Moths’ Extraordinarily Sophisticated Wing Design

Moths strike in the evolutionary arms race with sophisticated wing designs

The moth’s wings are ultra-thin, super absorbent, and exceptionally designed to distract attention. They could be the key to developing technological solutions to survive in a noisy world.

As indicated in a new study published today in PNAS, Researchers from the Bristol University discovered the precise construction of moth wings that enabled the species to evade its most pesky predator in a 65 million year old 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 layer on moth’s wings has developed exceptional ultrasound absorbing properties that allow acoustic camouflage in stealth against echolocating bats.

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

Earlier this year, the expert in behavioral acoustics and sensory ecology, Dr. Marc Holderied, and his co-researchers, how deaf moths developed ultrasound-absorbing scales on their bodies that enabled them to absorb 85 percent of the incoming sound energy that bats use to recognize them.

The need for survival meant that moths developed a 1.5mm deep protective barrier that acts as a porous sound absorber. However, such a protective barrier would not work on the wings, where the increased thickness would affect the moth’s ability to fly. A key characteristic of acoustic metamaterials is that they are much smaller than the sound wavelength they act on, making them much thinner than conventionally constructed sound absorbers.

In this latest study, the Bristol team, led by two lead authors, Dr. Thomas Neil and Dr. Zhiyuan Shen stated that moths took a life-saving step further, creating a resonance absorber that is 100 times thinner than the wavelength of the sound it absorbs, allowing the insects to maintain their lightness while reducing the potential for bats to recognize the echo of their wings in flight.

When examining the sophisticated cross-sectional images of sound captured using ultrasound tomography, the team found that moth wings have evolved into a resonance absorber that provides effective protection against echo-localizing bats. The results could greatly aid the efforts of materials scientists, acousticians and sonar engineers to develop bio-inspired sound absorbers with exceptional performance at deep sub-wavelengths.

“Most amazingly, moth wings have also devised a way to get a resonance absorber to absorb all of the bat frequencies by adding another amazing feature – they build many of these resonators, individually tuned to different frequencies, into a range of Absorbers, which together generate broadband absorption as an acoustic metamaterial – the first known in nature, ”said the lead researcher Dr. Holderied. “Such broadband absorption is very difficult to achieve in the ultra-thin structures of the moth’s wings, which is what makes it so remarkable.”

This goes well beyond the limits that can be achieved with classic porous absorbers such as those currently used for sound absorption in office environments where large, thick materials are used.

Dr. Holderied added, “The Promise is one of the much thinner mufflers for our homes and offices. We’d be approaching a much more versatile and acceptable muffler wallpaper than bulky absorber panels.”

This latest study builds on the team’s earlier work on acousto-mechanics of individual scales and shows how the more traditional sound absorption by scales on moth bodies with much thinner structures on wings can be achieved to ensure acoustic protection of the entire organism.

Reference: “Moth wings are acoustic metamaterials” by Thomas R. Neil, Zhiyuan Shen, Daniel Robert, Bruce W. Drinkwater and Marc W. Holderied, November 23, 2020, Procedure of the National Academy of Sciences.

This study was supported by the Research Council for Biotechnology and Life Sciences (BBSRC, BB / N009991 / 1), the Research Council for Engineering and Physical Sciences (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 more than 68,000 views, the paper – moth thoracic scales as a stealth coating against bat biosonar – is the most downloaded paper on Royal Society Interface magazine.