Acoustical Evolution Increases Battle Between Predator, Prey

Moth wing structure, composition absorb bat echolocation calls.

4-Jun-2021 2:30 PM EDT, by Acoustical Society of America (ASA)favorite_border

Thomas Neil, University of Bristol

Newswise — MELVILLE, N.Y., June 9, 2021 — In the evolutionary battle between hunter and hunted, sound plays an integral part in the success or failure of the hunt. In the case of bats vs. moths, the insects are using acoustics against their winged foes.

During the 180th Meeting of the Acoustical Society of America, which will be held virtually June 8-10, Thomas Neil, from the University of Bristol, will discuss how moth wings have evolved in composition and structure to help them create anti-bat defenses. The session, “Moth wings are acoustic metamaterials,” will take place Wednesday, June 9, at 1 p.m. Eastern U.S.

Nocturnal moths are under intense selection pressure by the bats that hunt them. Some moths have developed a form of acoustic camouflage by evolving structures on their bodies and wings to absorb ultrasound. This decreases the strength of the bat’s echo return and gives the insects a better chance of survival.

“The wings of a moth will produce strong echoes to a hunting bat owing to their large size,” Neil said. “As such, it is important the moth cloaks the wing with sound-absorbing material, so it matches the acoustic camouflage brought about by the fur on the body. The only way to create the much thinner sound absorber allowed on the wings is by developing a resonant absorber, and we discovered moth wings have evolved this approach.”

The scales covering moth wings allow as much as 70% of the sound hitting them to be absorbed. More amazingly, these scales are individually tuned to different frequencies, forming an array of resonant absorbers, which together create broadband absorption by acting as an acoustic metamaterial — the first known in nature.

Neil said the strategies identified in moths have already been partially explored from a theoretical and technical standpoint. The advantages of using a metamaterial design for sound absorption is the absorbers can be much thinner than the wavelength of the sound they absorb. In the case of the moths, their absorbers are 100 times thinner than the wavelength of a bat’s cry.

“In theory, we could take inspiration from the moths and build sound absorbing panels made from lots of differently tuned resonating paddles, with the goal of achieving sound absorption that is on par with traditional sound absorber panels but being just a fraction of the width,” Neil said. “With this approach, we would be getting close to a much more versatile and acceptable sound absorber wallpaper rather than the typically bulky absorber panels we use today.”