Aug. 8, 2003 — A long-standing question about a type of water bug has been answered: how do water striders propel themselves across the surface of still water?

Previous models of water-strider locomotion theorized that the insect moved forward by creating minute surface waves that carry momentum backward. But in a study in the journal Nature, researchers at the Massachusetts Institute of Technology created a robot water strider that showed otherwise.

Animal locomotion boils down to essentially one principle: to move forward, animals must push against something backward.

Land animals push against the hard ground; fish push against water; birds push against air.

But water bugs are stuck somewhere between water and air. The wave theory worked for adult water striders, which can move faster than the tiny waves they might generate. Young water striders, however, don't have the leg length to outpace such waves.

To conduct the experiment, the researchers, David Hu, Brian Chan and John Bush, collected water striders from local ponds and kept them in aquariums. The insects reproduced every few weeks, providing them with fresh juvenile striders to observe.

Study of the tiny bugs revealed that water striders' legs, like the oars of a rowboat, create swirling vortices that carry momentum beneath the surface of the water. Fish and flying animals also use vortices for locomotion. But the insects' vortices are an unusual "U" shape, unlike fish vortices, which are circular.

The robot, fashioned partly out of elastic thread, confirmed this.

"It is the rearwards motion of these vortices, and not the surface waves, that propel the animal forwards," wrote CalTech engineer Michael Dickinson in an article accompanying the study.