Row, Row,
Row Your Bug
Colorful Experiments Solve Water Strider
Mystery: Their Middle Legs Serve as Oars
By Guy Gugliotta
Washington Post Staff
Writer
Thursday, August 7, 2003; Page
A06
Scientists have deciphered the mystery of how the tiny
water strider -- a familiar denizen of your local pond -- gets
around: It uses its middle set of legs as oars and rows along the
water's surface at a rate of almost 31/2 feet per second.
In a series of colorful experiments reported in today's
issue of the journal Nature , a team of researchers led by
Massachusetts Institute of Technology mathematician John W.M. Bush
used high-speed video and blue-dyed water to track striders as they
darted past the camera, leaving vortices as neat as those made by a
sculler in a boat race.
The experiment answered a question that had long puzzled
scientists, who had assumed that striders developed momentum using
the tiny waves they generated as they flapped their legs across the
water's surface.
"When you look at them, they move so fast that you can't
see the vortices, so all you see is the waves," Bush said in a
telephone interview. "But the baby water striders' legs aren't big
enough to generate waves. There had to be something else."
The Bush team's experiments -- to understand and perhaps
replicate the way animals move -- evoked a scientific tradition at
least as old as classical Greek mythology, when the inventor
Daedalus fashioned wings so he might fly as a bird.
With burgeoning interest and funding from security
agencies, the discipline has blossomed in recent years as
"biomimetics" -- building devices that mimic the movements of
creatures such as lobsters, crabs, scorpions, lampreys, flies or
honeybees, and using the devices in applications ranging from
mine-clearing to reconnaissance and eavesdropping.
"There's no doubt that goals have shifted toward
biologically inspired robotics," said California Institute of
Technology bioengineer Michael H. Dickinson, an expert in insects'
flight behavior. "But for people like John and myself, the main
target is to figure out how the world works, and if that leads to
novel devices -- that's great."
The key in mathematically describing the way animals move
is to keep in mind Newton's Third Law of Motion, Dickinson
explained. For every action there is an equal or opposite reaction,
or, as Dickinson phrased it in an explanatory article accompanying
the Bush team's research, "To move forwards, animals must push
something backwards."
Water striders presented an interesting problem. They are
tiny insects -- about 0.4 inches long -- whose six legs are covered
with thousands of fine hairs that make them virtually waterproof.
They float on the surface of standing water through surface tension
-- like standing on a trampoline.
The strider's stubby forelegs and slightly less stubby back
legs are used primarily as stabilizers, floats or even skis.
Propulsion comes from the two long middle legs. "A lot of animals do
a lot of swimming or flying," Bush said. "But what's so nice about
this research is that water-walking is on the interface."
Striders are hard to analyze because in one second they can
travel about 40 inches -- 100 times their body length -- making it
impossible for the human eye to follow them. Most people can see
them only when they are stationary, and only because the adults
leave tiny surface ripples, called "capillary waves," that attract
the eye.
Knowing that the young are incapable of generating
capillary waves, Bush's team needed a way to map exactly what
happens when the striders skitter across the water.
The team members used a video camera capable of taping the
striders' movement at 500 frames per second. They spiked the water
with microscopic particles that reflected light in such a way that
they could show current patterns generated by the insects.
Finally, the team sprinkled the top of the aquarium with a
layer of blue dye that would scatter as the strider ran across it,
thus describing the surface movement. The aquarium was lit from the
bottom.
The result was a set of elegant color photographs that show
clearly spaced whorls marking the spots where the striders' legs
caught the water and "rowed" their bodies forward. The whirlpools
eerily mimic the characteristic "puddles" left by oarsmen as they
drive their shells forward. Capillary waves, Bush said, are simply a
side effect.
"I'm impressed," said Northeastern University biologist
Joseph Ayers, who has modeled the motion of lobsters in the sea.
"The way these folks used these particles to look at the structure
of flow vectors is really interesting, and in terms of pretty
science, the images are like [Van Gogh's] 'Starry Night.' "
To show "proof of concept," the team tested its research by
building a mechanical water strider with stainless steel legs,
elastic muscles, an aluminum body and a spring-loaded driver to move
the legs. It was 10 times the size of the lab striders, but only
half the size of the eight-inch Asian giant water strider. It worked
fine, Bush said.
"I like to apply mathematics to things that are poorly
understood and explain them," he said. "As far as practical
applications, nothing leaps out at me, but it's a beautiful aspect
of the natural world."
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