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Robot answers the mystery of how insects walk on water

By Steve Connor

07 August 2003

The mystery of how some insects can walk on water has been solved with the construction of a lightweight robot that performs the same feat.

Using high-speed photography and the mathematical principles of mechanical engineering, researchers discovered the secrets of the pond skater - a long-legged creature that can skim across water - called water striders in the US.

The robot strider, made of stainless steel wire coated in a water-repellent, is powered by an elastic thread that pulls its legs backwards and forwards across the water's surface, just like the real insect.

Until now it was thought that the legs of water striders created tiny waves which propelled the insect forward. But that could not explain why young striders with much shorter legs can walk, given that their legs do not move fast enough to generate the smallest ripples.

David Hu, Brian Chan and John Bush of the Massachusetts Institute of Technology in Cambridge, Massachusetts, have found that the tips of the insect's legs generate invisible U-shaped vortices under the water which propel the strider forward as each vortex moves backwards.

Because of this, the water strider is able to obey Newton's third law of motion which, in simple terms, states that in order for animals to move forward, they must have to push something backwards.

The research, published in the journal Nature, describes how the water strider floats on water using surface tension to keep it from sinking. By moving its legs like the oars of a rowing boat, the strider generates hemispherical vortices which shoot tiny jets of water backwards.

Michael Dickinson, a mechanical engineer from the California Institute of Technology in Pasadena, said that the study finally explained one of the more enduring images of summer - the sight of water striders skimming over a pond. "Despite their small size, the legs of water striders are analogous to the oars of a rowing boat, which also move forwards by sending a series of vortices backwards through the fluid," Dr Dickinson said. "Through their use of vortices, water striders share general features with animals flying above and swimming below them."

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