When Jayne Yack speaks, she knows that her butterflies can hear her. They’re listening with their wings.
Yack, a professor at Carleton University, studies a group of butterflies called nymphalids, which include well-known species like monarchs, morphos, emperors, and admirals. Many members of this group have ears at the base of their wings. If one lifted its top pair of wings in the air, “the ear would be in what you think of as the armpit,” Yack says.
The ears consist of membranes that are stretched taut over oval holes, and that vibrate when incoming sounds hit them. Those vibrations trigger electrical signals in the insects’ nerves, which Yack can record. In this way, she has shown that the ears are especially sensitive to low frequencies, like those found in human speech. “When we’re recording from a butterfly and we’re talking, its nerves are just firing like crazy,” she says. “Moths don’t hear us; they’re tuned to high frequencies. But butterflies can.”
Over the years, Yack noticed that one group of nymphalids—the satyrines, or browns—has weird veins on the top pair of wings. Veins are common to all butterfly wings; they’re air-filled tubes that don’t carry blood, but instead provide structural support. They’re usually very thin, but the satyrines have one on each wing that’s bizarrely bloated, like a single piece of penne on a plate of spaghetti. Scientists have described these inflated veins before, but Yack noted that they lie very close to the satyrines’ ears. Maybe, she suspected, they help the insects hear.
To test that idea, she sent specimens of the common wood nymph to Natasha Mhatre at the University of Toronto, who studies acoustic communication in insects. She played noises at the butterflies’ ears while shining lasers on them. By analyzing the reflected laser light, she could work out how much the ears move in response to different sounds. “We can get a pretty good appreciation of what the butterfly is hearing,” Yack says.