The way water striders skim across the surface of ponds has earned them the nickname “Jesus bugs.” But this seemingly miraculous ability—a signature of the mostly freshwater-dwelling Gerridae family of insects—has a perfectly natural explanation.
Surface tension helps make the water strider’s leaps and glides possible. Water molecules are attracted to each other more than to the air above them, causing water to behave as if it was covered by an elastic membrane.
The insects can stand on the surface because they’re light, about one-hundredth of an ounce, and that weight is distributed over a relatively wide area by four of their six legs.
HIGH AND DRY
Water striders also have remarkable feet that are basically water-repellent, according to Christine Goforth, an aquatic entomologist who blogs at thedragonflywoman.com and is the senior manager of citizen science at the North Carolina Museum of Natural Sciences.
“They have lots of little hairs on their feet that are hydrophobic,” she says. The chemical composition of these hairs keeps the water strider’s feet from breaking the surface. The hairs also cover the insect’s body, preventing rain or splashes of water from soaking it and weighing it down.
In addition, the claws on many species of water strider are positioned farther up their legs than on most insects, like “fingers coming out of our wrists,” Goforth says. At such a remove, the claws are less likely to touch the water and break the surface tension.
If a water strider gets submerged, the tiny hairs on its body and legs trap air, providing enough buoyancy to bring the insect to the surface. In the meantime, water striders can breathe air trapped in bubbles. They also can maneuver beneath the surface. “They have really spindly legs so they’re terrible swimmers, but they can do it,” Goforth says.
Water striders don’t face much competition from other insects for the food that falls onto or rises up to the water’s surface. They eat mosquito larvae, beetles, butterflies, and other insects—alive or dead—using their two short front legs to grasp their prey. “The whole surface of the water would be completely covered in gunk if you didn’t have organisms like water striders,” Goforth says.
Most water striders live in ponds or marshes, where the water is calm, or on the edges of slow-moving creeks and streams. But one group, the Halobates, lives on the ocean, hundreds of miles offshore.
Only a few other creatures, such as the pygmy gecko and the basilisk lizard, can travel on top of the water, although some must run rather than walk because they’re too heavy to be supported by surface tension. The water strider’s unusual fluid dynamics have even inspired researchers at MIT to fashion a mechanical version from a soda can, steel wire, an elastic band, and a pulley. The Robostrider moves according to the same principles, but its creators admit it isn’t nearly as elegant as the real thing.