How moths evolved a kind of stealth jet technology to sneak past bats

Moth wings have special scales that absorb sonar emitted by hunting bats

Moth wings have special scales that absorb sonar emitted by hunting bats

The cabbage tree emperor moth (Thomas Neil)
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A species of silk moth has evolved special sound absorbing scales on its wings to absorb the sonar pulses from hunting bats. This is analogous to the special coatings on stealth aircraft that allow them to be nearly invisible to radar.

"It's a battle out there every night, insects flying for their lives trying to avoid becoming a bat's next dinner," said Dr. Marc Holderied, the senior author on the paper and an associate professor in the School of Biological Sciences at the University of Bristol.

"If you manage to absorb some of these sound energies, it would make you look smaller and let you be detectable over a shorter distance because echoe isn't strong enough outside the detection bubble."

Many moths have ears that warn them when a bat is nearby. But not the big and juicy cabbage tree emperor moths which would ordinarily make the perfect meal for bats.

Holderied wanted to know what mechanism they employ to defend themselves against their hunters.

Stealthy wings

He discovered that the silk moth has evolved a thin layer of specialized scales covering its forewings that absorb sound instead of reflecting it back to the sound source.

Normally, sound absorbers have to be thick and porous to cut out echoes, Holderied explained.  

3D nanoCT image of a single wing scale of the cabbage tree emperor moth (Marc Holderied & Zhiyuan Shen)

The thick fur on the cabbage tree emperor moth's body makes for a perfect sound absorber, but to achieve the same thing on the wing, it would need 3 millimeters of fur, which would be too heavy for flight.

The moths, however, have developed the perfect solution to this problem by evolving elaborate scales on their wings that act as a "resonant sound absorber."  These scales are much less bulky than a traditional sound absorber.

The scales have moveable surfaces, so when a bat's ultrasonic waves hit them, the surfaces will resonate, absorbing the sound energy and converting it into kinetic energy.

Perfect tuning

The researchers found three resonant frequencies in the moth's wings that matched the entire frequency range that bats use for echolocation.

"It really looks like evolution was at work to tune these tiny resonators to exactly the frequency they would need to be at," said Holdereid.

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