify its call through resonance.
Unlike other bush crickets, Acanthacara acuta has evolved an extremely inflated pronotum (the hard plate-like region behind the head) which is ballooned towards the rear of the animal forming a chamber around the entirety of the wings. It has been suggested that this pronotal chamber acts as a Helmholtz resonator to amplify the signals being produced by the wings within.
By using state of the art computer modelling techniques with micro-CT scanning and laser Doppler vibrometry, the research has revealed that this pronotal chamber is indeed functioning as a Helmholtz resonator which, critically, resonates at the frequency of the call produced by the wings. This matching of frequencies between the wings and the pronotal chamber results in a resonant system which greatly amplifies the produced call.
To further clarify the role of the pronotal chamber, 3D prints of the animals were used under the same experimental conditions and yielded the same results, indicating that it is the size of the chamber which causes the resonance, and not the material properties of the chamber walls. Although, a further study of the removed chamber revealed that the chitinous material of the chamber itself also provides resonance at the calling song frequency.
This means that the song system of A. acuta actually consists of three separate resonators, the wings, the Helmholtz resonator formed by the pronotal chamber and the chamber material itself.
Acoustic calls are extremely important in the mating systems of these insects and A. acuta has evolved an elaborate morphological amplifier to enhance their own signals’ effectiveness. This amplification not only makes the act of singing more efficient, but is also greatly beneficial to animals who communicate with sound in acoustically noisy environments, such as the Ecuadorian rainforests in which these animals live.
The full paper is available to view online: http://jeb.biologists.org/content/220/16/2900
(Picture caption: 3D reconstruction of an A. acuta male with the pronotal chamber showing in red and the wings inside the chamber in blue.)