12 Feb 1998
Action and reaction is handy stuff. Action: a jet engine forces burned fuel backwards. Reaction: the plane zooms forward. Action: a bicycle tire pushes the road -- and the Earth -- back. Reaction: the bike moves forward.
Photos courtesy Robert Suter, Vassar College
Action and reaction is not just handy: it's about the only thing that makes anything move. It's as plain as Isaac Newton and his laws of motion.
So what's the deal with the simple water spider? This eight-legged marvel speeds across the water fast enough to escape predators, find prey, and chase off intruding spiders. But how does it move? It must be pushing the water backwards, but what is doing the pushing?
That kind of question interests Vassar College biology professor Robert Suter. Shunning the glamorous study of elephants and Tyrannosaurus rexes, Suter is fascinated by the small. He started this Lilliputian quest by looking at spiders that ride strings of silk into the sky and found that, in a decent breeze, a single 10-centimeter strand of silk could lift a small spider.
Now Suter has turned his attention to a spider that doesn't fly. This one literally walks on water. The fisher spider (Dolomedes triton), a pond-dwelling species found in much of North America, is one of about 15 spiders that can turn this trick. Two groups of insects, including water striders, can do likewise.
Like a seven-year-old boy, it hates water
Surprisingly, Suter is not trying to figure out why the organisms don't drown. That clever feat is well understood to be a result of surface tension, a force between a liquid and solid materials that resist being wetted by that liquid.
In the presence of surface tension, a liquid "tries" to minimize its surface area. This explains why water beads up on a waxy surface -- because that the lowers the surface area of the droplet. Surface tension even explains why wet sand makes better sand castles.
Objects like waxes and glass that resist being wetted by water are called "hydrophobic." The hydrophobic legs of fisher spiders and other water-walking arthropods create so much surface tension that they barely touch the water. Yet because the animal has weight, its legs makes dimples where they do contact the water, but the animal still stays on top.
No friction, no movement?
But since the legs barely contact the water, there's very little friction, and here's the rub, or lack of it. Friction is the mechanism land creatures use to push themselves forward. But with so little friction, the spider can't push water backward to start the action-reaction equation.
It's a paradox: the same phenomenon that allows the spider to stay on the surface seems to prevent it from moving. Yet since the spider does move, there must be a flaw in the reasoning -- a fly in the ointment.
Using calculations and high-speed photography, Suter discovered that spiders "row" across the water by exerting force on the dimples of water under their legs. The dimple creates drag on the water, allowing the leg to push the water backward while barely touching it. In reaction, the spider moves forward.
Drag is the same mechanism that moves a rowboat, Suter points out: an oar pushes water backward, and the boat reacts by moving forward. Like somebody rowing a boat, fisher spiders lift their legs on the forward stroke. Otherwise, they'd push water in both directions, and would stay in one place.
However, while an oar can work at any depth, the spider's leg must be at the surface: a submerged leg would not create the dimple, and would move much less water.
Doin' the locomotion
A fisher spider in a real hurry doesn't bother to row, Suter says. It "gallops" across the water, actually lifting itself off the surface briefly while sprinting at up to 75 centimeters per second. Galloping is usually a response to a threat, and it does not use the "dimple" mechanism we described above. Rather, they do a kind of slapping stroke that certain lizards use. "Most of the propulsive parts of the legs are momentarily submerged during galloping," Suter says.
When they're feeling lazy, fisher spiders have other ways of getting around, Suter says. They may stand on tiptoe and sail downwind, or prostrate themselves and stick two legs aloft to catch the breeze.
Suter is to first to admit that the study of water spiders has virtually no practical applications. So why bother? Because these small organisms live in a world with different rules, and that alone makes them worth the effort. "I'm interested in the way organisms do things when they live in a different physical environment." Suter says. "The rules are different, and the smaller you get, the more different the rules become."
Small organisms have a high ratio of surface area to volume, making them more responsive to surface forces like surface tension and adhesion. As size increases, Suter notes, the surface area to volume ratio decreases, making large organisms more responsive to gravity and inertia.
The all-important ratio also explains why we couldn't walk on water, even if our feet were hydrophobic. With our much greater mass, the perimeter of our feet -- where all surface tension takes place -- would have to be huge.
However, Suter has calculated that surface tension would support a half-kilogram spider. That's a body as big as your fist. Think about that next time you're watching some quarter-sized water striders dance across a lake.
— David Tenenbaum
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