Water scorpion (Nepa cinerea)

GenusNepa (1)
SizeLength: 18 - 22 mm (2)

Common and widespread throughout Britain (2).

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' (2). 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 (3). All true bugs have sucking mouthparts, known as a rostrum or 'beak' (2). 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 (3). Very young larvae seem to be 'tailless', as the tail grows at a different rate to the body (2).

This water bug is common and widespread throughout Britain, with the exception of northern Scotland, where it becomes rare (3).

Shows a preference for still waters in shallow ponds and lakes but is also found in brackish dykes and streams. In all cases, it requires plenty of aquatic weeds (2).

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. Air is taken in at the surface via the respiratory tail, and the bug is able to stay below the water for up to 30 minutes. Most individuals are unable to fly as the flight muscles are poorly developed, but occasionally they do fly, in order to colonise new ponds.

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 (2). The eggs have 7 long hairs, which touch the surface of the water and serve to supply the eggs with oxygen (3). 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 (2).

Not threatened at present.

Conservation action has not been targeted at this widespread species.

For more on invertebrates and their conservation see Buglife, the Invertebrate Conservation Trust at:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact: arkive@wildscreen.org.uk

  1. National Biodiversity Network Species Dictionary (Jan 2003): http://www.nhm.ac.uk/nbn/
  2. Southwood, T. R. E. & Leston, D. (1959) Land and water bugs of the British Isles. Frederick Warne & Co. Ltd., London.
  3. Olsen, L., Sunesen, J., & Pedersen, B. V. (1999) Small freshwater creatures. Oxford University Press, Oxford.