water strider

water strider, any insect of the family Gerridae (order Heteroptera), which numbers about 350 species. Water striders, often seen running or skating in groups over the surface of a pond or stream, are slender, dark coloured, and generally more than 5 mm (0.2 inch) long.

With their short front legs they capture insects that fall onto the water’s surface. In crowded conditions water striders have been known to prey on each other. The middle and hind pairs of legs are long, sometimes more than twice the length of the body. The middle pair is used for propulsion, and the hind pair is used for steering. The feet (tarsi) are covered with fine water-resistant hairs that enable the insect to remain on the water’s surface. Often two forms of the adult occur in the same species: one wingless, the other winged. Occasionally a third form with short wings appears.

Gerris is a cosmopolitan genus of this family. All water striders live in fresh water except those of the genus Halobates, which are considered the only true saltwater-inhabiting insects. They have been seen many miles from land on tropical and subtropical ocean surfaces, feeding on the fluids of dead floating animals.

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Male and female Gerris gracilicornis demonstrate a phenomenon known as antagonistic coevolution. Females have a shield that covers their genitalia, which protects them against forced copulation and is believed to allow for mate selectivity. To increase mating opportunities, males counterevolved a strategy of vibrational signaling that attracts both females and predators. During copulation the female floats on the water’s surface with the male mounted on her back; this leaves the female more susceptible to predators than the male. The males’ strategy is believed to intimidate females into quick copulation, since females that have experienced prior predator attacks consent to copulation more quickly than females that have not had predatory encounters.