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 the wings tattered to some degree, but this one was pristine, almost glowing among the branches and leaves. Luna moths are the only North American moth with elongated tails on the hindwings. Early records indicated the tails may have an antipredator function, serving as non-vital body structures that tear off during escape. This would be analogous to an insect losing a leg, an animal losing its tail, or just to mention one bizarre case, a gecko jumping out of its own skin. Although costly, the shedding of a body part is a successful strategy during the final stages of a predatory encounter. A leg or limb may be gone, but all is not lost.. even though in luna moths, of course, the tails cannot autotomize nor regenerate/regrow.
The use of visual deception is common in insects, either guiding the predator’s focus to the extremities or employing intricate mimicry to alter its perceived identity (e.g. antenna-like movements of the hindwing tails of lycaenid butterflies, and most famously, eyespot patterns). In luna moths, the primary predation pressures 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, studies have suggested a role as an acoustic lure— changing the moth’s acoustic flight signature to confuse echolocating bats. This is a cool and compelling explanation, but until the past decade, it remained experimentally untested. Through staged encounters between bats and moths with manipulated wing morphologies, several studies have now demonstrated that the presence of tails decreases prey localization and capture success by echolocating bats. Tailed moths were less likely to be caught and to receive a fatal blow (damage to vital organs) than non-tailed moths.
But as most science goes, the story becomes more complicated. Imagine that a bat generates a wave of incident sound that comes into contact with a moth that is flying in a perpendicular direction, then returns to the bat as an echo signal— like a pulse radar. The presence of tails increases the spatial range of detectable echoes. So assuming that a bat intercepts prey near the center of the echo signal, this would shift the relative position of the attack site along the moth’s body to the abdomen and hindwings, which are smaller targets and more expendable structures for survival than the forewings, head, and thorax. This explanation is a much less refined description of what one study suggests. Through modeling spatial localization of luna moths by bats through pulse-compressed echo envelopes, the spectral profiles illustrate how different parts of the moth’s body across varying orientations influence receiver (bat) signal intensity. For instance, if a moth is flapping erratically in space, acoustic snapshots by the bats would reveal stronger echoes produced by certain body parts than by others. Although the tail echoes are present, they are actually weak in comparison to those produced by the forewings and body. However, for bats operating under noisy auditory environments and swift-paced maneuvers, lower acoustic resolution together with tail echoes likely reduces target-tracking accuracy by bats.
Luna moths and other tailed saturniids are large prey items, in many cases even larger than their bat predators. Another study claims that instead of the tails producing weakly-detectable echoes, the spinning wavy tails themselves may be significant enough to produce what they call an “illusion of multiple targets.” According to this hypothesis, we would expect to see a concentration of bat attacks directly on the tails. Through more bat-moth staged interactions, the authors revealed exactly that. The majority of bat attacks were located on the moth’s body and tails, while only a quarter of the time bats targeted the region behind the moth’s abdomen (which is the shifted center of the entire echo profile). But given the small relative area of the echoic center, the observed difference does not seem convincing enough. It’s also likely that active evasion by moths and intercept strategy of the bats across multiple swoops dramatically influences the attack site, but who knows… maybe bats are good enough to make these things trivial!
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 re-occurring phenotypes include large hindwing lobes (e.g. in Rothschildia & the atlas moth) or short pointed tails, which I am much less familiar with. Exploring the potential antipredator function of tails and these other hindwing morphologies is fascinating; but because of the role wings play in flight and crypsis, it is difficult to tease these aspects apart.