For the first time, biologists have linked the ribbony “tails” streaming from big, green luna moths’ hind wings with, of all things, a cozy climate.
Those dangling wing tails rank among such evolution-was-drunk novelties as the narwhal’s single unicorn tusk or the peacock’s giant feather train. Wing streamers with twisting or cupped ends have evolved independently at least five times in the family of luna and other moon moths (Saturniidae), says behavioral ecologist Juliette Rubin, now at the Smithsonian Tropical Research Institute in Balboa, Panama. Her new data crunch of environmental factors links the ribbony tails with growing up in a long stretch of even temperatures, she and colleagues report May 7 in Proceedings of the Royal Society B.
Moon moths can grow wings big enough to cover a human palm “and often have beautiful colors,” Rubin says. This winged phase lasts only the final week or so of the moths’ lives, when they flirt and mate.
The streamers, however, don’t seem to matter for seduction, Rubin’s earlier experiments on the moth Actias luna showed. Instead, they help confound bats. “I really loved this ancient evolutionary story of this tension between this echolocating predator … and this prey animal that is evading these bats across time, across the night sky,” she says.
Unlike some other flying bat food, luna moths can’t hear a bat’s sonar clickings as it swoops and hunts around the night sky. Nor are these moths known to make any “Back off, bat!” sounds. Instead, the fluttery wing ends can fool bats into biting at the thin, scaled tail tissue instead of some more vital body part.
In the evolution of big, novel body parts, though, “generally there are these hazards,” Rubin says. Finding what those are hasn’t been easy. Hauling extra flappy wing bits doesn’t seem to make flight harder, although more experiments still might turn up a flight cost.
What may matter now more, she and colleagues propose, are the physical costs of growing the wings to begin with. Using iNaturalist photos posted by citizen scientists, Rubin and colleagues looked at where moths have extravagant wings and where they don’t. Antenna length gave Rubin a way to standardize size comparisons.
One result was expected: Having local insect-eating bats favored evolving longer wing tails. A less obvious, important factor favoring longer wing tails turned out to be prolonged warm and relatively steady temperatures. Those places let a moth larva spend time feeding. The super-nourished larvae then can grow super wings for a grand adulthood finale.
