Ed Yong in Not Exactly Rocket Science:
You don’t need a field guide to recognise a luna moth. This large insect, found throughout the eastern half of North America, is unmistakeable. It has a fuzzy white body, red legs, feathery yellow antennae, and huge lime-green wings that can stretch up to 4.5 inches across. And at the end of its hindwings are a pair of long, streaming tails that can double the moth’s length.
In 1903, an entomologist named Archibald Weeks suggested that the tails direct predators away from the moth’s body. “Again and again may predator bat or bird, in an effort to capture a moth or butterfly, successively tear away sections of the tails, of which a sacrifice can be readily afforded, without disabling it or retarding its flight,” he wrote.
He was roughly right. More than a century on, Jesse Barber from Boise State University has shown that the luna moth’s tails are the equivalent of eyespotson fish and butterflies. These distinctive markings are typically found on dispensable body parts like tails and outer wings. They serve to draw a predator’s attention away from more vulnerable regions; better to lose a tail than a head.
Eyespots are visual defences, and bats—the main nemeses of moths—are not visual hunters. They find their prey with sonar—they make high-pitched squeaks and visualise the world using the rebounding echoes. To divert a bat, you need something that makes distracting echoes.