INVERTEBRATES
Status: Common, Native Residents

WATER SCORPIONS

Description 

Water scorpions are not really scorpions, but insects with only 3 pairs of legs and 2 pairs of wings. Their name comes from
their specialized grasping forelimbs, superficially similar to the anterior 'pincers' of scorpions, and an elongate caudal
siphon or breathing tube, which conjures up the image of the scorpion's long stinging tail. In both cases, these features are
completely different from their scorpion counterparts. The forelegs of a true scorpion have a powerful pincer - similar to
that of a crab or lobster - at the tip. The forelegs of the water scorpions are likewise adapted for grasping prey, but lack
pincers; instead, they use a jack-knifing design with the outer segments folding into a groove to secure prey. The tail of a
scorpion has 6 rounded segments with a terminal venomous spine, and can be folded forward over the animal's back. The
tail siphon of the water scorpions is actually two straight filaments pressed against one another; the siphon is not jointed, can pivot only at the base, and does not sting. It is used to obtain air from the water surface, much like a snorkel.

The water scorpions belong to the insect order Hempitera, or the true bugs. Like all hemipterans the head is a long sucking
beak or rostrum, which conceals the mouthparts. The head of a water scorpion is very small and the rostrum projects forward. The large eyes project to the side. Within the order Hemiptera, water scorpions belong to the sub-order Heteroptera ('different wings') in which the anterior wings are stiffened to form protective wing cases and conceal the membranous posterior wings that are normally folded beneath. The wings fold at a slant across one another. The overlapping posterior regions remain membranous, with only the anterior part of the forewing being stiffened. The stiffening is brought about by tanning of the wing proteins by chemicals named quinones. Tanning of leather is essentially the same process; tanneries use quinones extracted from tree bark.

The Hemiptera are insects that grow through a series of molts but lack a distinct metamorphosis. The juvenile stages, or
nymphs, resemble small versions of the adult.

Like all insects, water scorpions possess antennae (feelers), but they are tiny and lie concealed at the base of the eyes. Two
genera may be found in South Dakota; both are widely distributed in North America. Nepa apiculata , the only North
American species in the genus, is a dark brown, strongly flattened water scorpion closely resembling a dead leaf. Adults are
fully winged, but apparently flightless. Nymphs are paler, are shorter bodied, and lack the long caudal siphon. Adults range
from 0.6 to 0.8 inches (16-20 mm) long with a 0.3 to 0.4 inch (8-10 mm) 'tail.'

Ranatra spp. are long water scorpions or water stick insects. Nine species are found in North America, but they are difficult
to identify. The most common species in our region is the pale buff Ranatra fusca. Ranatra spp. are longer and much more
slender than Nepa , with a long, tapering thorax and almost cylindrical abdomen . The outer 'jack-knifing' portion of the
forelegs is shorter than in Nepa (see Figure 1). Ranatra are pale buff in color. Adults will fly on warm days, lifting the
wings to reveal a red-topped abdomen. These slender insects are fairly common and widespread in slow-flowing waters
with dense vegetation. Adults are 1.2 to 1.4 inches (30-35 mm) long with a 0.4 to 0.6 inch (10-15 mm) 'tail.'

Distribution

Both genera occur in the state, but Nepa apiculata has an eastern distribution in North America and is quite scarce in the
Great Plains region. Ranatra fusca appears to be widespread in the state, though seldom abundant. There is little
information available on other species. Both genera are quite tolerant of polluted and deoxygenated waters, but are usually
associated with dense vegetation. Nymphs occur in the summer months, but adults may be found throughout the year.

Natural History

Water scorpions usually lurk motionless close to the surface, head down, clinging to twigs or pond weeds. Periodically, they
back up to the water surface to replenish air. Like the majority of insects, water scorpions are air-breathers. They carry a
submerged air bubble that serves as a renewable air supply. Air is trapped by tiny water-repellent hairs on the under surface
of the forewings and the underlying abdomen . The trapped air bubble connects with the surface through a series of hairs
between the two tail filaments of the breathing tube. When this breaks the surface, diffusion renews the oxygen content of the
air bubble. When under water, the animal's oxygen supply is gradually depleted as metabolism occurs. Oxygen and nitrogen
also dissolve out of the bubble into the surrounding water. If the water is deoxygenated, the rate of oxygen loss from the
bubble will be high and the animal will need to surface every few minutes. But in more typical habitats, with abundant living
plants close to the water's surface, very little will be lost from the air bubble to the surrounding water. In fact, as the oxygen
content of the bubble is depleted by the animal, oxygen from the surrounding water will start to diffuse into the bubble,
partly offsetting the metabolic consumption. In this way, the water scorpions are able to survive when trapped beneath ice.
during the winter. Under these conditions, the very low temperatures lower metabolism to a level where dissolved oxygen
levels provide for adequate gas exchange.

In the summer months, because of their dependence on the surface for air, water scorpions do not want to stray into deeper
waters. To ensure against this, they possess three pairs of pressure sensors called false spiracles on the underside of the
abdomen . These are visible as dark, oval discs (see Figure 2). Underneath these discs are air sacs of the insect's internal
respiratory system. These are compressed as the animal ventures deeper. Nerve endings attached to the walls of the air sacs
register the degree of deformation and thereby inform the water scorpion of the depth. Upward displacement of air in the
sacs also informs the animal of its posture in the water.

Their slow movements and excellent camouflage make water scorpions inconspicuous to potential prey species. These
include smaller insects such as the nymphs of mayflies, stoneflies, and water beetles, crustaceans such as freshwater shrimp
and hog lice (Isopoda), and small segmented worms. Water scorpions are ambush predators and cling to plants with the
second and third pairs of legs. The forelimbs are held out at the front. When prey approaches, the hindlimbs straighten,
swinging the water scorpion forward, and the victim is grasped by one of the forelimbs. Firmly held in the jack-knifing grip,
it is then pierced by the rostrum and injected with digestive enzymes. Partially digested tissues are then sucked up. The
rostrum of several water bugs, including the water boatmen and huge belostomatid bugs, can deliver an excruciating bite.
However, despite allegations in many texts, water scorpions are docile and are quite safe to handle.

Water scorpions can swim for short distances but seldom do so unless disturbed. When swimming, they use alternating
oar-like movements of the second and third legs to propel themselves in a somewhat jerky fashion.


Figure 2. Ventral Abdomen of Ranatra With Dark False Spiracles.


Ranatra Nepa


Figure 3. Eggs of Water Scorpions Showing Respiratory Horns.


In our area, the Nepidae produce only one brood per year; further south many species will have two broods. Both of our
genera breed in the spring. Males attract females by a quiet chirping, rather in the manner of a cricket. The sound is produced
by rubbing a roughened surface at the base of the forelegs against a tiny file on the front edge of the thorax, which also serves
as a resonator. After mating, the female lays several eggs, which are attached to aquatic vegetation. The eggs possess two
anterior horns, or spiracles, that serve the same function as the breathing siphon of the adults (see Figure 3). By penetrating
the water surface, these provide an air supply to the egg. Frequently, the eggs are inserted into the stems of emergent plants
such as cattails, with the respiratory horns projecting. If they are submerged, the eggs are able to respire by means of a thin
air film that remains trapped within the intricate lattice of the egg shell. This structure serves as a kind of permanent gill into
which oxygen diffuses from the surrounding water and from which carbon dioxide can diffuse out. The advantage of this
arrangement is that oxygen and carbon dioxide diffuse some 10,000 times more rapidly in air than in water, and gas exchange
across the egg shell is thereby facilitated. Eggs hatch in early summer into nymphs. These pass through five molts before
maturation.

Collecting And Keeping Water Scorpions

Nepa apiculata can be searched for in shallow waters at pond and river edges and collected by hand or by using a small net.
Ranatra spp. typically inhabit deeper waters and are best collected by sweeping a dip net slowly through dense vegetation
near the water surface. When captured, they usually lie motionless with the limbs extended anteriorly and posteriorly and are
easily overlooked as twigs or dead leaves. They are easy animals to maintain in an indoor aquarium. Use a gravel bed and
freshly collected rainwater or pond water with plenty of submerged vegetation (Elodea, Callitriche, Myriophyllum,
Ceratophyllum or Potamogeton are ideal). Plants can be secured beneath a few large stones. Make sure that some
vegetation reaches to within 0.8 inches (20 mm) of the water surface allowing the water scorpions to breathe. Small nymphs
and crustaceans provide ideal prey items. Use a well-lit location to promote plant growth but avoid direct sunlight. A few
freshwater snails are useful to prevent the accumulation of algae on the glass.

Conservation Measures

There are no restrictions concerning the collecting and keeping of aquatic insects. For this reason, and because they are so
easy to keep, they make ideal creatures for classroom study.

Glossary


Abdomen - the hindmost of the three major divisions of an insect's body: head, thorax, and abdomen. The abdomen
comprises twelve segments.
Caudal - posterior; of or pertaining to a tail.
Diffusion - the random movement of molecules due to their kinetic energy. It increases with increased temperature.
Nymph - the immature life-stage of an insect that goes through incomplete metamorphosis.
Resonator - something that increases or intensifies sound by sympathetic vibration.
Rostrum - modified mouthparts, characteristic of the insect order Hemiptera, which are used for sucking juices from plants
and prey animals.
Venomous - poisonous.

References
Hinton, H.E., 1961. The structure and function of the egg shell in the Nepidae (Hemiptera). Journal of Insect Physiology 4,
176-183.
Hungerford, H.B., 1922. The Nepidae of North America. Kansas Univ. Science Bulletin 14, 425-469.
Pennak, R.W., 1978. Freshwater Invertebrates of the U. S. (Second Edition). John Wiley and Sons.
Thorpe W.H. and Crisp. D.J., 1947. Studies on plastron respiration III. The orientation responses of Aphelocheirus
(Hemiptera, Aphelpocheiridae (Naucoridae)) in relation to plastron respiration; together with an account of specialized
pressure receptors in aquatic insects. Journal of Experimental Biology 24, 310-328.
Written and Illustrated by:
Dr. Jonathan Wright, Biology Dept., Northern State University, Aberdeen, SD. 1997.
Reviewed by:
Nels H. Troelstrup, Jr., Ph.D., South Dakota State University, Brookings, SD 57007.
Publication of the Water Scorpions fact sheet was funded by the South Dakota Department of Game, Fish and Parks,
Division of Wildlife, Pierre, SD.