Water Scorpions, Family Nepidae

Habits and Traits of Water Scorpions

Water scorpion (family Nepidae), climbs out of a forest river, Belize
David Maitland / Getty Images

Water scorpions aren't scorpions at all, of course, but their front legs do bear a passing resemblance to scorpion pedipalps. The family name, Nepidae, derives from the Latin nepa, meaning scorpion or crab. You don't need to worry about being stung by a water scorpion – it has no stinger.

Description:

Water scorpions vary in shape within the family. Some, like those in the genus Ranatra, are long and slender.

These are often described as looking like aquatic walkingsticks. Others, such as those in the genus Nepa, have large, oval bodies, and look like smaller versions of giant water bugs. Water scorpions breathe by means of a caudal respiratory tube formed from two long cerci that extend to the water's surface. So regardless of the body shape, you can recognize a water scorpion by this long "tail." Inclusive of these respiratory filaments, water scorpions range in size from 1-4 inches long.

Water scorpions capture prey with their raptorial front legs. As in all true bugs, they have piercing, sucking mouthparts, hidden by a rostrum that folds under the head (much like you see in assassin bugs or plant bugs). The water scorpion's head is narrow, with large side-facing eyes. Although they do have antennae, it's difficult to see them, as they're quite small and located beneath the eyes. Adult water scorpions do have developed wings, which overlap when at rest, but don't often fly.

Nymphs look much like adult water scorpions, though smaller, of course. The respiratory tube of the nymph is considerably shorter than in the adult, particularly in early stages of molting. Each water scorpion egg bears two horns, which are actually spiracles that extend to the water's surface and provides oxygen to the developing embryo.

Classification:

Kingdom – Animalia
Phylum – Arthropoda
Class – Insecta
Order – Hemiptera
Family - Nepidae

Diet:

Water scorpions ambush their prey, which includes other aquatic insects, small crustaceans, tadpoles, and even small fish. The water scorpion grasps vegetation with its second and third pairs of legs, just below the water's surface. It sits and waits for a potential meal to swim by, at which point it straightens out its hind legs, pushes itself forward, and grasps the animal tightly with its front legs. The water scorpion pierces its prey with its beak or rostrum, injecting it with digestive enzymes, and then sucks up the meal.

Life Cycle:

Water scorpions, like other true bugs, undergo simple or incomplete metamorphosis with just three life stages: egg, nymph, and adult. Typically, the mated female attaches her eggs to aquatic vegetation in the spring. The nymphs emerge in early summer and undergo five molts before reaching adulthood.

Special Adaptations and Behaviors:

The water scorpion breathes surface air but does so in an unusual way. Tiny water-repellent hairs under the forewing trap a bubble of air against the abdomen. The caudal filaments also bear these tiny hairs, which repel water and hold air between the paired cerci.

This allows oxygen to flow from the water's surface to the air bubble, as long as the breathing tube is not submerged.

Because the water scorpion breathes air from the surface, it prefers to stay in shallow waters. Water scorpions regulate their depth using three pairs of special sensors on their bellies. Sometimes referred to as false spiracles, these oval sensors are attached to air sacs, which are in turn connected to nerves. Any SCUBA diver can tell you that an air sac will be compressed as you dive deeper, thanks to the forces of water pressure which are amplified at depth. As the water scorpion dives, the air sacs become distorted under pressure, and nerve signals send this information to the insect's brain. The water scorpion can then correct its course if it inadvertently dives too deep.

Range and Distribution:

Water scorpions can be found in slow moving streams or ponds throughout the world, particularly in warmer regions. Globally, scientists have described 270 species of water scorpions. Just a dozen species inhabit the U.S. and Canada, most of which belong to the genus Ranatra.

Sources:

  • Borror and DeLong's Introduction to the Study of Insects, 7th edition, by Charles A. Triplehorn and Norman F. Johnson.
  • Lecture notes, Entomology for Teachers course, Dr. Art Evans, Virginia Commonwealth University.
  • Water Scorpions, Northern State University. Accessed February 19, 2013.
  • Water Bugs and Water Scorpions, Fact Sheet, Queensland Museum. Accessed online February 19, 2013.
  • Family Nepidae - Water scorpions, BugGuide.Net. Accessed February 19, 2013.
  • Guide to Aquatic Insects and Crustaceans, Izaak Walton League of America.