Moths use sound-reflecting wingtips to thwart attacking bats

Bats are nocturnal hunters, and though they can see pretty well in the dark, they mostly rely on echolocation to locate prey.

To track down flying insects, bats send out sound wave pulses. By fielding reflected sound waves, bats can discern the location and movement of an elusive target.

According to a new study, published Thursday in the journal Current Biology, some saturniid moths, or silk moths, have evolved folds and ripples along their forewings that help them elude attacking bats.

Previous studies have revealed sound absorbing moth wings, which help insects avoid detection, but the ripples and folds texturing the wingtips of silk moths actually reflect sound rather efficiently.

The silk moth approach to defense is to direct a bat's "acoustic gaze" toward a less vulnerable part of the body. The wingtip, apparently, is nonessential.

In the lab, scientists bounced sound waves off the wings of atlas moths, Attacus atlas, from thousands of angles, recording and comparing the returned echoes. Their experiments showed the forewing folds reflected the strongest sonic signals.

"We have demonstrated that the folded and rippled wingtips on the forewings of some silk moths act as acoustic decoys," corresponding author Marc Holderied said in a press release.

"Structurally, the wingtips act as acoustic retroreflectors, reflecting sound back to its source from numerous angles, meaning a bat would be more likely to strike the wingtip over the more vulnerable body of the moth," said Holderied, a professor of biosciences at the University of Bristol in Britain.

Several silk moth species boast long, twisting hind wings, or tail wings, that similarly produce powerful echoes.

But there are many species that don't have hind wings. Many of these have folds and ripples along their forewings.

"Conclusive support for the idea that the forewing reflector is an acoustic decoy comes from our finding that acoustic forewing decoys always evolved as an alternative to acoustic hindwing decoys, with there being no species known to possess both," said lead author Thomas Neil, a bioacoustics expert and postdoctoral researcher at Bristol.

Next, scientists plan to study the survivability of moths with different wingtip morphologies to determine whether distinct fold and ripple patterns offer advantages.

"The results of this study introduce another exciting aspect to the story of the bat-moths acoustic arms race," Holderied said.

"We have identified a novel form of acoustic defense amongst silk moths which may give them an advantage over hunting bats. Wider implications might include improved man-made anti radar and sonar decoy architectures," Holderied said.