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WATER-SCORPION, an aquatic hemipterous insect of the family Nepidae, so called from its superficial resemblance to a scorpion, which is due to the modification of the legs of the anterior pair for prehension, and to the presence of a long slender process, simulating a tail, at the posterior end of the abdomen. The common British species (Nepa cinerea) lives in ponds and stagnant water, and feeds upon aquatic animal organisms principally of the insect kind. 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.
In Nepa the body is broad and flat; but in an allied water-bug, Ranatra, which contains a single British species (R. linearis), it is long and narrow, while the legs are very slender and elongate. Certain exotic members of this group, sometimes erroneously referred to the Nepidae, but really forming a special family, Belostomidae, are of large size, a South American species, Belostoma grande, reaching a length of between 4 and 5 in.
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