Butterfly ears suggest a bat influence

Hungry bats whipping through the blackness to gulp down insects may have driven moths into daylight—creating what we know as butterflies—suggest Canadian researchers.

Night-flying butterfly, 4 centimeters across, can hear bats. Yack

The idea springs from their discovery of butterflies that have ultrasonic ears, explains Jayne E. Yack of Carleton University in Ottawa. Night-flying, tropical butterflies of the superfamily Hedyloidea have on their wings ears sensitive to high frequencies, Yack and James H. Fullard of the University of Toronto report in the Jan. 20 Nature.

Such ears can detect an incoming bat’s echolocation pulses in time for the insect to try zigzagging out of death’s way. Many moths have evolved such ultrasonic hearing, but researchers hadn’t detected it in butterflies, Yack notes.

She and Fullard hypothesize that bats’ evolution of echolocation more than 50 million years ago drove moths to protect themselves. Some developed ears that could detect bat ultrasound pulses; others adopted the daylight lifestyle of the butterfly. “In a sense, bats may have invented butterflies,” Yack suggests.

Malcolm J. Scoble of the Natural History Museum in London deems the suggestion “a very interesting idea” and “a reasonable speculation.”

Hedylids—a little mothy and a bit butterflyish—were long considered moths and placed in the inchworm group. In 1986, Scoble reclassified the 40 hedylid species as a superfamily of butterflies. “It caused a little bit of excitement among my colleagues,” he recalls, but has since won converts.

When Yack examined the hedylids in Canada’s national insect collection, some wing structures caught her attention. She had done her dissertation on moth ears and immediately recognized an ear. “It was one of those amazing moments when you see something that no one has seen before,” she remembers.

Microscopic views revealed a mothlike ear, about half a millimeter across, with a delicate drum membrane as transparent as cellophane, she says. Each wing has one ear.

To see how the ears work in living butterflies, Yack scoured Panama in search of hedylid populations that hadn’t vanished. Her first circuit, “a miserable trip,” was unsuccessful.

When she finally located enough butterflies to test, she played pulses of high-frequency sound at night. “They would go into these wild, crazy loops and spins and dives,” resembling a moth’s evasive flight, she says. When Yack interfered with hearing—by clogging the ears with Vaseline, for instance—the butterflies no longer responded.

“I’m quite convinced that they’ve found an ear,” comments William E. Conner of Wake Forest University in Winston-Salem, N.C. However, he cautions that it’s too early to say whether the ears remain from moth ancestors or have arisen more recently.

Both he and Ronald R. Hoy of Cornell University find it plausible that night-flying butterflies like the hedylids would evolve ultrasonic hearing to evade bats. “If you’re an insect and you fly at night, in principle you have a bat problem,” Hoy says.

Insects can solve that problem in a variety of ways, such as lurking in plants, being too tiny a mouthful to be worth chasing, or growing large enough to look formidable. Still, Hoy has found bat detectors in a wide range of night-fliers, such as crickets, katydids, locusts, praying mantises, green lacewings, and beetles. He says that metallic squeaks made by jingling keys under street lamps or porch lights will often send insects into a bat-avoiding plop.

Conner wonders whether the hedylids can make noises in the frequencies that they hear. The tiger moths he studies make ultrasonic clicks in response to the sound of a bat’s approach.

Debate flourishes over whether the clicks jam the bat’s echolocation system or advertise that tiger moths taste awful, the sonic equivalent of warning colors that unpalatable day-flying insects flash at birds.

If hedylids make ultrasonic noises, Conner asks, do they squeak as they flirt? He discovered that his tiger moths click to each other, their sounds outweighing pheromones in courtship. “They’re like acoustic fireflies,” Conner says.

Biologists generally agree that moths gave rise to butterflies, although the path that they took remains “a tantalizing enigma,” as Scoble puts it.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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