Paradox lost: answers and questions about walking on water.

The mechanism by which surface tension allows water striders (members of the genus Gerris) to stand on the surface of a pond or stream is a classic example for introductory classes in animal mechanics. Until recently, however, the question of how these insects propelled themselves remained open. One plausible mechanism-creating momentum in the water via the production of capillary waves-led to a paradox: juvenile water striders move their limbs too slowly to produce waves, but nonetheless travel across the water's surface. Two recent papers demonstrate that both water striders and water-walking spiders circumvent this paradox by foregoing any reliance on waves to gain purchase on the water. Instead they use their legs as oars, and the capillary 'dimple' formed by each leg acts as the oar's blade. The resulting hydrodynamic drag produces vortices in the water, and the motion of these vortices imparts the necessary fluid momentum. These studies pave the way for a more thorough understanding of the complex mechanics of walking on water, and an exploration of how this intriguing form of locomotion scales with the size of the organism.