Robot Helps Show That Water Striders "Row" on H20

James Owen
for National Geographic News
August 6, 2003

Water striders, the insects also known as pond skaters or Jesus bugs, have long baffled scientists who couldn't explain how these creatures "walk" on water.

Many people assumed the insects rode on miniature waves the strider's legs generated on the surface of a pond, river, or lake. However, a U.S.-based research team has learned that what the insects actually do is row. More important, the scientists discovered how they do it.

Water striders can't pierce the surface of a pond or stream otherwise they'd sink. Instead, the insects press down on the water's surface, creating little dimples around their feet. These dimples act like the blades of an oar, generating swirling underwater currents that propel the insects forward.

The theory was first proposed by a team of applied mathematicians and mechanical engineers from Massachusetts Institute of Technology, in Cambridge, Massachusetts. To confirm their hypothesis, the researchers built a model version of the insect. Christened "Robostrider," its movements closely mimicked those of a living water strider.

Water striders are narrow, light-framed insects often seen in ponds, rivers, and lakes. Belonging to the family Gerridae, they are the world's most advanced surface-dwelling water bugs. One of the few insects to conquer the oceans, some intrepid species venture hundreds of miles across becalmed tropical seas.

The insects rely on surface tension and water-repelling hairs to stay afloat. Their long middle legs drive them in a sculling motion, with the hind legs acting as rudder and brakes. This leaves the front pair free for snaring prey.

While water striders seem to skate effortlessly across water, the surface is like glue to most other insects. Once trapped, the struggling bug is located via telltale ripples. The water strider closes in, snares its meal, and sucks its prey dry.

Hydrophobic Legs

Scientists soon realized that the water strider's hydrophobic legs and undersides, coupled with its small size (typical length is about one centimeter/0.4 inch), are what keep it from drowning. What they couldn't understand is how the insect moves on water without breaking the surface.

"For something to move in a fluid it has to transfer momentum backwards," said research team leader John Bush, an expert in fluid dynamics. "So when you're swimming, for instance, you fling water backwards."

The only visible clue as to how water striders accomplish this are the surface ripples they make when darting about.

It was this observation, and the fact that scientists had not performed more careful experimental studies, which led people to erroneously think that the insects transferred momentum exclusively through surface waves, Bush said.

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