It's a bug's life: MIT team tells moving tale
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September 28, 2005
MIT mathematicians have discovered how certain insects
can climb what to them are steep, slippery slopes in the water's surface
without moving their limbs -- and do it at high speed.
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Welcome to the world of the tiny
creatures that live on the surface of ponds, lakes and other standing bodies of
water. There, "all the rules change," said David Hu, a graduate student in the
Department of Mathematics and first author of a paper on the work to appear in
the Sept. 29 issue of Nature.
For the last four years, Hu and John Bush,
an associate professor in the department, have been studying the novel
strategies these insects use to navigate their environment. To do so, they took
high-speed video of the creatures using a camera provided by MIT's Edgerton
Center, then digitized and analyzed the images.
In 2003, the two and
Brian Chan, a graduate student in the Department of Mechanical Engineering,
reported in Nature how some of these creatures walk on water. Both that paper
and the current one were Nature cover stories.
Now Bush and Hu are
describing how three species of insects are able to climb the slippery slopes,
or menisci, that arise when the water surface meets land, floating bodies or
emergent vegetation. Why would they want to leave the water? "There are many
reasons, such as laying eggs or escaping predators," said Hu.
Menisci
are all around us -- picture the slight upward curve of water in a glass where
it meets the side. "But we don't notice them because they're so small, only a
few millimeters in height," said Hu. But if you're a creature that's much
smaller than that, those slopes "are like frictionless mountains," Hu said.
"Plus, it's slippery."
In these conditions, the insects' normal modes of
propulsion won't work. Hu and Bush took high-speed video of insects trying to
ascend menisci with a running start and found they got partway up, then slid
back down.
The solution? The creatures adopt special postures that
create forces that pull them up the slope at speeds of almost 30 body lengths
per second (for comparison, an Olympian sprinter moves at about five body
lengths per second).
For example, Hu and Bush found that two species of
water treaders have retractable claws on their front and hind legs that allow
them to "grasp" the surface of the water and pull it into a miniscule peak. The
insect simultaneously presses down on the water with its central pair of legs,
forming dimples in the water surface that bear the creature's weight.
Because the insects are so small, these perturbations create forces that
suck them up the slope, similar to the way champagne bubbles rise to the edge of
a glass.
Bush explains that the insect is actually "generating tiny
menisci" with its front and hind legs. Since menisci are attracted to other
menisci, the cumulative effect is to pull the insect up and over the meniscus at
the water's edge.
Remember the champagne bubbles? Each essentially forms
its own meniscus, hence the attraction to the edge of the glass.
The
larva of the waterlily leaf beetle solves the same problem a different way. The
sluglike creature simply arches its back, creating menisci at each end. The
effect has the same end result, propelling the larva up the slope.
Bush
and Hu got involved in this work because they wanted to explain how these
creatures do what they do. Bush notes, however, that "the physics is also of
interest to people working in nanotechnology because they, too, are concerned
with problems at very small length scales."
Source: MIT
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