Water striders dash, leap and scramble across the water if they are disturbed, moving at a rate of about a meter per second. At other times they may move quite slowly. They do not swim. Instead they glide over the water surface by pushing horizontally against the water, much as a human sprinter pushes against starting blocks. They locate one another by means of the waves they generate as they move. They also seem to locate objects in their environment by means of waves.

Freshwater striders spend the winter in hiding under rocks or vegetation or lying on the bottom of a stream or pond. In the spring they become active, move to the surface and mate. Eggs are left by the female on submerged objects. After two weeks the nymphs hatch and move to the surface. Maturing to the adult stage takes a little more than a month.

The water striders I studied are skittish insects that apparently can see well enough to detect my approach. When I waded through streams, they were obviously also scared off by the waves I made. I learned to move slowly and then stand still for a long time so that they would eventually ignore me.

I caught several striders in a small, fine-mesh butterfly net. The long handle enabled me to avoid tipping off my presence. It was hopeless to try to chase them; they could easily outrun me in the water. When I caught one, I dropped it into a small glass jar. With this last I had to be quick; the insect could jump several centimeters in the air and out of the jar before I got the lid on.

I believe the striders I saw were all of the species Gerris remigis. Like all freshwater striders they had some kind of wing growth along their back. (Marine water striders are wingless.) The specimens I caught ranged in length from less than half a centimeter to about 1.5 centimeters. The immature ones were only a few millimeters long.

I rarely saw any striders along the edges of lakes, ponds or streams more than a few meters wide. On smaller streams the striders were absent from turbulent water and from places where algae covered the surface. They were also absent from regions with sparse vegetation. Areas with slowly moving water and some emergent vegetation were heavily populated with the insects. The water they seemed to like best was only a few centimeters deep. Presumably the striders congregate there because the water is too shallow for any fish large enough to eat them.

The striders were easy to detect. As soon as I waded into an appropriate habitat the water surface became a frenzy of small ripples as they scurried off. On my first explorations I found dozens of areas covered with small water striders. Then in a stream I happened on a pool busy with large ones.

For hours I sat on the shore or squatted in the water to watch their movements. Later I caught several of them and released them into a plastic container filled with water. If I took care to move slowly to avoid scaring them, they would stay on the water surface, dashing about, and would not attempt to climb over the edge.

I lay next to the container with a 20-power magnifying lens to examine how the insects manage to stand and to move on the water. Patience was called for. I waited until an insect moved into the focal plane of the lens. Then I could follow it for a short time before it became alarmed and skated off.

To photograph striders on streams I put a telephoto lens on my 35-millimeter camera, a single-lens reflex model. When I had an insect on the water in my plastic container, I hovered patiently and motionlessly with a close-up lens on the camera.

If you intend to photograph these insects in nature, you would do well to eliminate some of the light reflected from the water surface. Otherwise the photographs will be dominated by images of the surrounding trees. You can diminish the reflections by taping a polarizing filter over the lens. Mount the filter so that its axis is vertical.

All water striders have three pairs of legs. The front legs are usually short; they help to support the insect. On the striders I caught the other legs are much longer than the body. The two middle legs are primarily responsible for propulsion. The two rear legs may also aid in propulsion, but more often they serve for steering as the insect glides on the surface.

Each leg consists of several segments. From the body outward the segments are the coxa, the trochanter, the femur, the tibia and the tarsus. The tarsus itself may consist of several segments.

The long-legged water striders are distinguished by the position of a claw on the last tarsal segment, just above the end of the leg. The location of the claw may help the insect when it stands on the water, taking advantage of surface tension.

A stationary water strider rests its weight on all six legs. On the front and middle legs only the tarsus touches the water. On the rear legs the tibia and the tarsus touch. These sections are not submerged but lie in shallow depressions in the water surface. The depressions for the front legs are small. Those for the other legs are elongated because more of the leg is in contact with the water.

When I am close to a water strider with the sunlight at the right angle, the depressions are easily spotted. At other times they are apparent in the shadow the insect casts on the bottom of the stream or the container. There I see a slim shadow for the body itself and more prominent dark ovals at the end of the thin shadow of each leg. The ovals result from the bending of light rays by the curving of the water surface at the depressions.

With a flat water surface the rays of sunlight illuminating a given region would all refract into the water in the same way. Thus they would evenly illuminate the bottom of a stream or a container. When instead the rays pass through the depression surrounding a tarsus of the insect, they are sent off to. the sides. The shadow cast on the bottom is therefore considerably larger than the tarsus. These large shadow proved to be useful. When the insect moved a leg, I had better luck following the oval than the leg.

Often the striders glide slowly, presumably searching for food. At other times they dart over the water. The motion is always in a straight line. At the end-of a glide the insect stops, repositions one middle leg or both of them to reorient its body and then propels itself forward again.

Although I can follow the slow movements, I cannot see the details of the fast ones. In a slow movement the insect propels itself by moving its middle legs toward the rear. The rear legs are nearly stationary but might move slightly rearward too. The front legs appear to serve only as a support.

The faster movements have been recorded in high-speed motion pictures. Here the legs serve basically the same functions but the rearward motion of the middle legs lasts only about 20 milliseconds. The acceleration is large, about 10 times the acceleration of gravity. The front legs are momentarily lifted from the water surface, as is the upper end of the tibia on each rear leg. After the acceleration both sets of legs are returned to the water surface and the insect glides. The rear legs then serve as stabilizers to hold the glide straight. Eventually the kinetic energy of the movement is dissipated and the insect skids to stop. The energy is lost to the waves generated by the movement and to the friction between the insect and the water surface.

The propulsion from the middle legs involves several rotations. The tarsus and the tibia rotate about the joint between the tibia and the femur faster than the femur rotate-s about its attachment to the trochanter. These two rotations push the tarsus against the rear wall of the depression in which it initially rests. The resistance of the water to the push supplies the force of propulsion.

In this motion one can see two advantages of the long middle legs of the insect. First, the length provides a long lever arm for pushing against the water surface. A human oarsman likewise finds that a long oar makes for easier rowing.

The second advantage is that the long tarsus provides more friction with the water, increasing the effectiveness of a push of the leg backward. The front legs have shorter tarsi so that the friction there is minimized, since friction shortens a glide. Some species of water strider have fans deployed from the tarsi of the middle legs to aid the push against the water. Some have projections from the legs that effectively grab the water surface, as metal studs on a snow tire bite into the snow for traction.

Much of the support for the insect as it rests or glides arises from the surface tension of the water. Surface tension is the name given to the cohesion of the water at the surface. In textbooks this phenomenon is described in terms of the electrical attraction between the water molecules. Consider one molecule that lies on the surface of pure water. Electrical forces pull it horizontally toward its neighbors, but all those forces balance: out by symmetry. The molecule is also pulled downward by the neighbors just below it, but it cannot enter the bulk water because of collisions with those underlying molecules, all of which are in motion. Thus the surface is said to be in a state of tension because every molecule on it is subject to a net downward electrical force.

A second way of examining the surface is in terms of energy. Suppose the surface is to be expanded. If more surface is to be gained, molecules must be brought there from the bulk water. Such a molecule initially has no net force acting on it because on the average the forces from its neighbors balance out. At the surface, however, a net downward force develops. The movement from the bulk to the surface therefore requires energy from whatever is causing the expansion of the surface. In this description the surface is said to be in a state of tension because the water attempts to decrease its energy by contracting its surface.

The surface tension of water can be surprisingly strong. If a sewing needle is laid horizontally and carefully on water, the surface tension can hold the needle in place. The support comes from the depression of the surface produced by the weight of the needle. At first the needle starts to sink, curving the water surface, but then the surface begins to put a net upward force on the needle. Part of the upward force is due to buoyancy (because the needle has displaced some of the water). The rest of the upward force comes from the tension along the curved surface of the depression. When the demonstration is done right, the needle receives enough upward force to balance its weight, and so it floats.

Similar support is given to an insect resting its weight on the tarsi of its legs. In addition the insect has evolved other support mechanisms. Closely spaced fine hairs on the legs and body of many species of water strider trap a layer of air. The hair, legs and body may also be coated with a waxy material to resist wetting.

The layer of trapped air provides additional buoyancy. I demonstrated the presence of air by dunking a water strider. Underwater the air on its surface shimmered in the sunlight. When I released the insect, it quickly regained the surface and scampered off, obviously still dry.

Another source of resistance to wetting has recently been discovered in the oceanic water striders. They need it because they are more exposed to rain and splashing and rarely find any floating object they can climb on to dry off. They differ from the freshwater striders in the structure of the microtrichia, the microscopic fixed hairs on the surface of the body. In the freshwater species the microtrichia are shaped like pegs and jut outward from the surface of the body. In the oceanic species they are shaped like mushrooms, presumably to aid in trapping tiny air bubbles next to the body for additional buoyancy.

Water striders do sometimes become wet, which suggests that the waxy covering or the air trapped in the hairs can fail. I observed several water striders that seemed to sink progressively into the water. The sinking strider would climb onto a rock or a lily pad to dry off.

As I was trying to get close-up photographs of one strider I provoked it into skittering across the water in my plastic container for a long time. As it began to sink it moved to the side of the container, hoisted one leg onto the plastic and hung there for a while. If it did not find the side of the container, it groomed itself dry by rubbing one front leg along the length of the adjacent middle leg, both of which were lifted out of the water. Special hairs on the tibia of the front leg serve for removing water from the other legs in such circumstances.

The buoyancy, surface tension and resistance to wetting provide strong support for a strider when its legs are dry. The insect not only can stand on the water with all six legs but also can shift its weight to four legs while grooming. I often saw striders leaping several centimeters into the air. Although the downward push required for this maneuver created waves in the water, the insects did not break through the surface. Sometimes I saw two striders tangle in what appeared to be a fight. It always ended with one insect or both leaping into the air and then running off. Even in these leaps the insects did not break the water surface.

There is more to surface tension than I have said. Perfectly clean water is never found in nature. Even when water is thoroughly cleaned in the laboratory, its surface will be coated with a monolayer of other molecules after a few minutes of exposure to the air. Water found in nature certainly has such a top coating, which reduces the surface tension.

I believe another factor, surface viscosity, aids in the propulsion of a water strider. When the middle legs are brought rapidly to the rear, they push against the back wall of the depression they create. Because of surface viscosity the leg does not simply slide up and out of the depression in the course of the motion. It gets to give a good push. The viscosity is associated with the resistance of the top layer of molecules (the contaminating layer) to sliding over the layer of water slightly lower.

I once watched a water strider that was having difficulty in moving across a stretch of water. The water was almost stagnant and in one area was covered with a thick layer of scum that was obviously highly viscous. When I chased a strider into this area, it could no longer push off from the water and then glide. Its motion was more of a forward leap: it kicked off with its middle legs and flew through the air for several centimeters before it landed, immediately coming to a stop. After I chased it back into cleaner water it was able to glide again.

When a water strider glides, the fluid surface passing under its supporting tarsi must flow quickly into the curved shapes of the depressions. In the scummy region the high viscosity prevented this flow, but it also provided a firm support for the insect's spectacular leap.

A water strider is so agile that a female can move even while mating. One afternoon I watched a pair of the insects mating for almost an hour. They stopped only when I captured them. The male had mounted the female from the rear with his weight resting on the female and on his own rear legs. The male's front legs were around the female's body just forward of her middle legs, and his middle legs were swiveled upward. The arrangement left the female's legs entirely unhindered. As they mated the male was motionless. The female, however, continued to move slowly over the surface. She appeared to be incapable of rapid gliding but could still move in glides of a centimeter or so.

I had supposed the ability of a water strider to stand on the water and glide over it depended critically on the wide separation of the places where the legs touched the water. At each such place the curvature of the surface in the depression around the tarsus provides an upward force on the insect. What would happen if these several depressions were brought close to one another? The surface between nearby tarsi would be flatter and would provide less upward force. Hence I supposed that a water strider would be unlikely to bring its legs close together.

My supposition was wrong. Clearly the propulsion achieved by an insect demands that the middle legs be brought close to the rear legs. For an instant as a water strider is accelerating the front legs are even lifted off the water, requiring that the middle and rear legs fully support the insect. The tarsi of those legs are then quite close to each other. Yet the insect does not break through the surface.

The mating pair I monitored had most of their weight resting on the water directly behind the female. They may have been low in the water, but they were apparently in no danger of breaking through the surface layer even when the female brought her middle legs rearward for propulsion. Furthermore, the water striders of the family Velidae have short legs, and so their support regions are closer. The insects of this family are just as agile as the long-legged striders I studied.

The wave patterns the striders create when they are moving are striking. A simple push and glide builds up a circular group of waves that expand from the site of the push. The waves arise during the acceleration stage, when the insect forces each middle leg against the rear wall of the depression in which it rests.

The full rearward motion of each leg sends out a semicircle of waves to one side. Since the two legs are synchronized, the two sets of waves merge at the front and the back of the insect. The merger is nearly perfect. When I am close to a strider, however, I can see two wakes left in the group of waves expanding to the rear. I believe they arise from the rear legs.

It is difficult to analyze the production and propagation of these waves. The waves are somehow generated when a disturbance, such as the rearward push of the strider's legs, upsets the initially flat surface of a pool of water. Two forces, operating to restore the surface flatness, are responsible for the propagation of the waves away from the site of the disturbance.

When the wavelength of the wave is short (thus when the shape of the water surface is highly curved), surface tension is the important force. The waves are called capillary waves. For long wavelengths gravity is important, and the waves are said to be gravity waves. With intermediate situations one must consider both forces. In no instance is a wave traveling over water a simple sine wave, notwithstanding what is said in many textbooks.

The structure traveling over the surface is considered mathematically to be a superposition of many sine waves of differing wavelength. In the mathematical model they are called phase waves. Since the speed of each wave depends on its length, the waves do not travel in lockstep; hence they end up interfering with one another.

The result is often an impressive pattern. A disturbance such as an insect's kick sends out a short series of phase waves. Together they form what is called a wave group, which spreads out from the insect. On each side of the group the water is flat. Within the group some five crests can be seen. The distance between the crests is approximately the average wavelength of the phase waves making up the wave group.

The speed of the movement of the crests over the water is termed the phase speed. It differs from the speed of the wave group as a whole. When gravity dominates the propagation of the waves, the phase speed is larger than the group speed. Therefore the crests appear at the rear of the wave group, overtake the middle and disappear at the front of the group. Since their amplitude is greatest at the middle of the group, one can follow the change in the amplitude of a crest as it traverses a wave group.

The waves generated by a water strider are capillary waves, because their wavelengths are short enough for the waves to be dominated by surface tension as the restoring force. With capillary waves the phase speed is less than the group speed. Hence a crest in an expanding wave group from a water strider appears at the front of the group, grows in amplitude as the center of the group overtakes it, decreases in amplitude as the rear of the group reaches it and finally disappears. At any instant the wave group has approximately five crests, separated by a millimeter or so.

This wave group expands roughly as a circle until it runs into some obstacle or its energy is dissipated. After gliding the insect may generate another such pattern of capillary waves. If the insect's speed relative to the water is less than .23 meter per second, no waves are generated. That is the minimum speed at which waves can propagate over the surface of water. Insects moving at less than this speed leave the water undisturbed except for the wave group initiated by their propulsion.

If the insect glides faster than the minimum wave speed, it continuously generates waves. The pattern that results has two curious features. One feature is that the waves in front of the insect are more closely spaced than the ones in the rear. The second feature is that the wave pattern takes the shape of a V, with the apex at the insect's head.

In 1883 Lord Rayleigh demonstrated mathematically that a small object moving in water with a relative speed in excess of .23 meter per second sends out waves shorter in length toward the front than at the rear. He was not concerned with wave patterns from insects but with the pattern produced by a fishing line in a moving stream. The line served as an obstacle to the flow of water. (Although the line was stationary and the insect moves, the difference is irrelevant. It is the relative speed of the object and the water that matters.) The generation of waves by the obstacle builds up closely spaced wave crests upstream of the obstacle. They are parts of capillary waves. Downstream the perturbation of the water by the obstacle sends out widely spaced wave crests. They are parts of gravity waves.

In the V wave pattern around a water strider the capillary waves are outside the V, the gravity waves inside. The angle of the V reflects a mathematical relation of twice the minimum wave speed of .23 meter per second divided by the relative speed of the obstacle and the 'water. A water strider in a faster glide ends up generating a narrower V wake.

In 1972 R. Stimson Wilcox of the Australian National University demonstrated that the surface waves generated by water striders play an important role in their mating. He studied the genus Rhagadotarsus Breddin by capturing several specimens and installing them in a large tray of water. The male initiated mating by generating wave signals. It gripped a support in the water with its front legs while its middle legs (and possibly the rear ones too) created the waves.

Stimson discovered three main types of signal preceding copulation, all with a frequency of between 17 and 29 waves per second. The male could call for a female by sending out a group of from seven to 15 waves at high amplitude. Alternatively it could transmit a group of only two or three waves in what Stimson interpreted as more a signal for courtship If a female approached, the signal switched rapidly to a strictly courtship mode of 30 waves at low amplitude. When the female was within a ii few centimeters of the male, the female too would send such a signal. Copulation then began.

A male sometimes generated another signal of higher frequency in an apparent warning to other males to avoid the mating area. If two males closed on each other, they would fight, perhaps for some minutes. I saw several such fights as water striders circled and then leaped at each other, after which one would chase the other.

On all my trips up and down streams I found water striders apparently locating one another by means of the waves they generated. When a wave group reached a water strider, the insect paused and then oriented itself perpendicular to the wave crests and toward the source of the crests. Then it dashed a short distance toward the source. After waiting for a fresh set of waves to pass, it would again dash off in the direction of the source. I cannot believe the insect located the source entirely by sight; if it did, the occasional pauses would be unnecessary.

Striders also employ waves to find prey. When a fly fell into the water and flapped violently on the surface, a water strider located it by means of the resulting waves. Sight might have played a role when the strider got near the fly, but not before.

Although the strider's exploitation o f waves seemed obvious to me, I wanted a better test. I tried oscillating a twig in a stream, but only one strider came to investigate. It might have arrived there even if I had not been dabbling in the water. I tried a vibrator sold for massage. The oscillating part, which is fully encased in rubber, is powered through a wire running to two small batteries. (A vibrator run on household current would invite electrocution.) A switch on the battery holder varied the frequency of oscillation.

I waded into the center of a pool of water about six meters wide along a stream. Several water striders collected at the edges of the pool after sensing my arrival. I submerged part of the vibrator and set its oscillation at about 20 hertz. The striders immediately paused in their motion, turned toward me and within about three seconds raced to within a few centimeters of the vibrator. Neither the vibrator nor I looked like a water strider, and so the insects surely were attracted by the waves.

Insects of many other types live on or near the water surface in a stream. You might like to search for whirligig beetles. The capillary waves they generate were studied by Vance A. Tucker of Duke University, whose work provided the basis for my discussion of the wave from a water strider. Another interesting aquatic subject is the back swimmer. This insect travels upside down and just below the surface of the water by rowing with its legs.

Bibliography

WAVE MAKING BY WHIRLIGIG BEETLES (GYRINIDAE). Vance A. Tucker in Science, Vol. 166, No. 3907, pages 897-899; November 14, 1969.

COMMUNICATION BY SURFACE WAVES: MATING BEHAVIOR OF A WATER STRIDER (GERRIDAE). R. Stimson Wilcox in Journal of Comparative Physiology, Vol. 80, pages 255-266; 1972.

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