National Geographic : 2010 Apr
PHOTO: HIDETA NAGAI. GRAPHICS: DINARA SAGATOVA SCIENCE Up a Slippery Slope Gazing at a pond, you see a smooth surface. A tiny water- walking bug sees difficult terrain. Everywhere the water meets a leaf, a twig, or the shore, the sur- face curves up a fraction of an inch. The result is a meniscus, from the Greek word for "moon," whose crescent shape is much like the water's slope. As you climb a hill, friction between your foot and the ground pushes you upward. But most water-walking insects skate on tiny pockets of air, making for a very slippery world. So how does such a bug scramble up a towering meniscus to lay eggs on a leaf or escape a predator? MIT researchers John Bush and David Hu say it uses the same forces that clump breakfast cereal together in a bowl of milk. Watching high-speed videos, they saw that an insect approaching a meniscus reshapes the water surface below its body (graphics, below). Much as a trampoline gains energy when you stand on it, the surface of the water has more energy when a bug distorts it. Bugs use this energy to launch themselves up the meniscus---and away from the jaws of a hungry bass. ---Juli Berwald 0.08 inch Water Meniscus � Aquatic plant 1 The bug walks to the meniscus---the curved water surface that forms because water is attracted to the plant. 2 To reshape the surface, water- grabbing claws on front and back legs pull up, and middle legs push down. The surface gains energy. 3 Energy from the reshaped surface pushes the bug up the meniscus. Grabbing the leaf, it pulls itself off the water. A water strider distorts the surface of a puddle with its legs.