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
Water Stick insect
On an Oak leaf
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
)³ 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.
)²
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 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 larva
THE LARVA
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.