How do moths stay aloft? With their antennae, of course. When your wingspan is just three inches across, the slightest breeze becomes a gale, and knowing which way is up becomes a matter of life and death. Now, a research team reports that moths stabilize their flight by using their antennae as gyroscopic sensors.
Rotational inertia keeps a spinning top balancing on its tip: If you try to knock it over, the Coriolis force pushes it to the side instead. The size of that force depends on how fast the top is spinning. Engineers measure the corrective force on calibrated gyroscopes to keep aircraft and ballistic missiles on a level course. And flies stabilize their flight by using their club-shaped hind wings to detect these forces. But no one suspected that moths use a similar strategy. Their antennae are primarily known as super-sensitive odor receptors--used to sniff out females and food from miles away--and researchers had hypothesized that they assist in flight only by acting as air flow sensors. That untested idea had "become part of the lore," says biologist Sanjay Sane of the University of Washington in Seattle.
So when Sane began filming the movements of moth antennae with a high-speed camera, he wanted to understand how the animals convert the speed of air rushing by into nerve impulses. The millimeter-sized antenna movements were too small to support the flow-sensor hypothesis, but when Sane did the math, he realized they were large enough to register Coriolis forces. Sane took recordings from the Johnston's organ, a mechanosensor at the base of the antenna, and found that it was most sensitive to vibrations at twice the wingbeat frequency, just as his gyroscopic hypothesis predicted. With this signal, the moth could be measuring every twist and turn of its body as it soared through the night sky. As a final test, Sane sliced off the antennae and watched as the moths hovered in a flight chamber. "They start bumping around and crashing to the floor and flying backwards," he says. Some even flew upside down. Incredibly, when he reattached the antennae with a dab of superglue, the moths flew normally. The study appears in tomorrow's issue of Science.
Insect physiologist Neil Vickers at the University of Utah in Salt Lake City notes that a gyroscopic sense "is particularly helpful for animals that fly under low light, where stabilization from visual feedback takes longer to process." The study also answers earlier questions about why the Johnston's organ is so sensitive to vibration: A deflection the size of a single molecule is enough to trigger a nerve response.