Scorpions belong to phylum Arthropoda: their bodies are divided into segments, they are covered with a cuticle consisting of chitin and tanned proteins and they grow by molting. They belong to subphylum Chelicerata: they have a pair of preoral appendages, the chelicerae, and have no antennae. Like all members of the class Arachnida, they have a pair of pedipalps and four pairs of walking legs. Paleontologists believe that the common ancestors of scorpions were probably eurypterids or xiphosurans. The first scorpions date from the Silurian era (about 420 million years ago). The body of the scorpion has two parts: a prosoma or cephalothorax and an opisthosoma or abdomen, which is itself divided into two parts, a large mesosoma and a narrow metasoma (or tail) ending in the venom vesicle.
The prosoma is covered on the dorsal side by a “carapace,” which may be trapezoid or rectangular in shape and bears two medial eyes and two to five pairs of lateral eyes, together with the carinae or keels. The prosoma carries six pairs of appendages. At the very front are very small tripartite pincers called chelicerae, located in front of the mouth and armed with teeth. The pedipalps, ended by a chelae, and four pairs of walking legs follow. The ventral face of the prosoma is composed of the bases of the legs, the last two of which frame the sternum, a small structure thatmay be triangular or pentagonal (ormay take the formof a transverse bar).
The mesosoma bears seven dorsal plates or tergites, often decorated with carinae. On its vertical side, it bears a genital operculum formed from one or two valves, followed by a medial pectiniferous plate carrying the appendages characteristic of scorpions, the pectines. These paired mesosomal appendages consist of several parts, with variable numbers of laminae or teeth (from 1 to 58). They are followed by five large plates or sternites, the first four of which each carry a pair of slit-like (or rounded) respiratory orifices (spiracles) connected to four pairs of lungs. Flexible articular membranes connect each tergite or sternite to the next one, and pleural membranes connect the tergites and sternites laterally. The metasoma or tail consists of five narrow, mobile segments without pleural membranes. It ends in the telson or venom vesicle, which itself ends in the aculeus (or sting), a needlelike structure with two subterminal laterodorsal orifices.