Walking on water is not part of the normal human behavioral repertoire. But for some insects, it’s routine. Check out almost any pond or creek and you’ll see water striders, known to some as Jesus bugs, skating along the top with their cruciform shadows tracking them below. There’s a nice collective name for these creatures and others that exploit this liminal habitat: neuston, dwellers on the surface.
Although striders winter underwater, sheltered under rocks on the streambed, most of their lives take place on the surface film. It’s their hunting ground: these bugs are predators, always on the lookout for flying insects that fall into the water or aquatic types like mosquito larvae that swim up from below. Alerted by vibrations, the striders dash toward their prey, rowing with their middle pair of legs, then pounce and grab, mantis-like, with the front pair. They inject the victim with enzymes that dissolve its flesh, suck it dry and discard the husk.
The surface is also an arena for courtship. A male water strider selects an object where females can lay their eggs, then uses those versatile middle legs to send out ripples at a frequency of 10 to 30 per second. He alternates between a signal pattern that lures females and another that warns off rival males. The eggs hatch underwater, and the fully-formed nymphs swim up to begin their superficial lives.
How do they manage the water-walking trick, though? It seems inherently improbable. I agree with Sue Hubbell; “If water striders…didn’t exist and we were set the task of designing a new bug, I don’t believe we would ever come up with this one.” Entomologists figured the tiny hairs on each leg somehow acted as a hydrofuge, trapping air and holding water away from the foot, and thought the hairs got their water-repellent properties from a wax secreted by the insect. But that view was recently challenged by two scientists in Beijing, Lei Jiang and Xuefeng Gao, in an article in Nature entitled “Water-repellent Legs of Water Striders.”
According to Jiang and Gao, the key is the structure of the hairs, each of which is less than two-thousandths of an inch long. Scanning electron miscroscopy revealed that each hair, or microseta, is covered with elaborate grooves. The grooves are nanoscale; we’re talking seriously tiny here. The researchers say it’s the combination of microsetae and nanogrooves that hold the air, forming a cushion where leg meets water.
Using a quartz-fibre model of a strider’s leg, Jiang and Gao convinced themselves that wax alone couldn’t account for either the leg’s water resistance or its strong supporting force, about 15 times the insect’s total body weight. The legs, which can displace up to 300 times their own volume, allow the striders to ride out surface turbulence; when raindrops hit the water, the striders bounce. The microstructures also explain how water striders can pursue prey at a speed of 100 body lengths per second.
So? Well, Jiang is with the Chinese National Center for Nanoscience and Nanotechnology. And the authors note: “Our discovery may be helpful in the design of miniature aquatic devices and non-wetting materials.” Once again, the biomechanics of a small obscure creature may point the way to a technological breakthrough.
I was reminded, of course, of the gecko toes. A couple of years ago, researchers at UC Berkeley and Lewis and Clark College figured out how a tokay gecko—a mid-sized, highly vocal Southeast Asian lizard—could support itself by a single toe on a smooth vertical surface. The secret was setae, again: tiny hairs on the toetips, each divided into hundreds of thousands of pads called spatulae, 10 millionths of an inch across. When the pads contact a surface, something happens at the intermolecular level: Van der Waal forces come into play, creating a strong attraction between the molecules of the pad and those of the surface. The interaction generates a bond 1000 times greater than the lizard actually needs to cling to the wall. As it climbs, the gecko rolls its toe-hairs onto the surface, then peels them off like tape.
With an adhesive force so strong that a dime-sized patch of setae could support a 45-pound child (not that you would want to try this at home, of course), the spinoff potential is obvious. It’s probably just a matter of time until gecko velcro is available in stores. There’s a whole branch of research called biomimetics that looks at natural structures and processes for possible industrial replication. Another researcher has just invented a plastic lens that mimics the sophisticated eye of the octopus. And at this point we’re just scratching the surface.
I’ve always been troubled by the pragmatic argument for conservation, the case for saving species because they may turn out to be valuable new food sources, or to hold the key to a cure for cancer. (And if they don’t, we can waste them with a clear conscience?) But the water strider example suggests that the most unlikely creatures may have unexpected utility.
Speaking of walking on water, Tonia Hsieh at Harvard has just figured out how the Jesu Cristo lizard of Central America, also known as the basilisk, does it. Hsieh says a large upward-and-sideways force is produced each time the lizard’s fringed toes hit the water, countering its natural tendency to sink. As with the water strider, faith does not appear to be involved.