Many people long to live near water for the pleasures of swimming, boating, fishing, or just to contemplate.
But, says Manu Prakash, a scientist at Stanford with broad interests, the surface of an alluring lake or pond, the boundary where water meets air, can be far less inviting for certain insects.
Water is about 100 times as viscous as air, and insects that live on its surface must somehow negotiate movement in both realms. Most seem to choose one or the other. Some, like long-legged water striders, maintain a small measure of distance from the water with a cushion of air that lets them skate on the surface. Others, like whirligig beetles, embrace the water and swim.
The lily pad beetle, which Dr. Prakash and his colleagues report on in The Journal of Experimental Biology, has evolved a unique solution to moving in water and air at the same time.
It flies, beating its wings for propulsion much as it would if it were taking off into the air. But it remains tethered to the water by claws at the end of its legs that break through the surface and act as anchors so a strong wing beat won’t unexpectedly lift the beetle.
The movement is unusual, a bit like windsurfing, except that rather than having a sail to catch the wind, the beetle uses its wings to generate power.
Dr. Prakash runs a laboratory devoted to projects as varied as economical paper microscopes, water computers and biophysics. He conducted the study of the lily pad beetle with Haripriya Mukundarajan and Dong Hyun Kim at Stanford and Thibaut C. Bardon of the École Polytechnique in Paris. He described the research as “just good old natural history,” with a large dose of physics.
It began some years ago when he saw, on a pond, “something that moved so fast that I couldn’t actually see it.” He stayed for a couple of hours trying to track the culprit down and finally found the beetle, which eats holes in lily pads and zips from one to the other on the water’s surface.
He thought its technique was a great way to move around and had a hunch that the physics would be intriguing. And he was attracted, he said, to a creature that had “really claimed this harsh environment.”
With high speed video and mathematical analysis of the movements and forces involved, the team made several discoveries.
The beetle’s legs repel water, which keeps it afloat, except for claws at their tips, which penetrate the surface of the water and tie it to the surface.
As the beetle flies, or surfs, the up and down force of its wings and the attachment of the claws make the surface of the water bounce up and down like a trampoline. The tips of the wings almost touch the surface of the water, and the beetle copes with aerodynamics, surface tension and another kind of drag that appears only when the insect is moving faster than about nine inches a second. The beetle actually travels close to a foot a second. It’s called capillary wave drag.
The whole system, Dr. Prakash said, is “right at the edge of chaos,” mathematically speaking. A chaotic system is one so complicated that given the initial conditions, one can’t predict what is going to happen. One way or another, the beetle manages to navigate this environment.
The self-powered surfing actually demands more energy than fully airborne flight, which the beetle can also do. It seems to prefer this sort of locomotion. Dr. Prakash said that may be because it’s a good way to find new food sources, or because predators haven’t figured it out.
He said that he and his colleagues were now turning to a fly that seemed to do something similar but in the sea. There are, of course, ways that one could imagine applying the findings to robotics, but the research really was done, he said, for pure scientific curiosity.
One thing the remarkable beetle did not evolve is a braking system. In the lab, it runs into the edge of a dish and just topples over. On a pond, Dr. Prakash said, they seem to stop the same way.
“They hit the leaf,” he said.