Small Wings, Loud Songs

Crickets and bush-crickets use their wings to produce a wide range of often impressively loud courtship songs to attract distant mating partners. These communication signals are different for each species and highly variable in rhythm (the temporal structure) and pitch. Many species also produce like bats ultrasonic songs far beyond the human hearing range. Generally, songs are produced by rubbing a hardened edge on one forewing against a row of fine teeth on the underside of the opposite forewing, a process called stridulation. The vibrations produced during stridulation are amplified by specialized structures within the wing surface that are, like the body of a violin or guitar, adapted to resonate at the specific song frequencies. The wings have evolved in time to be small, lightweight and optimised resonators, therefore equipping these small insects with their own miniaturised, natural loudspeakers. When observing singing crickets and bush-crickets, one question springs to mind: How can these insects, using only their small wings, produce such loud songs? Or, more specifically: which morphological structures, biomechanical processes and material properties influence and ultimately define the acoustic properties the tuning of the sound producing wings in crickets and bush-crickets? To answer these questions, state-of-the-art bioacoustic and biomechanical methods (including lasers for the detection of minute surface vibrations, artificial intelligence for detailed wing motion analysis and nanomechanical exploration of material properties) will be used to study how the structure of a wing relates to the songs it produces. Knowledge gained during the project will be used to create three- dimensional and mathematical wing models based on real-life insect wings that will allow to simulate the stridulation process and the songs thereby created. Artificial manipulation of these computational models and their corresponding acoustic output will help to gain further knowledge of the mechanical potential and constraints within these miniature loudspeakers. Combining the study of real-life insect wings with the mechanical and acoustic insights gained from the virtual models has the implicit potential to help biologists to understand the processes shaping the acoustic evolution of insects and to guide engineers in the development of innovative, biomimetic miniature loudspeakers for, e.g. hearing aids.

Crickets and bush-crickets use their wings to produce a wide range of often impressively loud courtship songs to attract distant mating partners. These songs are different for each species, highly variable in rhythm and in frequency (pitch). Many species also produce - like bats - ultrasonic songs far beyond the human hearing range. Songs are produced by rubbing a hardened edge on one forewing against a row of fine teeth on the underside of the opposite forewing (called stridulation). The vibrations produced are amplified by specialized wing regions that are, like the body of a violin, adapted to resonate at specific song frequencies. These wings have evolved to be small, lightweight and optimised resonators, therefore equipping these small insects with their own miniaturised, natural loudspeakers. Observing these insects singing, one question springs to mind: How can they produce such varied and loud songs using only their small wings? In this project, we used state-of-the-art bioacoustic and biomechanical methods (including lasers for recording minute surface vibrations, high-speed video recordings, micro-computed tomography and computer simulations) to study these wings and to find out how the insects' songs are affected by wing geometry and morphology. As a result, we e.g. showed how an African cricket (called Pseudolebinthus gorochovi) evolved to produce loud high-frequency songs in the low ultra-sound. Such songs are highly unusual for crickets which normally sing at much lower pitches. Pseudolebinthus did not have to change its stridulation behaviour or drastically alter its wing morphology; the evolutionary thickening of a single wing vein was enough to shift the wings' vibration behaviour, so they preferentially radiated the high-frequency components of the song which are normally strongly suppressed in other crickets. This process is called harmonic hopping, and we found other cricket species performing similar frequency shifts using different morphological or behavioural mechanisms, a form of convergent evolution in song production. The results of our studies on insect wings and the knowledge gained from being able to simulate the physical vibrations of artificial wings using computer models was also used to recreate the acoustic world of a Jurassic forest. Here, we used exceptionally well-preserved wings of nine fossil bush-cricket species that lived together in the same habitat in Inner Mongolia approximately 165 million years ago. By analysing the song production organs and the shape of the wings and comparing them with data from modern species, we found that these animals already produced a variety of loud songs of different pitches, ranging from low frequencies to the ultrasound. Therefore, we could show for the first time that ultrasound communication already evolved in an acoustically diverse world in the Middle Jurassic, more than 100 million years before ultrasound communication in bats evolved.