Water striders are insects that can perform amazing feats of dexterity on water. They perch on the surfaces of ponds and slow streams as if on solid ground. And, as their name suggests, they can skim blithely across the water’s surface, just like surfers on a wave.

Not to be outdone by these humble creatures, John Bush and his colleagues at the Massachusetts Institute of Technology have tried to create a mechanical device that can do the same. The result is the Robostrider, a 9cm-long replica that looks remarkably like the real thing (although it is about nine times longer). It is fashioned out of aluminium and steel-wire, and powered by an elastic thread and pulley. Like a real water strider, the Robostrider relies on the surface-tension properties of water to stay afloat.

How it moves, though, is a different issue. For years, scientists have argued that water striders propel themselves by creating surface waves that carry momentum backwards. Newton’s third law (that for every action there is an equal and opposite reaction) then carries them forwards. This idea works for adult water striders. To generate suitable waves, an insect leg must move faster than about 25cm a second, which is the minimum speed of a surface wave. Exceeding this speed is not a problem for water striders with long legs. But if it were the whole story, then baby water striders, with much shorter legs, should be immobile on water—yet they skim along as easily as adults.

This conundrum, known as Denny’s paradox after the scientist who first noticed it, has puzzled researchers for almost a decade. Dr Bush and his colleagues believe they have found the answer. As they explain in a paper published in this week’s Nature, water striders typically stroke the water with their middle legs a couple of times each second. High-definition pictures captured on video cameras reveal that each stroke produces a pair of vortices beneath the water’s surface. It is these vortices, which move backwards in the wake of the insect, that are responsible for driving both adult and baby striders forward.

Like its inspiration, the Robostrider depends primarily on vortices for thrust, although it creates both vortices and surface waves in its wake. The researchers say that it can achieve speeds of about 30cm a second. Not bad, though real water striders have peak speeds of about one metre a second. And the Robostrider, they concede, moves less elegantly than its natural counterpart. Walking on water is impressive and not as easy as it seems.

The Economist