Water striders do the locomotion by 'sculling,' scientists discover

I imagine walking with mathematician John Bush during a summertime hike beside a pond. You pause as Dr. Bush points to the tiny water striders skittering across the surface.

"When I walk around ponds in the Northeast, I see water striders a lot, and I often wondered how they propel themselves," says the Massachusetts Institute of Technology professor.

As he looked into the question, he adds, he learned that the reigning explanation leaves an unsolved puzzle: If these tiny insects propel themselves in the way many researchers think they do, then baby water striders should go nowhere fast. They should be too tiny to use the waves they make to propel themselves anywhere. Yet a quick look at the pond shows young water striders can flit about the surface just as adroitly as their parents.

Joined by two students, Bush has attempted to solve this puzzle. The work has yielded a fresh explanation of the bug's ability to walk on water - and a mechanical "robostrider" to demonstrate it. The mechanical bug "is clearly less elegant than its natural counterpart," Bush concedes. "A water strider is incredibly fast and graceful. Ours is none of the above."

Ponds and streams support water striders and other aquatic bugs through surface tension - a force that turns the surface into something like a trampoline. When the feather-light bugs stand on water, their water-repellent legs depress the surface, but don't break through. When the bugs zoom across the surface, they leave a series of tiny waves in their wake.

Until now, many researchers interested in aquatic-bug locomotion have suggested that striders move forward as their busy legs transfer momentum to the water via these waves. But this led to the baby-strider conundrum.

Bush and his colleagues decided to take a closer look at the bugs' effect on water beneath the surface. Using high-speed movies, tanks of water colored with dyes, and striders gathered from local ponds, the team discovered that the fast-moving legs generate tiny vertical whorls or vortexes under the surface. When the team crunched the numbers to determine whether the surface waves or vortexes were the keys to momentum transfer, the vortexes won hands down.

The bugs, in effect, are "basically sculling," says Bush in a phone interview. "They use their central legs to row, but as they shoot fluids back, the 'oars' never break the surface." And over hundreds of millions of years, they have evolved the ability to sense just how much force to apply so that they can race across water at speeds as high as 4.9 feet per second without a splash.

To CalTech scientist Michael Dickinson, the work, reported in today's edition of the journal Nature, is an "elegant study" that helps "form a more cohesive picture of animal locomotion." Writing in the same edition, Dr. Dickinson notes that vortexes are the means by which birds move forward through air or fish through water.

For Bush, a water strider in full stride represents "a beautiful system that's poorly understood. I think we've made a contribution to understanding it."