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Fruit flies: Nature’s hovercraft

From Science: Scientists find that tiny changes in wing motion generate the torque required for fruit flies’ ability to quickly make turns.
/ Source: Science

On a high-tech stage surrounded by infrared video cameras grabbing 5000 frames per second, scientists record the fruit fly spin-and-hover shows performed daily at produce stands and their findings suggest that tiny changes in wing motion generate the torque required for these quick turns.

This research, appearing in the April 18 issue of the journal Science published by the American Association for the Advancement of Science, flies in the face of a long-held hypothesis and suggests that the inertia of the fly’s body as it turns, not friction, is the most critical factor that the aerodynamic force generated by the wings must overcome.

Scientists and engineers around the world are working to build tiny flying robots capable of the rapid flight maneuvers of flies. Such robots could be used for surveillance, search and rescue, environmental monitoring, mine detection, and planetary exploration. But when the science behind the biological model changes, what happens to the technology designed to mimic the old scientific ideas?

“The clever biomimetic researchers already know that getting a little robot flapping is the easy part. The hard part is mimicking the sensory control system that helps the fly turn and zoom around,” said Michael Dickinson from the California Institute of Technology in Pasadena, CA. “Knowing how the animals solve the problems can help the engineers. They may be able to design their way out of the problem if they know what to expect.”

Fruit flies have not made copying Mother Nature easy. They perform 90-degree turns called saccades, each in less than a tenth of a second, as they explore their environment and search for food. On a cross-kitchen trip from a ripe peach to a tomato, the wings change orientation throughout each flap. Consequently, the aerodynamic forces at work are also changing, and this makes studying the forces generated by the wings of insects and airplanes quite different. In fruit flies, the topside of the wing faces up during the downstroke, but then the wing rotates on its axis so that the underside faces up during the upstroke.

“This research does not define the end of the story but rather the beginning,” said Dickinson. “Armed with the knowledge of how their wings work, we can apply this tool kit to the difficult problem of understanding how insects control flight behavior and generate remarkable aerial feats.”

Inertia, not friction
A rotational version of inertia, that force that makes you fall forward when the subway car you’re riding stops short, dominates fruit fly flight according to this new study. This is a departure from the long-held idea that the friction between the air and the fly is the primary force at work. The results for the fruit flies hold true for many other flying insects that also must exert a force to start turning and then exert a counter force to stop turning.

Dickinson was amazed that the differences in forces required to start and stop the turns were so subtle and that the flies could control the forces responsible for the turns at such a fine scale. “Once we understood what the wings were doing, it made so much sense,” said Dickinson.

These findings demonstrate the high performance level of the fruit fly sensory system. Dickinson explained that when fruit flies make their signature quick turns, their bodies rotate too fast for their visual systems to keep up. Therefore, a non-visual sensory system is probably involved in flight control.

“It has to be the halteres, the fly’s shrunken, second pair of wings. They are both sensitive and fast enough to order the signal to stop turning,” said Dickinson who noted that scientists have long believed that halteres were used for stability.

Recording and studying fruit fly acrobatics at high resolution required some calculated and technical fly-watching. The scientists lured flies toward a visual target laced with vinegar using three video cameras recording infrared light at 5000 frames per second.

“If you blasted fruit flies with visible light, you would never get a saccade. Steven Fry [the first author on the paper] went through a lot of work to get the flies to saccade under experimental conditions and to get data that we really believed,” said Dickinson. “Our ability to see the flies did not interfere with their ability to see the world

From these movies, the scientists collected information on the tilt of the wings as the flies spun their bodies at blazing speeds. A robot fly named “Bride of Robo Fly” that can fly forward and make turns in its home, a tank of mineral oil, reenacted the sharp-turning fruit fly performance in slow motion using wing data from the video. Both oil and air are fluids in engineering terms, and the forces generated by the robot were recorded and used to describe the physics of live fruit fly flight. The robot data allowed the researchers to make the leap from wing position to the aerodynamic forces generated by the wings.

Next, the researchers superimposed the detailed force info generated by the robot fly back to the video. The green arrows in the video represent the aerodynamic force created by the flapping wings. A Web version of this video is available for download here.

The researchers worked hard to see and feel the world as if they were fruit flies and this approach opened the door to discovery.

“You can’t interpret what a biological system is doing using a Star Trek brain-in-a-jar approach. No system is operating in isolation. For every little output element in a fly there are hundreds of inputs. The flight system is sensory biased,” Dickinson explained.