The water scorpion is not a true scorpion, but is a large, aquatic insect that belongs to the order of insects known as the Hemiptera or bugs. Its flattened body is greyish brown on the upper surface; the wings, which are held close to the body, and the abdomen below the wings are pink. All true bugs have sucking mouthparts, known as a rostrum or beak. The water scorpion's first pair of legs are very powerful, and are used to seize prey, but the most notable feature is the remarkable tail, a thin projection from the rear of the bug which is used as a siphon, to take in air. Very young larvae seem to be tailless, as the tail grows at a different rate to the body.

The water scorpion, Nepa cinerea, got its name because of its long tail, but the tail is a breathing tube (actually two tubes stuck together), rather like a snorkel, not a sting. The body length is 20 - 23 mm, and the tail is usually around 10 mm long. It is dark brown and flat, so well camouflaged and is often mistaken for a dead leaf.

These bugs, like all Hemiptera, have back wings with thick, opaque tips that are more membranous than the wing base. Some of them, like the giant water bug, have a special appendage at the base of their wings that allow the insect to keep its wings tightly closed against its body. This adaptation to aquatic life makes the insect a better swimmer.

Habitat

It can be found in weedy, stagnant ponds, shallow lakes, fens and occasionally weedy streams, and tends to lurk around in vegetation waiting for prey to pass within reach.

Most individuals are unable to fly as the flight muscles are poorly developed, but occasionally they do fly, in order to colonise new ponds.

Behaviour

The water scorpion is a poor swimmer, and tends to move around mainly by walking. It preys on water beetles, mosquito larvae, small fish and a range of other aquatic animals, which are caught by the powerful, pincer-like fore legs. It injects a powerful digestive enzyme into the prey to overcome it quickly. They are poor swimmers. Adults can fly, but rarely do so. It is about 3.5 cm in length including the tail. When taken out of the water it pretends to be dead, or crawls quickly away. The female lays her eggs underwater in the stems of vegetation just below the water surface.

Bug is able to stay below the water for up to 30 minutes.

Nepa cinerea is a species of water scorpion (Nepidae), found in Central Europe and the British Isles. It lives in ponds and stagnant water and feeds upon aquatic animals, especially insects.

Respiration in the adult is effected by means of the caudal process, which consists of a pair of half-tubes capable of being locked together to form a siphon by means of which air is conducted to the tracheae at the apex of the abdomen when the tip of the tube is thrust above the surface of the water. In immature forms the siphon is undeveloped and breathing takes place through six pairs of abdominal spiracles. The eggs, laid in the stems of plants, are supplied with seven filamentous processes which float freely in the water.

The water scorpion is active throughout the year; adults can be found in winter moving around under ice or under stones. Mating takes place from April to late May, and females lay their eggs shortly after. The eggs are laid at night, just beneath the surface of the water, amongst algae or in the stems of water plants. A female can lay as many as 32 eggs in one night. The eggs have 7 long hairs, which touch the surface of the water and serve to supply the eggs with oxygen. After 3-4 weeks, the larvae emerge from the eggs; they spend most of their time in the shallow water at the margins of ponds. They tend to remain still, with the tail protruding from the water, waiting for suitable prey to pass by, which they seize with their strong front legs. Bugs undergo a type of development known as incomplete metamorphosis in which the larvae progress through a series of moults. The stages between moults are known as 'instars'; there are 5 instars in this species, and it takes 6-8 weeks for water scorpion larvae to develop into adults.