Scorpion
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always ending with a stinger. The evolutionary history of scorpions goes back 435 million years. They mainly live in deserts but have adapted to a wide range of environmental conditions, and can be found on all continents except Antarctica. There are over 2,500 described species, with 22 extant (living) families recognized to date. Their taxonomy is being revised to account for 21st-century genomic studies.
Scorpions primarily prey on insects and other invertebrates, but some species take vertebrates. They use their pincers to restrain and kill prey. Scorpions themselves are preyed on by larger animals. The venomous sting can be used both for killing prey and for defense. During courtship, the male and female scorpion grasp each other’s pincers and move around in a “dance” where the male tries to maneuver the female onto his deposited sperm packet. Most species give live birth and the female cares for the young as their exoskeletons harden, transporting them on her back. The exoskeleton contains fluorescent chemicals and glows under ultraviolet light.
The vast majority of species do not represent a serious threat to humans, and healthy adults usually do not need medical treatment after being stung. Only about 25 species have venom capable of killing a human. In some parts of the world with highly venomous species, human fatalities regularly occur, primarily in areas with limited access to medical treatment. Scorpions with their powerful stingers appear in art, folklore, mythology, and commercial brands. Scorpion motifs are woven into kilim carpets for protection from their sting. Scorpius is the name of a constellation, and the corresponding astrological sign is Scorpio; a classical myth tells how the giant scorpion and its enemy, Orion, became constellations on opposite sides of the sky.
Morphology
Scorpions range in size from the 8.5 mm (0.33 in) Typhlochactas mitchelli of Typhlochactidae, to the 23 cm (9.1 in) Heterometrus swammerdami of Scorpionidae. The body of a scorpion is divided into two parts or tagmata: the cephalothorax or prosoma, and the abdomen or opisthosoma. The opisthosoma is subdivided into a broad anterior portion, the mesosoma or pre-abdomen, and a narrow tail-like posterior, the metasoma or post-abdomen. External differences between the sexes are not obvious in most species. In some, the metasoma is more elongated in males than females.
Cephalothorax
The cephalothorax comprises the carapace, eyes, chelicerae (mouth parts), pedipalps (which have chelae, commonly called claws or pincers) and four pairs of walking legs. Scorpions have two eyes on the top of the cephalothorax, and usually two to five pairs of eyes along the front corners of the cephalothorax. While unable to form sharp images, their central eyes are amongst the most light sensitive in the animal kingdom, especially in dim light, and makes it possible for nocturnal species to use starlight to navigate at night.The chelicerae are at the front and underneath the carapace. They are pincer-like and have three segments and sharp “teeth”. The brain of a scorpion is in the back of the cephalothorax, just above the esophagus. As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but has a long ventral nerve cord with segmented ganglia which may be a primitive trait.
The pedipalp is a segmented, clawed appendage used for prey immobilization, defense and sensory purposes. The segments of the pedipalp (from closest to the body outwards) are coxa, trochanter, femur, patella, tibia (including the fixed claw and the manus) and tarsus (moveable claw). A scorpion has darkened or granular raised linear ridges, called “keels” or “carinae” on the pedipalp segments and on other parts of the body; these are useful as taxonomic characters. Unlike those of some other arachnids, the legs have not been modified for other purposes, though they may occasionally be used for digging, and females may use them to catch emerging young. The legs are covered in proprioceptors, bristles and sensory setae. Depending on the species, the legs may have spines and spurs.
Mesosoma
The next four somites, 3 to 6, all bear pairs of spiracles. They serve as openings for the scorpion’s respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical or oval according to the species. There are thus four pairs of book lungs; each consists of some 140 to 150 thin lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles hold the lamellae apart. A muscle opens the spiracle and widens the atrial chamber; dorsoventral muscles contract to compress the pulmonary chamber, forcing air out, and relax to allow the chamber to refill. The 7th and last somite does not bear appendages or any other significant external structures.
The mesosoma contains the heart or “dorsal vessel” which is the center of the scorpion’s open circulatory system. The heart is continuous with a deep arterial system which spreads throughout the body. Sinuses return deoxygenated blood or hemolymph to the heart; the hemolymph is re-oxygenated by cardiac pores. The mesosoma also contains the reproductive system. The female gonads are made of three or four tubes that run parallel to each other and are connected by two to four transverse anastomoses. These tubes are the sites for both oocyte formation and embryonic development. They connect to two oviducts which connect to a single atrium leading to the genital orifice. Males have two gonads made of two cylindrical tubes with a ladder-like configuration; they contain cysts which produce spermatozoa. Both tubes end in a spermiduct, one on each side of the mesosoma. They connect to glandular symmetrical structures called paraxial organs, which end at the genital orifice. These secrete chitin-based structures which come together to form the spermatophore.
