They Walk on Water

It hasn’t rained that much this month. Streams are very low, and fish and tadpoles are concentrated into smaller, deeper water, leaving a bonanza of potential food for Great Blue Herons (Ardea herodias) that are hunting. On a walk recently along a quieter, calmer streamside area, movement caught my eye. Lots of water striders were practically bumping into one other as they skimmed across the water’s surface. I made a video of their activity.

These water striders have been here since the beginning of April, when I was rather surprised to see them out so early. Because they overwinter as adults, those had probably just emerged from dormancy and had spent the winter in nearby leaf litter. Many of the water striders that emerge now are wingless and so unable to fly. But those that hatch in late summer and fall have wings and can fly back to their overwintering sites.

Water striders show polymorphism, meaning “many forms.” If their pond or stream begins to dry up for the current, wingless generation, the next generation will have wings, enabling them to colonize new, more suitable habitat.

The insects I saw are probably the Common Water Strider (Aquarius remigis), a species found throughout North America. The scientific name translates very aptly as “water rower.” It is a member of the family Gerridae, one of over 1,700 species worldwide, and in the order Hemiptera, the true bugs.

Water striders are known for their ability to walk on water, hence the nickname “Jesus bug.” They do that by using the high surface tension of water combined with long, hydrophobic legs to distribute their weight. Their legs and “feet” are hydrophobic because they are loaded with tiny, water-repellent hairs. These hairs also capture tiny bubbles of air which help them float. There are also fringed hairs along their middle legs that thrust the water strider forward. The back legs help to steer and brake. This buoyancy allows them to skim across the water’s surface at relatively fast speeds for their size.

A water strider is a highly efficient predator. Its front legs can sense vibrations that prey make in the water, such as the “snorkel” of a mosquito larva poking through the surface to breathe. It will then grab the prey with its front legs. Like all members of the Hemiptera, water striders have piercing, sucking mouthparts. Digestive enzymes break down prey into a liquid and the water strider then sucks up the juices. This efficient insect predator does its part in the balance of nature. And for some of us, anything that eats mosquito larvae can’t be all bad.