Scientists have deciphered the mystery of how the tiny water strider -- a familiar denizen of your local pond -- gets around: It uses its middle set of legs as oars and rows along the water's surface at a rate of almost 3-1/2 feet per second.

In a series of colorful experiments reported in the journal Nature, a team of researchers led by Massachusetts Institute of Technology mathematician John W. M. Bush used high-speed video and blue-dyed water to track striders as they darted past the camera, leaving vortices as neat as those deposited by a sculler in a boat race.

The experiment answered a question that had long puzzled scientists, who had assumed that striders developed momentum using the tiny waves they generated as they flapped their legs across the water's surface.

"When you look at them, they move so fast that you can't see the vortices, so all you see is the waves," Bush said in a telephone interview. "But the baby water striders' legs aren't big enough to generate waves. There had to be something else."

The Bush team's experiments -- to understand and perhaps replicate the way animals move -- evoked a scientific tradition at least as old as classical Greek mythology, when the inventor Daedalus fashioned wings so he might fly as a bird.

Water striders presented an interesting problem. They are tiny insects -- about 0.4 inches long -- whose six legs are covered with thousands of fine hairs that make them virtually waterproof. They float on the surface of standing water through surface tension -- like standing on a trampoline.

The strider's stubby forelegs and slightly less stubby backlegs are used primarily as stabilizers, floats or even skis. Propulsion comes from the two long middle legs. "A lot of animals do a lot of swimming or flying," Bush said. "But what's so nice about this research is that water-walking is on the interface."

The team members used a video camera capable of taping the striders' movement at 500 frames per second. They spiked the water with microscopic particles that reflected light in such a way that they could show current patterns generated by the insects.

Finally, the team sprinkled the top of the aquarium with a layer of blue dye that would scatter as the strider ran across it, thus describing the surface movement. The aquarium was lit from the bottom.

The result of the experiment was a set of elegant color photographs that show clearly spaced whorls marking the spots where the striders' legs caught the water and "rowed" their bodies forward.