Organism of the Day
Water Scorpion, Ranatra sp.
June 29, 2012
Water scorpions—at first glance, swimming twigs with four legs and a cruel set of pincers—in fact belong to the order Hemiptera, or true bugs. Despite their vicious appearance, they are not capable of harming anything larger than a mayfly nymph. However, the stick-insect disguise and the violent ambush of their prey surely make this slow-moving insect a true terror in the aquatic invertebrate community.
Water scorpions have thin, elongate brown bodies 30-35 mm long, with a tail that adds an additional 10-15 mm to their length. Their front two legs have been modified to grasp prey, working on a “jack-knifing” design to fold in and pin their prey close to their mandibles. The thin, spindly, rear four legs work together to propel the insect forward when it is threatened using oar-like, jerky movements. Unlike the tail of a real scorpion, the tail of species in the genus Ranatra is not dangerous. It is merely a pair of straight, flexible siphons, jointed only at the base, that serves (in a simplistic sense) as a snorkel. Water scorpions, being air-breathing insects, carry a bubble of air with them between their forewings and their abdomen. When the tip of the long tail breaks the surface, diffusion via the tail between the bubble and the surrounding air renews the water scorpion’s oxygen supply. However, oxygen also dissolves into the bubble from the surrounding water. Thus, in highly oxygenated water with many plants close to the surface, the water scorpion will hardly ever have to surface for air. In the winter, water scorpions are able to survive in this way, even when trapped underneath ice, because the low temperatures dampen their metabolism to the point where the oxygen that dissolves into their air bubble is sufficient for survival.
Water scorpions typically spend their time clinging, head down, to twigs or pond weeds with their back two pairs of legs. When prey approaches, water scorpions straighten their hind pairs of legs, propelling them towards their prey while grasping it with their modified forelimbs. Once the prey (typically aquatic insect nymphs, crustaceans, or segmented worms) is firmly grasped by the forelimbs, the water scorpion will inject it with digestive enzymes—much like a spider—and then suck out its partially digested tissues. Despite their almost exclusively aquatic lifestyle, water scorpions can and will fly. On warm days, they have been known to lift off from ponds, displaying fully developed wings that are usually kept pressed flat against their cylindrical backs.
In our area, water scorpions have 2 broods per year in the springtime. Male water scorpions produce chirping noises, much like a cricket, to attract females. After mating, the female will lay several eggs attached to aquatic vegetation. The eggs have two anterior horns, or spiracles, that penetrate the surface of the water, functioning in the same way as the tail does on the adults. If the egg is completely submerged, oxygen can still diffuse through the egg shell into a thin lattice of air contained within the egg. In the early summer, the eggs hatch into nymphs which closely resemble the adults. After going through 5 molts, but without a distinct metamorphosis, they become fully-formed adults.
Water scorpions of the genus Ranatra can be found throughout eastern North America. They occur in quiet or still bodies of water, typically in shallow waters with abundant plant life. However, they are also quite tolerant of deoxygenated or polluted waters. This water scorpion was captured on the edge of the Station Pond, clinging to the aquatic vegetation.