for National Geographic News
November 3, 2004
It's easy to understand why water striders—those six-legged insects that float and glide across the surface of ponds, streams, and lakes—are sometimes called Jesus bugs or pond skaters.
Less clear is how the bugs manage to stay afloat without drowning, especially in turbulent conditions brought by rainstorms or moving water. That is, until now.
Studying the insects under powerful microscopes, researchers in China have found that water strider legs incorporate thousands of microscopic hairs. Measuring about 50 micrometers—or less than two-thousandths of an inch—long, the hairs are scored by miniature groves. These groves trap air, increasing water resistance of the water's striders legs and overall buoyancy of the insect.
"Normal hydrophobicity [or water repellency] may support them resting on water," said Lei Jiang, a researcher with the Chinese Academy of Sciences in Beijing, China, and study co-author.
"Slight touches or disturbances may break that balance. However, the air cushion at the leg-water interface can [enable] them to locomote quickly and stably on the water surface—even in rainstorms or other disturbances."
Jiang noted that other animals developed features, such as the feathers on a duck, that work in similar ways. However, most are far less effective at promoting super water-repellence, he said.
High Water Mark
Common to ponds, rivers, and lakes, water strides (Gerris remigis) are considered the most advanced surface-dwelling water bug found in nature.
The insects, which measure about a centimeter (two-fifths of a inch) long, have two pairs of long, thin legs that they use to float and travel on water surfaces. They also sport an additional pair of smaller frontal legs, which can be used to grab prey.
The water strider's hairy legs work to keep it afloat. The hair-and-trapped-air combination has such water-resistance qualities that the insects legs can create 4-millimeter (0.16-inch) dimples in a water surface without breaking through.
The water-resistant legs displace some 300 times their own volume—the source of the insect's remarkable buoyancy. Jiang and colleague Xuefeng Gao found that the water strider's legs are so buoyant, they can support 15 times the insect's weight without it sinking.
This excess floatation capacity may allow the insects to bounce on water surfaces, much like a rubber ball on a cement sidewalk, to avoid drowning during a downpour.
The superfloatation also helps the bugs to skate across water surfaces in search of prey with remarkable speed. The insects can dash at speeds of a hundred body lengths per second. To match them, a 6-foot-tall (1.8-meter-tall) person would have to swim at over 400 miles an hour (644 kilometers an hour).
Water striders not only travel quickly, but they venture far afield. Scientists say some bugs have traveled hundreds of miles across calm tropical oceans.