Water scorpions in the genus Ranatra (10 spp. in North America) look a lot like underwater walkingsticks. What initially seem to be antennae stretching forward from the body are actually their grasping, mantislike (“raptorial”) forelegs, used for seizing small aquatic prey. Like all insects, there are 3 pairs of jointed legs. The slender, needlelike appendage at the tip of the abdomen is not a stinger; instead, it is a pair of half-tubes that, when held together, become a sort of snorkel. The insect, an air breather, rests hind-end-upward with the tip of this tube projecting above the water surface; thus it can stay submerged for long periods.
Nepa apiculata, another water scorpion and the only member of its genus in all of North America, has a different body shape: it is oval, flattened, and blackish; its powerful, grasping forelegs are immediately noticeable, the hindlegs secondarily so. As with other water scorpions, its two-parted abdomen-tip breathing tube is long and slender.
Similar species: Giant water bugs (family Belostomatidae) look a little like Nepa apiculata, but they don’t have the needlelike “butt snorkel”; also, their mid and hind legs are flattened. Water measurers (family Hydrometridae) and water striders (family Gerridae), both skinny and sticklike, both “walk on water,” using the surface tension to keep from sinking; water scorpions rarely go above the water. True scorpions are arachnids and are not related at all to water scorpions, which are 6-legged insects.
Length (not including appendages, including “tail”): to 4 inches; usually about 1½ inches (Ranatra spp.); to about ¾ inch (Nepa sp.).
Habitat and Conservation
Water scorpions prefer still or slow waters with detritus and muddy bottoms, as in ponds, lakes, and slow sections of streams. In these calm waters, they lurk among plant stalks and submerged vegetation, waiting for prey to pass by. Water scorpions in genus Ranatra move slowly, when they move at all, and resemble a piece of grass or other stalk of debris. Like many other true bugs, water scorpions possess two pairs of wings. The first pair is hardened and forms a cover for the membranous second pair, which is used (rarely) for flight, usually at night.
Water scorpions prey upon aquatic insects and other invertebrates, tadpoles, and very small fish. Their method is much like that of land-based assassin bugs: The powerful forelegs grab prey, and a jab from the knifelike beak administers a digestive juice that disables the victim and liquefies its insides. Then, the tubelike mouthparts (a hallmark of all the true bugs) are used like a straw to suck the nutritious juices out of the prey. Water scorpions are said to eat enormous numbers of mosquito larvae.
Like other predatory true bugs (such as assassin bugs, giant water bugs, and backswimmers), water scorpions can potentially bite a person and deliver a painful bit of digestive salivary fluids (venom) in the process. The species in our area are not known to be harmful. However, we recommend handling them with care, using forceps or holding them in the middle of their stiff body, away from the head, so they cannot bite.
In spring, water scorpions, like other true bugs, begin life as eggs, then hatch and grow. They molt a number of times before becoming a mature adult in their last stage of life. The immature stages look a lot like small versions of the adults (they do not pupate). The full adults have completely formed wings. Immature stages show wing “buds.” Apparently most water scorpions can live for at least a few years.
Edwin Way Teale, in “Autumn Across America,” wrote of finding a water scorpion (R. fusca) fluttering about in the desert-like Badlands of South Dakota. As he held the specimen between his fingers, it made high-pitched squeaks as it rubbed its forelegs against roughened patches on its body. Then it stiffened and held still, playing dead. When Teale tossed it into the air again, it took flight and fluttered away.
The sticklike appearance and slow-motion behavior of water scorpions in genus Ranatra is a camouflage that works in two ways. First, it enhances the ambush hunting strategy, helping the insect surprise its prey. Second, it conceals the insect from the many fish, birds, frogs, and other animals that would otherwise prey upon it.