While 90% of the Gerridae are freshwater bugs, the oceanic Halobates makes the family quite exceptional among insects. The genus Halobates was first heavily studied between 1822 and 1883 when Buchanan-White collected several different species during the Challenger Expedition. Around this time, Eschscholtz discovered three species of the Gerridae, bringing attention to the species, though little of their biology was known. Since then, the Gerridae have been continuously studied due to their ability to walk on water and unique social characteristics. Small gerrids have frequently been confused with the other semiaquatic bugs, the Veliidae. The most consistent characteristic used to separate these two families are internal genitalia differences. Since internal genitalia require specific training and tools to identify, it is almost impossible to tell a member of the Gerridae apart from a member of the Veliidae by external visual cues. One must study their habitat and behaviors to properly differentiate the two without looking at their specific anatomy.
The family Gerridae is physically characterized by having hydrofuge hairpiles, retractable preapical claws, and elongated legs and body.
Hydrofuge hairpiles are small, hydrophobic microhairs. These are tiny hairs with more than one thousand microhairs per mm. The entire body is covered by these hairpiles, providing the water strider resistance to splashes or drops of water. These hairs repel the water, preventing drops from weighing down the body.
Water striders have two antennae with four segments on each. Antennal segments are numbered from closest to the head to farthest. The antennae have short, stiff bristles in segment III. Relative lengths of the antennae segments can help identify unique species within the family Gerridae, but in general, segment I is longer and stockier than the remaining three. The four segments combined are usually no longer than the length of the water strider head.
The thorax of water striders is generally long, narrow, and small in size. It generally ranges from 1.6 mm to 36 mm long across the species, with some bodies more cylindrical or rounder than others. The pronotum, or outer layer of the thorax, of the water strider can be either shiny or dull depending on the species, and covered with microhairs to help repel water. The abdomen of a water strider can have several segments and contains both the metasternum and omphalium.
A front, middle, and back pair of legs occur. The front legs are shortest and have preapical claws adapted to puncture prey. Preapical claws are claws that are not at the end of the leg, but rather halfway through, like mantids. The middle legs are longer than the first pair and shorter than the last pair and are adapted for propulsion through the water. The hind pair is the longest and is used for spreading weight over a large surface area, as well as steering the bug across the surface of the water. The front legs are attached just posterior to the eyes, while the middle legs are attached closer to the back legs which attach midthorax but extend beyond the terminal end of the body.
Some water striders have wings present on the dorsal side of their thorax, while other species of Gerridae do not- particularly Halobates. Water striders experience wing length polymorphism that has affected their flight ability and evolved in a phylogenetic manner where populations are either long-winged, wing-dimorphic, or short-winged. Wing dimorphism consists of summer gerrid populations evolving different length wings than winter populations within the same species. Habitats with rougher waters are likely to hold gerrids with shorter wings, while habitats with calm waters are likely to hold long-winged gerrids. This is due to potential for damage of the wings and ability for dispersal.
Wing polymorphism is important to the variety and dispersal of the Gerridae. The ability for one brood to have young with wings and the next not allows water striders to adapt to changing environments. Long, medium, short, and nonexistent wing forms are all necessary depending on the environment and season. Long wings allow for flight to a neighboring water body when one gets too crowded, but they can get wet and weigh a water strider down. Short wings may allow for short travel, but limit how far a gerrid can disperse. Nonexistent wings prevent a gerrid from being weighed down, but prevent dispersal.
Wing polymorphism is common in the Gerridae despite most univoltine populations being completely apterous (wingless) or macropterous (with wings). Apterous populations of gerrids would be restricted to stable aquatic habitats that experience little change in environment, while macropterous populations can inhabit more changing, variable water supplies. Stable waters are usually large lakes and rivers, while unstable waters are generally small and seasonal. Gerrids produce winged forms for dispersal purposes and macropterous individuals are maintained due to their ability to survive in changing conditions. Wings are necessary if the body of water is likely to dry since the gerrid must fly to a new source of water. However, wingless forms are favored due to competition for ovarian development and wings and reproductive success is the main goal due to the selfish gene theory. Overwintering gerrids usually are macropterous, or with wings, so they can fly back to their aquatic habitat after winter. An environmental switch mechanism controls seasonal dimorphism observed in bivoltine species, or species having two broods per year. This switch mechanism is what helps determine whether or not a brood with wings will evolve. Temperature also plays an important role in photoperiodic switch. Temperatures signify the seasons and thus when wings are needed since they hibernate during winter. Ultimately, these switching mechanisms alter genetic alleles for wing characteristics, helping to maintain biological dispersal.