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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.

But Bush and his colleagues were able to disprove this theory by using high-speed video, particle-tracking studies, and dyed water to examine water strider movements more closely.

The results of this research, published this week in the scientific journal Nature, show that with each stroke of their middle legs water striders create hidden, underwater currents. These swirling vortices carry enough momentum to propel the insect at high speed.

Bush said: "This is how rowboats work. The oar goes in the water and scoops fluid backwards. Water striders do the same, except they don't break the surface. The dimples they create act like the blades on an oar."

Michael Dickinson, a professor of bioengineering, added: "It is the rearwards motion of these vortices, and not the surface waves, that propel the animal forwards."

Fast Movers

Water striders can cover 100 body lengths in one second. This is equivalent to a six-foot-tall (1.8-meter-tall) human swimming at speeds of over 400 miles per hour (644 kilometers per hour), a velocity faster than many jet aircraft.

Basing its design on their observations, the researchers built a working replica of the insect. Like its inspiration, Robostrider creates surface ripples and hidden vortices as it moves across the water.

Fashioned from lightweight aluminum, Bush admits this mechanical model doesn't quite match the grace and speed of its natural counterpart, and travels only half a body length per stroke.

He added: "It's fair to say our Robostrider isn't nearly as elegant as the real thing, but it does work."

Having solved the mystery of the water-walking Jesus bug, Bush is turning his attention to locomotion in other waterborne creatures.

He said: "It's a fascinating world, completely dominated by surface tension. People have done a lot of work on birds and fish, but this is in between—neither flying nor swimming."

So take a good look at those other water bugs out there. Their secret could soon be out.
 

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