These experiments revealed that the veins are, in effect, part of the ears. When Mhatre cut them open, the ears became less sensitive in general, and to low-pitched sounds in particular. In the same way that our fleshy outer ears gather sounds and focus them toward our eardrums, the common wood nymph’s bloated veins focus sounds onto their ears.

That’s useful because these butterflies are very small—just a couple of inches from wing tip to wing tip. Small membranes aren’t very good at detecting low-pitched sounds, or at working out where those sounds are coming from. The veins seemingly compensate for this problem. “They’re hearing aids for small butterflies,” Yack says.

This is a new means of hearing that had never been described before, says Fernando Montealegre-Z of the University of Lincoln: “I remember that during my entomology training, back in the ’90s, we were told that satyrine butterflies could be identified by the swollen vein. We didn’t know why this vein was swollen, and I would have loved to have heard an explanation.”

Insects have really gone to town on the evolution of interesting ears. There are species with ears on their torsos, antennae, wings, knees, and legs. And in most cases, scientists know very little about how these organs work.

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For example, why would butterflies need to hear low frequencies in the first place? Most butterflies are silent, so they’re certainly not listening to one another. “We don’t really have answers,” Yack says, “but the most convincing hypothesis is that they’re listening to the incidental noises that predators make.” They could be listening for flapping wings, or feet stepping on branches, or feathers rustling through grass. It’s perhaps telling, Yack says, that butterflies are sensitive to the same low pitches as rabbits, lizards, and other small prey animals.

But satyrine hearing is unusual in one crucial way. Across the animal kingdom, most ears are tuned to particular frequencies, while satyrine ears react similarly to sounds across a very wide range of low pitches. “It’s very rare that you see a response that flat,” Mhatre says. “This would make a really good microphone.”

The ideal microphone accurately represents the sounds around it, without preferentially amplifying certain pitches over others. That flatness is very hard to achieve—and yet the satyrines have done it. Mhatre thinks their secret lies within their swollen veins. These structures aren’t completely hollow. They contains lots of very thin membranes that are arranged like honeycombs, or “like lots of soap bubbles stuck together,” Mhatre says. Perhaps these membranes help flatten the sounds that are amplified by the veins.

As far as anyone knows, these special hearing abilities are reserved for the satyrines’ swollen wing veins, but with such a wide variety of insect ears, that might not be true. Adriana Briscoe, who studies butterfly vision at the University of California at Irvine, is excited about the possibility. “This discovery makes me want to run back to my lab and start rifling through my butterfly collection in search of hearing organs, to figure out how widespread they are,” she says.

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