Solved: A mystery of flight

Research: A fruit fly study could spawn robots that conduct military surveillance and probe outer space.

April 18, 2003|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

Using a robotic insect, high-speed video cameras and a few drops of vinegar, scientists say they have answered a question that has puzzled them for decades - how flies perform stunning aerial acrobatics when they hover near a dessert tray or a piece of fruit.

A team from California and Switzerland says its two-year study shows that fruit flies use sophisticated steering mechanisms as they spin through the air - wings flapping at 200 beats per second - and shift course to avoid a cat's paw or a fly swatter.

The research provides the most detailed look yet at how flies move in the air - a key issue for scientists working on robotic insects that may someday probe outer space, search for lost hikers and conduct military surveillance.

"It's amazing that this animal works the way it does," said Ronald Fearing, a professor at the University of California at Berkeley, who has a Navy contract to build a robotic fly.

The study by researchers at the California Institute of Technology and the University of Zurich, published in today's issue of Science, says a fruit fly's spiraling maneuvers are the result of rapid-fire spins and counterspins, a sophisticated balancing act that would be the envy of any fighter pilot.

Three-dimensional images of the fly's aerial feats, created by cameras snapping 5,000 images per second, show that flies make complicated turns in one direction and then in another, using internal "gyroscopes" to battle inertia and keep themselves on course.

The findings, based on the laws of physics that govern the fly's flight patterns, apply to houseflies and to most other flying insects, said Michael H. Dickinson, a professor of integrative biology at Cal Tech and a co-author of the study.

"It shows that these animals are a lot more complicated and much more sophisticated flight-control mechanisms than we first thought," Dickinson said.

The scientists' methods were as interesting as their findings.

The researchers placed thousands of nearly starved fruit flies in a Plexiglas cube about 1-foot square. They trained three high-speed video cameras on a spot near a drop of vinegar on a cylinder in a corner of the cube, which provided a lure. Then they created three-dimensional images of the flies as they performed their spins - known as saccades - to reach the vinegar.

The team re-created those flight patterns by submerging a computer-driven, robotic "fly" about the size of a small dog in a 2-ton tank of mineral oil in the basement of Dickinson's California lab.

Measuring ripples in the mineral oil, which provided the same resistance to the robot wings as air did to the flies' wings, researchers were able to re-create the precise aerodynamic forces that influenced the insects' flight.

The fruit fly, which goes by the scientific name Drosophila melanogaster, may be the world's most-studied organism. The fly's genetic properties have been observed for 90 years because its two-week life cycle makes it easy to map succeeding generations. Researchers studying the human genome have mapped the fruit fly's 12,000 genes.

The fly's abilities as an aviator have also attracted scientists, who tether them to strings and sticks and photograph their flight patterns. (How do you tether a fly? By chilling it into stillness, zapping it with ultraviolet glue and putting it on a stick.)

Dickinson and his co-authors say this is the first time that the flies' wings, bodies and the aerodynamic forces that influence them have been studied in such detail in free flight.

"The key question was, `How does an insect control its flight once it gets in the air?' To answer that, you have to look at the whole dynamic of the process," said co-author Steven N. Fry, a researcher at the University of Zurich. "It wasn't enough to just look at how the wings moved or how the body moved. You have to understand the aerodynamic forces involved."

Fry said the study focused on the two forces that an animal must battle when it takes flight: viscosity, which is the resistance created when an object flows through a fluid or through the air; and inertia, which is the tendency of a body to continue moving or to stay at rest until it is acted on by force.

Of the two, inertia is the dominant force, researchers say. And that means insects that fly are smarter than scientists suspected.

"The fact that the animal is being dominated by inertia when it makes its turns means it must have a more-sophisticated control system in its brain and in its sensory apparatus than we thought. It has to have a way to tell itself to compensate, to stop turning," Dickinson said.

Scientists say the finding should help advance work on flying insect robots that might be used by the military for surveillance or by rescue crews to search debris after an earthquake.

Fearing, who hopes to have a prototype robotic fly in the air by the end of the year, said the study detailed the complexity of the fly's flight maneuvers and underscored the need to build robotic wings that work precisely together.

"It means with a robotic, if there's any variation in the two wings, it could easily spin out of control," Fearing said.

"In terms of moving through the air, flies are some of the most maneuverable things that ever existed," said Robert J. Full, a professor of integrative biology at the University of California, Berkeley who studies how animals move. "It can't hurt to understand how they work."

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