Why large animals cannot walk on water the way insects do

Settled in 2003 by D. L. Hu, B. Chan, J. W. M. Bush.

Mark Denny highlighted a scaling paradox: the surface-tension force available to support a water-walking animal grows only with the length of its legs' contact line (scaling as size^1), while its weight grows with volume (scaling as size^3), so surface tension alone should become hopelessly inadequate at anything much larger than insect scale, and yet natural history seemed murky on exactly how the largest water striders got away with it. Hu, Chan and Bush combined high-speed video and force measurements to show water striders don't rely on surface tension pushing directly downward but instead use their legs to generate subsurface vortices via rapid strokes, transferring momentum to the water in a way that works even without breaking the surface. This explained both how existing insects manage it and why the strategy hits a hard ceiling around tens of centimeters in body length, consistent with the largest known water striders.

The same cubic-versus-linear scaling law that keeps elephants from being scaled-up mice also draws an exact line on a pond's surface beyond which no animal, however cleverly built, can walk on water.

Explanations that circulated

• Water striders are simply too light for gravity to matter, so there is no fundamental size limit
• Surface tension support scales the same way as body weight with size, so in principle an animal of any size could walk on water with long enough legs

References

• The hydrodynamics of water strider locomotion, David L. Hu, Brian Chan, John W. M. Bush, Nature, 2003