To hover without crashing, an insect must be able to compensate for drift caused by, say, a cross breeze or a mistimed wing flap. Under daylight, researchers believe that sharp-eyed insects such as dragonflies maintain a steady flight position by watching their surroundings for cues. But vision can be unreliable in the shade or at night, leading other insects such as flies to rely more on tiny forces felt during flight.
Now add moths to that list. Using three high-speed video cameras, researcher Sanjay Sane of the University of Washington and colleagues observed tiny, repetitive forces shaking the antennae of a hawk moth as it hovered over a flower. The timing of the forces—twice every wing beat—indicated they were equivalent to the force that holds up a gyroscope or causes a swinging pendulum to twist under Earth's rotation. The moth's wing beats rattled its antennae, which in turn resisted any motion of the insect's head.
The researchers found that motion-sensitive cells in the base of the moth's antennae were picking up these forces and likely forwarding them to its brain's flight control circuitry, using the gryoforces to filter its own motion from its surroundings and make midair course corrections.
To confirm the idea, they clipped the moth's antennae just above the base. Suddenly the insect began smacking into walls [see video here]. But when the researchers reattached the antennae with superglue, much of its lost stability was restored [compare flight after reattachment to normal flight]. "We didn't realize how dramatic it would be. We thought it'd be more subtle," Sane says.
The result, reported online February 8 by Science, suggests that flies, moths and probably other insects have all converged on the same solution for drift, says insect physiologist Mark Frye of the University of California, Los Angeles.
Another principle of insect flight holds that forward-moving honeybees and other fliers get their orientation from the motion of the ground below them. From an insect's point of view, a spot on the ground would seem to zoom past (because it is the bug that is moving) at an angular speed set by the ratio between the insect's ground speed and its height. (It is like the view from a plane: the closer it is to the ground, the faster the ground appears to zip by.)
To study the idea, researchers at France's CNRS (National Center for Scientific Research) and the University of the Mediterranean, in Marseille, built a small, 100-gram helicopter with a 30-centimeter-wide rotor and tethered it to a pole [see image above]. A downward-pointing camera recorded the craft's angular speed and adjusted its height to keep that speed constant. So if the helicopter approached the ground, for example, it would register a faster angular speed and ascend to throttle back.
In a Current Biology paper appearing online February 8, the scientists report they reproduced several features of insect flight: The helicopter ascended from the ground after an initial thrust pitched it forward; descended in a strong wind and followed a diagonal line while landing; and it crashed when the researchers removed any sign of contrast from the ground. Although reports conflict, bees may sometimes crash when flying over still water for the same reason: with no ground as a cue, their apparent speed is zero so they descend to gain speed. (Sane notes that moths also "crash" to take drinks.)
"I have never heard of bees descending and crashing into water that is mirror-smooth, but I wouldn't be surprised," Frye says, because experiments have shown that bees do seem to fly by monitoring apparent motion. He says the authors have taken a proposed rule of insect flight "and then incorporated it into a control scheme for a stable flying robot—and it works. That is very cool indeed."
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