Nature Note – Pogo sticking on the water surface

Lily-pad beetles use small hops to move over the water. Image from

The world is a different place when you’re small. As an example, consider how insects interact with the surface tension of water compared to a person. Surface tension is the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules. The same property that makes belly flops painful for people, allows water striders to walk atop a pond.

Now Manu Prakash, an assistant professor of bioengineeringbat Stanford University has discovered a new means by which lily-pad beetles move over the water surface. The insects are in the genus Galerucella; other species from the genus have been introduced in the Lake Champlain watershed to help control the invasive plant purple-loosestrife. The insects’ legs get trapped in the water and they can’t generate enough force to break the surface tension. Instead, with each beat of their flapping wings they alternately pull away from the water or sink into it. When they sink, the surface tension resists their motion forcing them out of the water and they go bouncing from lily-pad to lily-pad as if they’re on a pogo stick, but never leaving the surface film.