How Insects Fly
When a bee zips across your garden, it blurs by at jet speed and turns on a dime like no airplane in the world. And when a fly zooms through your kitchen and sniffs a possible snack, it can stop short in midair and hover like a helicopter to check it out. If disappointed, the fly can twirl in an aerial loop-the-loop and land upside down on the ceiling. Then it takes off backward, and flies sideways out the open window. "Insects don't just stay in the air," says entomologist (scientist who studies insects) Michael Dickinson at the University of California at Berkeley. "They perform aerial maneuvers, fly up, down, and sideways, and respond to changes in wind speed and direction."
For many bugs, flying is the only way to travel, providing access to food that other creatures can't reach, and a swift escape from predators. More than 99.9 percent of the 900,000 species of insects on Earth are classified as Pterygota, or winged insects. Entomologists say these invertebrates (backboneless animals) were the first creatures to fly, dating from the Carboniferous period about 360 million years ago.
But for centuries, scientists have puzzled over exactly how insects perform their amazing acrobatic aeronautics. Though they haven't deciphered every secret, researchers have made new discoveries-and are now applying the principles of insect flight to the designing of sophisticated new planes.
The real secret to how insects fly lies in the wings, both in their design and in how they're used. Most insects rely on two pairs of wings, which join or overlap so they work together as a single pair. Insect wings are one of nature's lightest structures, lacking bone and muscle; they're made of chitin, an extremely tough material that also composes an insect's hard outer skin. Chitin is a polysaccharide, a chemical compound that forms fibrous molecules (in which hydrogen atoms bond to produce extra strength). A network of veins also lends insect wings extra support.
Wings on insects, bats, birds, and airplanes share a similar shape, called an airfoil: they're curved on top and flat on the bottom (see diagram, p. 15). Air rushing over the wing has to travel farther because of the curvature, so this air moves faster than air below the wing. Since fast-moving air exerts less pressure than slow-moving air, the difference creates suction, called lift. Lift is what pulls a wing-and a plane or critter-skyward. Each downward wing flap creates more lift, propelling the creature up and forward.
Entomologists now find that the lift produced by insect wings defies traditional laws of aerodynamics. Charles P Ellington and his colleagues at the University of Cambridge in England are trying to figure out how bees manage to launch their cumbersome bodies into the air with relatively tiny wings. Most flying creatures are lightweight so that they need the least amount of muscle power for liftoff-a 68-kilogram (150-pound) person would require flight muscles 1.8 meters (6 feet) thick in order to fly!
But the researchers found that insect flight is far more complex than previously thought. Large-bodied insects lift off by flapping their wings very rapidly: for bees and flies, about 200 times per second. Some midges and wasps flap their wings up to 1,000 times per second! What powers such energy? Strong muscles in the midsection, or thorax. For their size, they're the most powerful muscles known in nature.
Michael Dickinson has also discovered that insect wings don't just flap up and down.
On the upstroke, insect wings move differently from those of most other flying creatures-in a kind of figureeight motion. As the insect wing nears the end of a forward stroke, the wing rotates backward, twisting upside down, parallel to the ground. This rotation accelerates (speeds up) the flow of air over the wing.
This means that insect wings generate a burst of lift and speed from the upstroke as well as the downstroke-unlike the wings of birds or bats, which derive most of their flying power only from the downstroke. "Such elaborate wing movements create miniature tornadoes that send bugs soaring by sucking the wings upward," says Jane Wang, a physicist (scientist who studies motions and forces) at Cornell University in Ithaca, N.Y.
The rotation, combined with lightning-fast wing flaps, whips air flowing over the top of an insect wing into a swell of curling vortexes, or whirling spirals of air. Vortexes act like air streams that flow from a propeller, and prove "crucial for insects to hover," says Wang. "Air swells help bugs get lift, thrust to turn, and maneuver." When insects flap their wings downward, some air passing over the wings rolls along the entire front edge in a vertical spiral that grows as it sweeps along the wingtip.
Ellington's team documented wing movements of hawkmoths and the air currents they whip up. The scientists tethered a hawkmoth to the end of a wind tunnel, then blasted it with smoke. Using a strobe light to freeze motion, they snapped 3-D pictures of the moth flapping wildly in the gale.
But insects are also nature's helicopters, with their wings acting as helicopter blades. To fly forward, bugs tilt their bodies forward, pulling air from in front and pushing it out behind as they flap wings back and forth. To hover, they tilt their bodies upward and fan wings horizontal to the ground, blowing air straight down so they can hang in midair. But, says Dickinson, there's still a lot of mystery surrounding insect flight. "We don't know what the animal does to initiate and control such forces, and it's going to take a lot more work by scientists with different specialties to find out."