Nanostructured scales give moths a stealth coating

The cabbage tree emperor moth (Bunaea alcinoe) would be an ideal meal for bats. The palm-sized insect is large, nontoxic, and deaf, so it cannot hear an approaching predator. But the moth is not without its defenses. Scientists suspected the critter used some kind of acoustic camouflage. Now they have learned the moths hide from their hunters’ echolocation thanks to nanostructured scales on their wings. The discovery could guide the creation of ultrathin sound-absorbing materials.

“You can think of this as the same approach stealth airplanes use against radar,” says Marc W. Holderied, a University of Bristol biologist who led the research effort with colleague Zhiyuan Shen. “You absorb some of the energy that hits you rather than reflecting it and thereby you look smaller than you actually are,” Holderied says. “Smaller means you cannot be detected over that great a distance.”

Holderied, Shen, and colleagues examined a single scale from the moth’s wing and found that it resonates at three different frequencies spanning the range bats use for echolocation. They then created a computer model of the scale’s multilayered, porous, nanoscale structure. When they filled one layer of the pores in the model, the resonating frequencies shifted away from the bats’ echolocation range, indicating the porous structure was key (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1810025115).

Bodo Wilts, an expert in bio-inspired materials at the University of Fribourg, says the Bristol team’s work offers “yet another example of how nature outperforms current applications.” The Bristol group is trying to make stealth materials that mimic the moth scales, using nanostructures to tailor sound absorption.

Ping Sheng, a nanoscience expert at Hong Kong University of Science & Technology adds, the “ultrasound absorption at precisely the bats’ ultrasound frequency is, in my opinion, a definitive signature of evolution at work.”