Moths strike in the evolutionary arms race with a sophisticated wing design

Composite image of the moth Antheraea pernyi (top) and the butterfly Graphium agamemnon (bottom) showing photographs on the left and ultrasound (tomography) on the right. Notice how moth wings have weaker echoes (acoustic image) than butterfly wings.
Marc Holderied and Thomas Neil

False color 3D representation of a 0.21 mm x 0.28 mm wing section of the Lasiocampa quercus moth showing the structure, diversity and arrangement of base (orange) and cover scales (blue and yellow).

Simon Reichel, Thomas Neil, Zhiyuan Shen and Marc Holderied

As revealed in a new study published today in PNAS [date tbc], researchers at the University of Bristol discovered the precise construction of the moth wings that allowed the species to evade its most problematic predator in a 65-million-year evolutionary arms race.

Using a range of analytical techniques, including aerial cross-sectional imaging, acoustic mechanics, and refractometry, the team at the School of Biological Sciences in Bristol found that the very thin scaling layer on the moth wings developed extraordinary ultrasonic absorption properties. which provide invisible acoustic camouflage against bats’ echolocation.

What makes the team’s discovery even more extraordinary is that they have identified the first known natural acoustic metamaterial. A metamaterial traditionally describes an artificial composite material designed to exhibit physical properties that exceed those available in nature. Naturally occurring metamaterials are extremely rare and had never previously been described in the world of acoustics.

Earlier this year, behavioral acoustics and sensory ecology expert Dr. Marc Holderied and his co-researchers reported how deaf moths developed ultrasound absorption scales on their bodies that allowed them to absorb 85% of the incoming sound energy that bats use to detect them.

The need to survive has caused the moths to develop a protective barrier 1.5 mm deep that acts as a porous sound absorber. However, such a protective barrier would not work on the wings, where the increase in thickness would hamper the flight ability of the moths. A key feature of acoustic metamaterials is that they are much smaller than the wavelength of the sound they are acting on, allowing them to be much thinner than traditionally built acoustic absorbers.

In this latest study, the Bristol team, led by early authors Dr Thomas Neil and Dr Zhiyuan Shen, reveals that the moths have gone a step further, creating a resonant absorber that is 100 times thinner than the wavelength of the sound. it absorbs, thus allowing insects to maintain their lightness while reducing the ability for bats to detect the echoes of their wings in flight.

By examining sophisticated cross-sectional images of sound captured using ultrasound tomography, the team found that moth wings have evolved to create a resonant absorber that is effective protection against echolocated bats. The findings could significantly strengthen the efforts of materials scientists, acoustics and sonar engineers to design bio-inspired acoustic absorbers with exceptional deep wavelength performance.

“Surprisingly, moth wings have also developed a way to have a resonant absorber absorb all the bat frequencies, adding another amazing feature: they assemble many of these resonators individually tuned to different frequencies into a series of absorbers. which together create broadband absorption by acting as an acoustic metamaterial, the first known in nature, “said lead researcher Dr Holderied. “Such broadband absorption is very difficult to achieve in the ultrathin structures of the moth wings, which is what makes it so extraordinary.”

This goes far beyond the limits achievable with classic porous absorbers of the type currently used to absorb sound in office environments that use large and thick materials.

Dr Holderied added: “The promise is one of much thinner sound absorbers for our homes and offices, we would be approaching a much more versatile and acceptable sound absorbing ‘wallpaper’ rather than bulky sound absorbing panels.”

This latest study builds on the team’s previous work on individual stair acustomechanics and shows how the more traditional sound absorption by stairs on moth bodies can be achieved with much thinner structures on the wings to provide acoustic protection for the whole. body.