Navigating
your way out of a puddle might not sound like a challenge, but to a
tiny insect it is akin to scaling an extremely steep and slippery
mountain. Two mathematicians have outlined exactly how little insects
manage this feat: they turn surface tension to their advantage and
'surf' up the edge with scarcely any effort.
The
mountainous ridge of water at a puddle's edge arises thanks to
capillary attraction, which causes the liquid to shoot up where it
touches the surrounding land. This feature, called a meniscus, is also
commonly seen around the edge of a glass of water.
Large insects such as water striders (Gerridae),
which are often found on pond surfaces, are fast enough and big enough
simply to hurdle a puddle's meniscus when they come to it. But smaller
insects can't do this.
Instead,
insects that are just a few millimetres long deform the surface of the
water with their legs, creating forces that shoot them to the top of
the watery hill.
It's a very striking means of locomotion.
John Bush, Massachusetts Institute of Technology
The
insects are taking advantage of the same effect that causes pieces of
cereal to come together in a bowl of milk, says Bush. Objects that
deform a liquid and increase surface tension tend to attract each other
once the deformed regions overlap, because this minimizes the overall
amount of deformity, and therefore tension.
"It's
a very striking means of locomotion," says John Bush of the
Massachusetts Institute of Technology in Cambridge, Massachusetts, who
studied the phenomenon with his colleague David Hu. "It's rare. It may
even be unique."
Wet and wild
Bush and Hu studied three insect species: Mesovelia and Microvelia, which walk on water as adults, and larvae of the beetle Pyrrhalta,
which perform a similar trick. They used high-speed video to capture
the bugs in action, and then analysed the performance. They report the
results in Nature1.
The
insects all use their legs to pull the water's surface up at the front
and rear, while pressing downwards with their middle segments. Insects
that live on ponds and puddles tend to have bodies that are generally
water-repellent. But the insects in this study possess retractable
'wetting' claws that attract and hold the water's surface, allowing
them to pull it upwards and out of shape.
The
uplifted bit of water under the insect's foot then becomes an area of
particularly high tension, as is the portion of the puddle's meniscus
at the very edge, where it is steepest. Like bubbles on the surface of
a glass of champagne, these two areas of high tension attract each
other in order to lower the overall tension of the water surface. This
attraction pulls the insect to the top of the hill.
"They scamper onto the menisci and are sliding down under gravity, then they lock into position and travel up it," says Bush.
Bush
adds that the insect's front legs are mostly responsible for the
effect. Their middle legs press down to support their weight and
prevent them sinking, while the rear legs pull upwards again for
balance, preventing the insect from flipping over in a backwards
somersault.
The
technique makes for a speedy manoeuvre. The insects surf up the
meniscus at around 30 body lengths per second; the fastest human
sprinters only manage about five.
It's a very striking means of locomotion. 
