The Water Strider

My favorite river surface wafter is the aquatic insect known as the water strider. I actually believe that the name "water wafter'' would be far more appropriate for this insect. They only appear to stride because their huge, double pair of extended legs look as if they were made to do just that. On a calm day on the river, several species are noticeable with elongated body sizes ranging from a quarter of an inch to well over an inch long. Most in our area are classified as belonging to the genus Gerris. Some are winged and some are wingless. Water striders are restful to watch. Their effortless movements on the water surface are matched by no other aquatic animal.

I usually find individuals of the larger species in small groups. When I approach them in my boat they simply glide out of the way with a casual wafting stroke or two. Members of the smaller species congregate in large surface colonies. I can spot a dark, active mass on the water from quite a distance. If the colony turns out to be in the path of my boat, they become agitated, jumping frantically up and down. I have heard many amused children describe them as "popcorn bugs.'' As I pass by they calm down, spin about on the water for a couple of minutes and then reassemble in another tight resting colony.

The best way to observe them is to quietly watch several large ones from a distance of a couple of yards. They have two basic types of movement. The first is a slow, wafting motion created by simply rowing the middle pair of legs and letting the hind pair trail behind. This is a delightful movement to watch, as the small suspended body is sent gliding over the surface film until it slows to a rest again. This motion is employed when the insect is confronting fellow water striders. A rapid jump, engaging the hind legs as well, is used to escape danger or to grab its prey, which include mosquito larvae and other smaller insects that either rise to the surface or fall from the air.

The strider looks as if he has only four legs, forming a giant X pattern on the water. But all insects have six legs and the water strider has two shorter additional legs that are folded up front for grabbing prey. The insect's claws or feet are not at the ends of its legs, but are located farther back so as not to interfere with its delicately balanced water ballet. The ends of the legs that touch the water are covered with tiny hairs that increase the surface area of contact but are small enough not to break the surface tension of the water. So, in reality, the waft is in the hair! Shaving the legs of a water strider would be a disaster of titanic proportions. The gentle pressure of the hairy legs creates a dimple in the water surface. If seen in direct sunlight, these four dimples cast an enlarged, rounded shadow onto the river bottom if the river is shallow. To the observer, each rounded shadow may appear as a foot pad.

This aquatic insect communicates by making tiny ripples on the water surface with its legs. It is thought that there are two basic signals by the male at frequencies of between ten and thirty vibrations per second. One is used to attract females for mating, the other to show territorial aggression to intruding males. The location and type of other surface objects, including prey, are also discerned through its surface radar. When prey is detected, the water strider zooms over and grabs it with its two front feet and stabs it with its beak. Digestive juices then flow into the victim that actually dissolve its internal organs. This delicious brew is then sucked back out by the strider leaving an empty carcass to cast aside like jetsam.

While water striders appear serene from my dominant perspective above the water, I sometimes like to imagine how this remarkable animal must look from the perspective of an even smaller insect caught in the surface film. A huge, stilted column comes gliding up on the horizon. A mechanical sounding click issues forth each time it rows its middle legs in my direction. Huge, severe looking eyes scan the river. I freeze, hoping not to agitate the water and give away my presence. But I cannot hold my breath any longer and I let out a gasp. Shock waves fan out across the surface and the strider takes two quick hops and sends its javelin-like bill deep into my helpless, wriggling body. I am liquefied ...

Photo by Charles Lewallen: water strider