By RICK BORCHELT

A newly emerged male luna moth waits for dusk to run the bat gauntlet in search of a mate.
Credit: Rick Borchelt

In the darkness of a humid summer’s night, there’s an arms race going on: It’s bats versus moths, and right now the moths are winning.

At least, luna moths are winning.

Luna moths (Actias luna) are large — as big as your spread hand — bright green and best known for their extravagantly long tails, several times the length of their bodies. Lunas belong to a worldwide group of giant silk moths known as moon moths. Our luna moth has modest tails by comparison to other members of the moon moth clan.

Luna moths are nocturnal, for the most part, with very poor eyesight, so it’s unlikely the tails are purely ornamental or attractive to potential mates. In fact, luna moth males find their females by scent rather than sight.  

The tips of luna moths’ streaming tails are cupped so that they spin during flight.
Credit: Courtesy of Judy Gallagher

Luna moths are also rather haphazard flyers, bobbing and weaving and fluttering rather than powering through fields and forests, using their tails as rudders to steer their flight. 

Why then, scientists wondered, do all the moon moths have those long, trailing tails on their hindwings?

Turns out, it’s an elaborate acoustic ruse. 

Bats and moths have been embroiled in an evolutionary game of one-upmanship for eons. Bats that feed on flying moths find their prey through echolocation, emitting rapid-fire, high-frequency squeaks that bounce off of a moth in the air, get picked up again by the bat’s sensitive hearing, and provide a remarkably accurate auditory “picture” of where the prey item is, how it’s moving and how best to grab it. 

Some moths are able to hear bat signals and evade capture with a characteristic drop-and-roll maneuver when the bat closes in. Other moths have developed their own faux bat squeaks or other sounds to confuse the bats’ sensitive echolocation abilities. And still others broadcast using their own sonar click language to signal to bat predators that the moth tastes bad should be left alone.

But moon moths are unique in having evolved false targets for bats to aim at.  

The luna moth’s twisted tails twirl in such a way as to fool the bats’ biosonar, creating the impression of a large, meaty target at the end of the tail, rather than where the moth’s actual body is. So, when bats ping their prey with squeaks, and attack, all they get is a mouthful of wing scales or, at best, a piece of hindwing while the moth flutters away otherwise unharmed. (Watch bats being fooled by the twirling hindwings of luna moths at tinyurl.com/m872jm98.)

Other members of the moon moth tribe have long, streaming tails as well, sometimes much longer than luna moths’.
Courtesy of Beitrage zur Schmetterlingskunde, public domain.

Most of the world’s moon moths are also in genus Actias, a Latinized version of Actaeus, the mythological first king of Attica and father-in-law of Cecrops, the serpent-king of Athens. Cecrops lends his Latin name to another of our local giant silk moths, the cecropia moth. Most of the world’s 60 Actias moon moths are tropical and get their common name from the translucent windows or faux eyes on the wings that resemble full or crescent moons. 

The European equivalent of our North American luna moth is the Spanish moon moth (Graellsia isabellae), a species found high in the Pyrenees where it feeds on pine trees. It has survived in its pine forest habitat since the Ice Age, when it sought refuge among the pines as the world grew colder. But the mountain refugia the Spanish moon moths found then have changed little in the millennia since the great ice sheets retreated, and the moths have survived here ever since. Whether they will continue to do so in a warming world is unclear. 

The Madagascan comet moth (Argema mittrei) takes the deception one step further. The streaming hindwings of this species are the longest of any species in the moon moth group, sometimes as long as eight inches. In addition to luring bats into striking at the end of the long tail, comet moths have scales on their main wings that absorb and dampen the biosonar pings, making the moth’s true body almost invisible to bats. 

Male comet moths are the ones who fly widely in search of mates; females are more sedentary, since they’re already near the host plants they grew up on and can simply lay their eggs there once being fertilized. To compensate for the greater travel risks he takes, the male’s tails are about twice as long as the female’s. 

Luna moths are actually well camouflaged unless they settle down against a flat tree trunk.
Courtesy of Rick Borchelt

All this deception might work at night with bats, but what about during the day? One could think that these long tails would stand out in the environment during daylight and make luna moths easy pickings for the birds, lizards and mice that like nothing better than a luna lunch. But it turns out the tails are actually pretty good camouflage, too, breaking up the typical moth silhouette and making it surprisingly hard to see the luna moth at rest. 

Watch for adult luna moths throughout the spring and summer at porch or street lights — they are inordinately attracted to artificial lights. And be sure to leave your autumn leaves on the ground instead of raking them up: Unlike our other giant silk moths, the caterpillars of luna moths spin their cocoons on the ground in a blanket of leaves rather than on a shrub or tree branch.  

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Rick Borchelt is a botanist and science writer who gardens and writes about natural history at his home in College Park. Reach him with questions about this column at rborchelt@gmail.com.