Luna Moth

Last night I spotlighted a freshly-eclosed male luna moth (Actias luna) resting on the edge of a leaf. The majority of saturniid moths I find in the wild have wings tattered to some degree, but this one was pristine, glowing among the branches and leaves. Luna moths are the only North American moth with elongated tails on the hindwings. Early records indicate the tails may have an antipredator function, serving as non-vital body structures that tear off during escape. This is analogous to an arthropod losing a leg, an animal losing its tail, or bizarre cases like a gecko that jumps out of its own skin. Although costly, shedding a body part is a successful strategy to survive a predatory encounter. A leg or limb may be gone, but all is not lost… though in luna moths, the tails cannot regenerate nor autotomize.

Full moon after a penumbral lunar eclipse

Visual deception is common in insects, guiding a predator’s focus to the extremities or employing mimicry to manipulate its identity (e.g., lycaenid hindwings & eyespot patterns). For luna moths, their primary predation pressure doesn’t come from diurnal visual predators. It comes from bats. So, instead of the tails being a visual lure to redirect a predator’s gaze, they are an acoustic lure. The moth’s acoustic flight signature changes to confuse echolocating bats. This is a compelling explanation, but until the past decade, it remained experimentally untested. Through staged bat-moth encounters with manipulated wing shapes, several studies have now demonstrated that moth tails hinder prey localization by bats. Tailed moths were less likely to be caught and to receive a fatal blow than non-tailed moths.

Two years later, a second freshly-eclosed luna moth whose tails almost stretched all the way to the ground. The transparent lime color of luna moths almost seems fluorescent when hit by the green light of the forest.

But as most science goes, the story becomes more complicated. Imagine that a bat generates a sound wave that comes into contact with a moth that is flying in a perpendicular direction. The presence of tails increases the spatial range of detectable echoes. If a bat intercepts prey near the center of the echo signal, this shifts the relative position of the attack site to the moth’s abdomen and hindwings. Taking this a step further, what if a moth is flapping erratically in space? Acoustic snapshots by bats reveal that although tail echoes are present, they are weak in comparison to those produced by the forewings and body. Flight orientation therefore strongly dictates which parts of the moth’s body produce the highest intensity of acoustic cues. Not surprising that sensory perception in nature is dynamic and often unpredictable! Another hypothesis is that the spinning wavy tails themselves may be big enough to produce what is called an “illusion of multiple targets.” Bats would perceive the main body of the moth as one entity, and the tails as smaller alternative targets. Just as schools of fish avoid barracudas or herds of ungulates escape a lion, simultaneous tracking of multiple prey by a predator reduces capture success.

Hindwing eyespots with many color accents, easily overlooked from a distance
Multiple origins of hindwing tails in Saturniidae, from Barber et al. 2015 & Rubin et al. 2018

The evolution of exaggerated tails has occurred two times in silk moths (Saturniidae): in the luna moth tribe (Saturniini), and in the Arsenurine Copiopteryx. Two other recurring phenotypes are large hindwing lobes (e.g. in Rothschildia & the atlas moth) and short pointed tails. Exploring an antipredator function for hindwing tails is fascinating, but because wings function in both flight and crypsis, it will be difficult to tease these two aspects apart.

All photographs taken in situ [1]

Eastern firefly (Photinus pyralis) ‘stars and comets’ lighting up the trees underneath the big dipper. This species has earned the common name “big dipper firefly” due to its typical patrolling behavior of swooping down and lighting up on the ascent. Four stacked images of 30 second exposures.
Zebra swallowtails (Photographium marcellus) have the longest tails of any North American butterfly. Although the tails do not function in evasion of echolocating bats, they still direct attacks away from the body for visually-oriented predators like birds.

2 replies to “Luna Moth

  1. Something else that occurred to me is that the body shape might return to a bat as just another bat. The long tails mimic the protruding hind legs of a bat in flight.

    1. I think their shape and flight pattern should be sufficiently different for bats to discriminate between them. But I don’t know if anyone has looked at whether bats sometimes mistake other bats for insects!

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