Invertebrates
Water Strider
Gerridae · also called pond skater, water skipper, Jesus bug
The water strider does something that looks like a magic trick and is really a physics lesson: it stands, walks, and sprints on top of the water without sinking. Look closely and you can see the four small dents its legs press into the surface, each one a tiny trampoline holding it up.
Overview
Water striders are a family of true bugs, the Gerridae, found on the surface of almost every pond, ditch, and slow stream in the world — and, uniquely among insects, on the open ocean (the genus Halobates). They are slender, long-legged, and dark, and they eat insects that fall onto the water, including mosquito larvae.
Held up by hairs
Each leg is coated in thousands of microscopic water-repellent hairs that trap air and keep the leg from piercing the surface. The insect’s small weight is spread across the surface film, which pushes back where each leg dimples it.
Rowing and reading ripples
The middle legs row like oars; the hind legs steer. The strider finds prey by feeling the ripples a struggling insect makes, and it sends its own ripple signals to rivals and mates. It can also jump off the water without a splash — a trick copied by water-walking robots.
Status
Least Concern — abundant on fresh water worldwide, with the marine species common across the tropical seas.
Water Strider: common questions
How does a water strider walk on water?
It uses surface tension, the skin-like force at the boundary of the water caused by water molecules pulling on each other. That force is weak, so most animals break through it, but the water strider is very light and its legs are built specifically not to break it. Each leg is covered in a dense pile of tiny hairs, and each hair is itself grooved at the nanometre scale, so the leg is extremely water-repellent and traps a cushion of air. When the strider stands, its legs press small dimples into the surface without piercing it, and the upward push of the stretched surface film across all those dimples balances the insect's weight. Its legs also spread its weight over a wide area, further reducing the pressure at any point.
Do water striders bite people?
Practically never, and it would barely register if one did. Water striders are true bugs with piercing mouthparts, which they use to stab small prey and suck out the contents, and a large one handled roughly might give a tiny prick, but they are not aggressive toward people, do not seek out anything human-sized, and carry no venom or disease of concern. They are entirely harmless to swimmers and are in fact useful, since they eat mosquito larvae and pupae at the surface and clean up dead insects.
How do water striders find their prey?
Mainly by touch, through the water surface. When an insect falls onto the water and struggles, it sends out concentric ripples, and the water strider's legs are sensitive enough to detect those waves and work out the direction and rough distance of the source, then row toward it. Vision helps at close range. The same channel is used for communication — water striders drum and vibrate their legs to produce ripples at particular frequencies that carry specific meanings, a courting male and a rival male, for instance, use different signals, so the pond surface is both a hunting ground and a message board.
Where do water striders go in winter?
In cold climates the adults leave the water in autumn and overwinter on land, tucked into leaf litter, under bark, among grass tussocks, or in other sheltered, frost-protected spots near the pond, with their metabolism slowed right down. They do not survive frozen into the ice. In spring they return to the water, mate, and lay eggs on floating or submerged vegetation, and the new generation matures over the summer. In warm regions water striders may be active year-round. The marine species simply stay on the ocean surface throughout the year.