Extraordinarily sophisticated wing design by Moths

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). Credit: Image courtesy of Simon Reichel, Thomas Neil, Zhiyuan Shen, and Marc Holderied

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

Ultra-thin, super absorbent, and extraordinarily designed to distract attention, moth wings could be the key to developing technological solutions to survive in a noisy world.

As revealed in a new study published today in PNAS, researchers of 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.

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. Credit: Image courtesy of Marc Holderied and Thomas Neil

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.

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). Credit: Image courtesy of Simon Reichel, Thomas Neil, Zhiyuan Shen, and Marc Holderied

In this latest study, the Bristol team, led by early authors Dr Thomas Neil and Dr Zhiyuan Shen, reveal that moths have gone one step further, creating a resonance absorber that is 100 times thinner. the wavelength of the sound absorbent, thus allowing the insects to maintain their lightness while reducing the possibility that the bats 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.

Reference: “Moth Wings Are Acoustic Metamaterials” by Thomas R. Neil, Zhiyuan Shen, Daniel Robert, Bruce W. Drinkwater and Marc W. Holderied, November 23, 2020, Proceedings of the National Academy of Sciences.

This study was supported by the Biotechnology and Biological Sciences Research Council (BBSRC, BB / N009991 / 1), the Engineering and Physical Sciences Research Council (EPSRC, EP / T002654 / 1) and the Diamond light source (MT17616).

These latest findings build on previous studies by the Bristol team published in the Journal of the Royal Society in February 2020. With over 68,000 views, the paper – Thoracic Moth Scales as Invisible Coating Against Bat Biosonar – is the most downloaded paper on the Royal Society Interface magazine.