Why Do Luna Moths Have Such Absurdly Long Tails?

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 eyespots on 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.

That, according to Barber, is what the luna moth’s tails do. They are “auditory deflectors”. Bat distractors.

Barber pitted luna moths against bats in a dark room, and filmed their encounters with infrared cameras. Under normal circumstances, the bats only managed to snag 35 percent of the moths. But if Barber cut off the insects’ tails beforehand, the bats caught 81 percent of them. That’s not because they become worse fliers—in fact, the tails don’t seem to affect their aerial abilities at all.

When bats aim their sonar at insects, they analyse the rebounding echoes for the distinctive signatures of beating wings. But the luna moths tails, which spin behind them as they fly, also produce echoes that resemble wingbeats. To the bat, they either sound like a very conspicuous part of their target, or like a different target entirely. As a result, they fumble their attacks.

When bats attack, they usually use their wings and tail to scoop an insect towards their faces, so they can deliver a killing bite to their victim’s body. But when bats attack luna moths, they aim about half their attacks at the tails. That’s a mistake—only 4 percent of those attacks succeed. Sometimes, the bat misses the moth entirely (see above). Other times, it bites off a tail while the moth escapes—down one inessential body part, and still alive (see below).

The tails also make the luna moths bigger, which might make them harder for the bats to handle and dispatch. But when Barber pitted bats against the polyphemus moth—an even bigger species that lacks tails—he saw that the predators killed 66 percent of their targets. The luna moths, despite being smaller, were harder to catch. “Clearly, tails provide an anti-bat advantage beyond increased size alone,” Barber wrote.

It’s possible that female moths also judge the health and quality of a male by looking at the size of his tails. But this doesn’t fit with the moths’ behaviour. Female moths spend most of their time hiding in protected nests and drawing males to them by releasing pheromones. They also mate with the first males they find, so there’s no evidence that they’re choosy—much less that they choose on the basis of tail length.

Luna moths belong to a group of large moths called the saturniids—a group that contains members like Copiopteryx and Eudaimonia, with even more extreme tails. By comparing the tail lengths of 113 saturniid species, Barber showed that these moths have evolved long tails on at least four separate occasions. He now wants to know if these other species are also good at foiling bats.

Reference: Barber, Leavell, Keener, Breinhoff, Chadwell, McClure, Hill & Kawahara. 2015. Moth tails divert bat attack: Evolution of acoustic deflection. PNAS http://dx.doi.org/10.1073/pnas.1421926112