||Insects hold key to walking on
Thursday August 7, 2003
The secret of how to walk on water is out. A paper in this week's Nature explains how water striders (Gerridae) perform their aquatic feat. The research also reveals Robostrider - a mechanical water strider.
Water striders are small insects that live on ponds and rivers. Their long hairy legs keep them afloat, but their ability to walk on water had left researchers perplexed. Many thought they move forwards by creating surface waves that carry momentum backwards. This is reasonable for adults but, theoretically, baby water striders should be immobile, yet they skim along as easily. Clearly, there is more to water strider motion than meets the eye.
Now, John Bush and colleagues at MIT have used high-speed video and particle-tracking studies to observe their movement more closely. The insects' legs act like oars, they find. A water strider's legs create swirling vortices that carry momentum beneath the surface of the water. "It is the rearwards motion of these vortices, and not the surface waves, that propel the animal forwards," says Michael Dickinson from Caltech.
The researchers have designed and built a lightweight, 9cm replica of a water strider. Like its inspiration, Robostrider generates tiny waves and vortices as it moves along the water surface. It too transfers momentum backwards via the rowing motion of its legs. "Robostrider travels half a body length per stroke in a style less elegant than its natural counterpart," the team report.
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