MIT mathematicians have discovered how certain insects canclimb what to them are steep, slippery slopes in the water's surfacewithout moving their limbs -- and do it at high speed.
Welcome tothe world of the tiny creatures that live on the surface of ponds,lakes and other standing bodies of water. There, "all the ruleschange," said David Hu, a graduate student in the Department ofMathematics and first author of a paper on the work to appear in theSept. 29 issue of Nature.
For the last four years, Hu and JohnBush, an associate professor in the department, have been studying thenovel strategies these insects use to navigate their environment. To doso, they took high-speed video of the creatures using a camera providedby MIT's Edgerton Center, then digitized and analyzed the images.
In2003, the two and Brian Chan, a graduate student in the Department ofMechanical Engineering, reported in Nature how some of these creatureswalk on water. Both that paper and the current one were Nature coverstories.
Now Bush and Hu are describing how three species ofinsects are able to climb the slippery slopes, or menisci, that arisewhen the water surface meets land, floating bodies or emergentvegetation.
Why would they want to leave the water? "There are many reasons, such as laying eggs or escaping predators," said Hu.
Menisciare all around us--picture the slight upward curve of water in a glasswhere it meets the side. "But we don't notice them because they're sosmall, only a few millimeters in height," said Hu. But if you're acreature that's much smaller than that, those slopes "are likefrictionless mountains," Hu said. "Plus, it's slippery."
In theseconditions, the insects' normal modes of propulsion won't work. Hu andBush took high-speed video of insects trying to ascend menisci with arunning start and found they got partway up, then slid back down.
Thesolution? The creatures adopt special postures that create forces thatpull them up the slope at speeds of almost 30 body lengths per second(for comparison, an Olympian sprinter moves at about five body lengthsper second).
For example, Hu and Bush found that two species ofwater treaders have retractable claws on their front and hind legs thatallow them to "grasp" the surface of the water and pull it into aminiscule peak. The insect simultaneously presses down on the waterwith its central pair of legs, forming dimples in the water surfacethat bear the creature's weight.
Because the insects are sosmall, these perturbations create forces that suck them up the slope,similar to the way champagne bubbles rise to the edge of a glass.
Bushexplains that the insect is actually "generating tiny menisci" with itsfront and hind legs. Since menisci are attracted to other menisci, thecumulative effect is to pull the insect up and over the meniscus at thewater's edge.
Remember the champagne bubbles? Each essentially forms its own meniscus, hence the attraction to the edge of the glass.
Thelarva of the waterlily leaf beetle solves the same problem a differentway. The sluglike creature simply arches its back, creating menisci ateach end. The effect has the same end result, propelling the larva upthe slope.
Bush and Hu got involved in this work because theywanted to explain how these creatures do what they do. Bush notes,however, that "the physics is also of interest to people working innanotechnology because they, too, are concerned with problems at verysmall length scales."
Hu will be defending his thesis on Sept. 28.
This work was sponsored by the National Science Foundation.
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