The twisted tail of luna moths' acoustic camouflage

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Luna moths, which are common to the central and eastern US, feature a long twisted tail(Credit: Andy Reago & Chrissy McClarren/Flickr)

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Camouflage is common in the animal kingdom, but blending in with the patterns, colors and textures of their surroundings won't help prey evade predators that hunt using echolocation. Researchers at the University of Washington and John Hopkins University have studied how luna moths have evolved twisted tails as a form of acoustic camouflage that bounces sound waves in all directions to keep bats from homing in on them.

With no tail, the echoes bouncing off a moth in flight give a bat a bullseye to aim for. But the researchers suspect that the twisted tail of the luna moth reflects the sound waves in all directions, making it much harder for a predator to get a clear lock on its target.

"The interesting thing about these tails is they are not just extensions – there is a twist toward the end," says Wu-Jung Lee, first author of the study. "We think that twist could be a key for how the tails function acoustically."

To test the theory, the researchers tethered luna moths in place, and pinged them with high-frequency sound waves designed to mimic the ultrasonic pulses of bats. Each of these chirps of sound was only three milliseconds long and beyond the range of human hearing. A microphone, placed 2 m (6.6 ft) from the moth, recorded how the sound waves bounced back, while video cameras captured the moth in flight visually.

The researchers analyzed the echoes and the video footage to learn about the role the tail's twists play in messing up the moth's echo cloud. That ball of returning sound varies depending on how the moth's wings, mid-flight, are positioned when the chirp strikes it: if they're running perpendicular to the incoming sound, a lot more will bounce back than if they're parallel, where most of the sound simply passes by. A bat in pursuit might take the average of those varying echoes and shoot for the center.

"A moth is a very complicated object in space," Lee says. "It could be difficult for a bat to track each individual point of the echo cloud. It would be much easier for it to say, 'There's a ball of echoes coming back, I'm going to hit the center of it, and maybe I'll catch something'."

But the moth's tail throws a spanner in that plan. With its twists and turns, an echo will be returned no matter which angle the sound hits, shifting the echo cloud away from the body of the moth. The study found that 53 percent of the time, the echo center fell beyond the insect's abdomen, meaning a pursuer would be much more likely to miss the target.

The acoustic images returned were sharpened to improve the resolution, creating an approximation of how the insect may appear to a bat on the prowl. But because the type of signal processing being used by the bats is unknown, Lee says the images may not be completely accurate.

However, the results support a parallel study, which found that bats are 47 percent more successful at hunting luna moths that have lost their tails. How the tails influence flight, and whether different tail structures afford other moth species the same kind of defenses, will be the subject of future research.

"This study provides part of the clue, but we don't have the full answer yet," says Lee.

The research was published in the Journal of the Acoustical Society of America.

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