Water scorpions are not really scorpions, but insects with only 3 pairs of legs and 2 pairs of wings. Their name comes from their specialized grasping forelimbs, superficially similar to the anterior 'pincers' of scorpions, and an elongate caudal siphon or breathing tube, which conjures up the image of the scorpion's long stinging tail. In both cases, these features are completely different from their scorpion counterparts. The forelegs of a true scorpion have a powerful pincer - similar to that of a crab or lobster - at the tip. The forelegs of the water scorpions are likewise adapted for grasping prey, but lack pincers; instead, they use a jack-knifing design with the outer segments folding into a groove to secure prey. The tail of a scorpion has 6 rounded segments with a terminal venomous spine, and can be folded forward over the animal's back. The tail siphon of the water scorpions is actually two straight filaments pressed against one another; the siphon is not jointed, can pivot only at the base, and does not sting. It is used to obtain air from the water surface, much like a snorkel.
The water scorpions belong to the insect order Hempitera, or the true bugs. Like all hemipterans the head is a long sucking beak or rostrum, which conceals the mouthparts. The head of a water scorpion is very small and the rostrum projects forward. The large eyes project to the side. Within the order Hemiptera, water scorpions belong to the sub-order Heteroptera ('different wings') in which the anterior wings are stiffened to form protective wing cases and conceal the membranous posterior wings that are normally folded beneath. The wings fold at a slant across one another. The overlapping posterior regions remain membranous, with only the anterior part of the forewing being stiffened. The stiffening is brought about by tanning of the wing proteins by chemicals named quinones. Tanning of leather is essentially the same process; tanneries use quinones extracted from tree bark.
The Hemiptera are insects that grow through a series of molts but lack a distinct metamorphosis. The juvenile stages, or nymphs, resemble small versions of the adult.
Like all insects, water scorpions possess antennae (feelers), but they are tiny and lie concealed at the base of the eyes. Two genera may be found in South Dakota; both are widely distributed in North America. Nepa apiculata , the only North American species in the genus, is a dark brown, strongly flattened water scorpion closely resembling a dead leaf. Adults are fully winged, but apparently flightless. Nymphs are paler, are shorter bodied, and lack the long caudal siphon. Adults range from 0.6 to 0.8 inches (16-20 mm) long with a 0.3 to 0.4 inch (8-10 mm) 'tail.'
Ranatra spp. are long water scorpions or water stick insects. Nine species are found in North America, but they are difficult to identify. The most common species in our region is the pale buff Ranatra fusca. Ranatra spp. are longer and much more slender than Nepa , with a long, tapering thorax and almost cylindrical abdomen . The outer 'jack-knifing' portion of the forelegs is shorter than in Nepa (see Figure 1). Ranatra are pale buff in color. Adults will fly on warm days, lifting the wings to reveal a red-topped abdomen. These slender insects are fairly common and widespread in slow-flowing waters with dense vegetation. Adults are 1.2 to 1.4 inches (30-35 mm) long with a 0.4 to 0.6 inch (10-15 mm) 'tail.'