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Water scorpion, Nepa cinerea 19-06-1999
vergroting Water scorpion
Nepa cinerea
(Nepa rubra)
This waterbug is much more common than the related Water Stick insect. The flattened and broadened insect seems completely different at first sight, but a closer look reveals the similarities: the heads are almost the same, the front legs are transformed into prehensile organs and both have the long breathing tube at the tip of the abdomen. In side view the likening is more obvious, though the Water stickinsect is much more slender.
Water scorpion, side view 19-06-1999
Water scorpion
Water Stick insect, side view
Water Stick insect

On an Oak leaf
Water scorpion on a leaf 21-8-2006
The name Water scorpion is very old, and the peculiar form of this bug indeed make it look like a real scorpion. The front legs were (are) often held for a pair of forceps instead of legs. Even in the first Systema Naturae of 1735 Linnaeus put Nepa next to the real Scorpion, (with the synonym: Scorpio aquat.) and erroneously with four legs. The long breathing tube at the tail looks like a large sting, and many people think it's a scaring animal. In reality is a pretty harmless little insect. But it is a merciless hunter that lives from murder by stealth, just like the Water Stick insect. The favorite surrounding of the Water scorpion is shallow water with waterplants and a camouflaging mud layer. Very often it crawls a bit under the mud layer on the bottom.
The tip of the breathing tube is brought above water, a bit like a snorkel. In that position it may lay like dead for hours, waiting with front legs held wide, as if it were a well camouflaged trap for a passing prey. When an animal of suitable size passes by, the front legs close with lightning speed, the victim is caught and sucked out. This waterbug is able to give a nip to humans with its beak, but in fact they almost never do, and it's not that painful.

Appearance. Not exactly the most beautiful animal of the ditch, with its flat body and the color that may range from a dull brown to ash gray. (cinerus - ashes, cinerea - ash coloured). But the back of the abdomen has a nice scarlet or orange red color, only visible by lifting the wingcases. In that way you may see wings on some specimen. Whether Water scorpions are able to fly still is not known for sure, though it is generally held for impossible, because the flying muscles are transformed to strange breathing organs, sometimes called trachea lungs. This waterbug stores an air supply under its wingcases, which is ventilated by means of the long breathing tube. This tube consists of two seperatable filaments lying close together, locked by a 'zipper' of bristles. The both filaments are grooved on the inner side, so together they form a tube. The tip is brought against the surface, so that an air duct is formed. Short bristles at the tip spread out on the water surface. Because of the air under the wingcases, the Water scorpion is lighter than water and is able to float, as if it were suspended form the surface on the tip of the tube, in the same manner as mosquito larvae and some waterbeetle larvae. When the tip is damaged, this floating isn't possible anymore and the Water scorpion must hold the tip a little above the surface. The head is small, with bulgy eyes and a short, flexible beak. The front legs have a broadened thigh (femur) with a groove on one side, in which the shin (tibia) fits, and so form a very effective clamp. The Water scorpion holds it's front legs in a more horizontal plane the the Water stick-insect

Water scorpion onder het ijs Water scorpion under ice
Water scorpions are able to swim, but very sluggish. They prefer to walk on mud or between waterplants. Winter is no problem for them: they may survive in an air bubble in solid ice. With some luck you may see one crawling upside down under the surface of a frozen pond, like the specimen on the picture at left. That scorpion stopped when sensing an air bubble at the tip of the abdomen, and used it for refreshing its air supply. Water scorpions mate in spring, the (smaller) male sits in on the female in the strange slanted position that many bugs take when mating. The eggs are deposited in waterplants at the surface. They have seven little hairs at the top, and at first the eggs form a string with each next egg laying in the 'crown' formed by the hairs of the previous egg. Later, when standing single, the hairs spread out above the water surface, probably to supply the egg with fresh air. The eggs hatch in May and June

Water scorpion, larve
enlargement Water scorpion larva
young larva of Water scorpion 4,5 mm 28-06-1999
Like all bugs the Water scorpion is an insect with incomplete metamorphosis (hemimetabolism), and so the newly hatched larvae already are much like the adult. The main differences are that they have no wing(case)s and no breathing tube, instead they have a pointed tip on the abdomen. They have a louse-like appearance. On the body are long hairs, which help attaching dirt to camouflage the defenceless creature. But they are as good in hunting as their parents are, and grow fast. The younger larva at right measures a 4 mm, the one at left 9 mm. After five molts they are fully grown. Only after the last molt the wings and long breathing tube are formed.

Nepa cinerea or rubra? The names Nepa cinerea and Nepa rubra are synonyms: the same animal. The name Nepa rubra was (and still is) used a lot, but apparently somewhere in the many revisions of the last year cinerea proved older and thus the valid name. Both names are from Linnaeus' Systema Naturae of 1758: N. cinerea with the note: "in European regions", N. rubra: "in warm regions" (in Calidis regionibus). Later it turned out that these two species are one and the same.

1) WATERWERELD: About the Water scorpion.
2) Nice article with pictures (French).
3) Systema Naturae (1735) - Wikipedia
4) Digital scans of a.o. Systema Naturae (University of Göttingen)

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© G.H. Visser 02-04-2008
rev. 09-11-2008

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