Vortical motion. Whirlpools push a water strider across a watery surface.

How to Walk on Water

Water striders have long baffled scientists with their ability to deftly skate atop ponds and rivers. Now a study shows that the delicate insects paddle with their legs to whip up whirlpools that thrust them forward. All without getting even a drop of water on their bodies.

For at least a decade researchers thought that water striders are pushed along by short waves created in their wake as their legs slide around across the water's surface. If this theory were true, baby striders whose legs are too slow to generate waves shouldn't budge. But the infants don't have a problem with water walking, and scientists were left perplexed. This puzzle intrigued John Bush, an applied mathematician at the Massachusetts Institute of Technology.

Bush and colleagues filmed the bugs in a laboratory aquarium using a high-speed video camera. They noticed that the insects shifted their two middle legs in a rowing motion as they danced across the water. When the group then added a blue dye to the water, they saw swirling eddies directly below the creatures' moving legs. The vortex effect was even more obvious when the researchers put small trackable particles into the pool and watched the patterns they made under the striders. Combining these observations with a few mathematical calculations revealed the answer: By paddling and pushing as hard as they could on top of the water without penetrating it, the striders created strong semicircular currents that pushed them forward, the team reports in the 7 August issue of Nature.

To put their theory to the test, the group built an aluminum strider about 10 times the size of a normal strider. Robostrider was able move just like its live counterpart, making swirling whirlpools below its quickly revolving legs without getting wet. The subsurface vortices turn out to have even more momentum than the waves that the adults make in their wake, Bush adds. And even the smallest striders, which can't make waves, can still stir up the semicircular currents, Bush calculates.

"It's a very nice piece of work that contributes a real understanding of what propels these organisms," says Steve Childress, an applied mathematician who studies hydrodynamics and locomotion at New York University.

Related sites
Applied Mathematics at New York University
John Bush's Web site at MIT

Posted in Math