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They skate gracefully on thread-like legs over glass-smooth water. From spring into autumn, water striders glide back and forth, here and there, on the surfaces of quiet ponds and slow-moving streams.
Water striders belong to the insect order Hemiptera (true bugs) and the family Gerridae. Of the 45 to 60 water strider species in North America, about 12 call Wisconsin home. The slender, dark-colored insects range from a half-inch to an inch long. Females are generally larger than males.
When water striders skate into view a natural question is how do they perform the enviable feat of walking on water? First, recall a discussion from Physics 101. Calm, smooth water has a tension on its surface created by weak, asymmetrical, attractive forces between water molecules. Thus light objects will float on water and heavier objects break the surface tension and slip below.
If you can catch an elusive, quick-darting strider (a challenge in itself), look at its four back legs that are used for locomotion. Use a hand lens to look at the position of the tarsal claws. On "typical" insects, each pair of tiny tarsal claws is located at the tip or apex of the insect's leg. On water striders, the claws are positioned a millimeter or so farther back up the leg – far enough that they do not penetrate the water and break the surface tension when the insect moves. A second adaptation is each leg is covered with many fine, velvety-looking, hydrofuge hairs. These water-repelling hairs prevent the legs from becoming wet. Wet legs would also break surface tension.
Now that we know how water striders stay afloat, notice how they move in two different ways – a slow, graceful glide and skating motion, or a quick spurt and dart. The glide or skate is accomplished when the insect rows with its middle legs while its rear legs trail and steer like a rudder. Water striders skate when facing upstream against a current or when orienting to other objects on the surface like food or other striders. Darting uses both pairs of back legs for propulsion. Quick movements are necessary when pursuing insect prey, eluding danger or when males grab females.
Water striders obtain food by waiting for living or dead insects to drift by or by responding to vibrations produced by struggling insects caught on the surface. Sensory receptors on the tips of their legs pick up the vibrations and help the striders orient and zero-in on potential meals. The striders dart into action, grab their victims with strong, raptorial front legs, pierce the victims' bodies with their beak and inject digestive enzymes. The enzymes dissolve and liquefy the victim's internal organs which are then sucked up. The exoskeleton is discarded.
Striders also mate on the water surface. Depending on the species, males may defend small circular territories while waiting for females to drift by or they may lure females by tapping the water with their legs to create "good vibrations" to which the females respond. Males may just pounce on females. Once the pair gets together, after an initial struggle and rebuke by the female, mating is fairly quick, but the pair can remain together for several hours. After mating, the female lays her eggs on submerged rocks and logs. Nymphs hatch in about two weeks and swim to the surface where they must break the surface tension and climb on top of the water. Over the next few weeks, nymphs molt five times before becoming adults. Wisconsin species may produce one or two generations of striders per year. Water striders overwinter as adults under stones at the pond bottom and emerge the following spring when mating commences.
Water striders spend most of their short lives on the water, but life is not endless days of calm water and sunshine. Danger is ever-present. If a wave breaks the surface tension or wets the striders' legs, the fragile insects may slip or be dragged underwater and drown. To survive this misfortune, the insects must crawl onto shore, a rock or vegetation to dry their legs before venturing back onto the water. Meanwhile, insect-eating birds flit overhead while hungry fish and frogs lurk below. Back swimmers, other predaceous aquatic insects, respond to movement on the surface and prey on water striders, especially mating pairs. To fend off these dangers, water striders often rest on floating vegetation to reduce their visibility and vulnerability.
Water striders are really cool insects to study. I was surprised to discover that the shiny, silver racing stripes on the sides of one large species are not colored markings, but rather are composed of fine, silver, hydrofuge hairs that glisten in the sunshine. So wade right in and take a closer look at one of the only insects capable of walking on water.
Anita Carpenter wades in to take a closer look at Wisconsin wildlife near her Oshkosh home.