|
Water striders have long baffled scientists with
their ability to deftly skate atop ponds and rivers. Now
a study shows that the delicate insects paddle with
their legs to whip up whirlpools that thrust them
forward. All without getting even a drop of water on
their bodies.
For at least a decade researchers thought that water
striders are pushed along by short waves created in
their wake as their legs slide around across the water's
surface. If this theory were true, baby striders whose
legs are too slow to generate waves shouldn't budge. But
the infants don't have a problem with water walking, and
scientists were left perplexed. This puzzle intrigued
John Bush, an applied mathematician at the Massachusetts
Institute of Technology.
Bush and colleagues filmed the bugs in a laboratory
aquarium using a high-speed video camera. They noticed
that the insects shifted their two middle legs in a
rowing motion as they danced across the water. When the
group then added a blue dye to the water, they saw
swirling eddies directly below the creatures' moving
legs. The vortex effect was even more obvious when the
researchers put small trackable particles into the pool
and watched the patterns they made under the striders.
Combining these observations with a few mathematical
calculations revealed the answer: By paddling and
pushing as hard as they could on top of the water
without penetrating it, the striders created strong
semicircular currents that pushed them forward, the team
reports in the 7 August issue of Nature.
To put their theory to the test, the group built an
aluminum strider about 10 times the size of a normal
strider. Robostrider was able move just like its live
counterpart, making swirling whirlpools below its
quickly revolving legs without getting wet. The
subsurface vortices turn out to have even more momentum
than the waves that the adults make in their wake, Bush
adds. And even the smallest striders, which can't make
waves, can still stir up the semicircular currents, Bush
calculates.
"It's a very nice piece of work that contributes a
real understanding of what propels these organisms,"
says Steve Childress, an applied mathematician who
studies hydrodynamics and locomotion at New York
University.
--APARNA SREENIVASAN
Related sites
Applied Mathematics
at New York University
John Bush's Web
site at MIT | 
