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Small insects paddle through air

Just as a kayaker uses a paddle to push his or her kayak through the water, small insects use their wings to push themselves through the air, using the force of drag. Unlike birds who use the force of lift to fly through the air, insects swim through the air, pushing it behind them like a fish pushes aside water.

[High speed video (8000 images a second or 40 frames per wing beat) of free-flying mosquitoes; filmed at Cornell University in 2009.]

Birds and airplanes use lift - the pressure difference between the top and bottom of a wing - to fly through the air. Most animals going through water, though, use the force of drag to propel themselves along: they reach out and push the water behind them.

To small animals, like tiny flying insects, the air that makes up Earth's atmosphere can feel thick and heavy like water feels to humans. They need more of a push than lift alone to move through the thick atmosphere.

Researchers Leif Ristroph, John Guckenheimer, Z. Jane Wang and Itai Cohen, of Cornell University, and Attila J. Bergou, of Brown University, analyzed 140 movies of fruit flies (Drosophila melanogaster) moving in forward flight.

What they found is that small insects push air aside with their wings as a kayak paddle pushes aside water. Imagine a kayak paddle entering the water at a low angle of attack - fairly horizontal relative to the water. Then imagine the kayaker pushing it backwards: it becomes more vertical in the water, pushing more water aside. Small insects' wings do the same thing, changing their angle of attack to push themselves through the air.

The wing's movement forward is at a low angle of attack, and there is little drag produced. Once it moves back, however, it changes to a higher angle of attack, producing more drag and pushing away more air.

The angle can also be changed to produce the same amount of drag going both backward and forward enabling the insect to hover like a swimmer uses his or her arms to tread water. The scientists created a computer model of the insect's wings and saw the same behavior reproduced on the computer.

Just for good measure, they also used their computer model to see how the density of the fluid surrounding the insect would affect its flight, simulating flight through water and through the Martian atmosphere. Martian air is less dense than air on Earth while water is a thousand times more dense than air.

Because density affects both the forward movement and the drag forces equally, the velocity was relatively unchanged for both scenarios. Flight speeds through the Martian atmosphere were about the same as on Earth and flight through water was slightly slower.

The authors argue that this research is evidence there may be an evolutionary link between modern insects and their swimming ancestors. They theorize the insects' swimming behavior in air may be left over from their predecessors' swimming behavior in water.

This research was published yesterday in the APS journal Physical Review Letters.


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