Scorpions are the most primitive members of all Arachnida. They have changed little since their first appearance in the Silurian period (395,000,000 years ago). Initially aquatic, they became completely terrestrial at the end of the Devonian period and the beginning of the Carboniferous period (345,000,000 years ago). Today they are exclusively land animals. They live for an average of two to six years. Their life span in nature is primarily determined by predation. Scorpions have an articulated body. In all, there are 18 body segments (somites), the first 6 of which have fused. The head and the thorax are fused to a cephalothorax or prosoma (lit. "fore-body"). Each of these fused segments has a single pair of appendages. At the very front there are small tweezer-like chelicerae, the second segment has a pair of pincers (pedipalps) and the others have altogether 4 pairs of legs. The abdomen is divided into a thick pre-abdomen (mesosoma: lit. "middle-body") with seven segments and a thin articulated post-abdomen or tail (metasoma: lit. "rear-body"). This tail has 5 segments and ends in a knob-like appendix (telson) with a sharp curved sting (aculeus), where the two venom glands have their outlet. The sting is used to kill prey, for defence against aggressors and in some species it also has a role in the courtship display. Ventrally on the first segment of the abdomen there is a small sternal plate. Caudal from the sternal plate is the genital opening which is covered by a small cap (operculum). Behind this, on the third abdominal segment, is a small basal plate on which there are two protuberances which form a typical V-shaped comb (the so-called "pecten"). This is a sensory organ only found in scorpions. Scorpions have a primitive nervous system. There is one pair of median eyes and two to five pairs of lateral eyes. These eyes are simple (ocelli). There are no compound faceted eyes like the large eyes of many insects. There is an open circulation. The blood flows via tissue sinuses. The elongated heart has seven ostia and pumps blood containing haemocyanin (which is a haemoglobin-like oxygen-carrying pigment that contains copper, not iron). The creatures breathe via 4 book lungs which are connected to the outside world via spiracles (respiratory openings) ventrally on the 3rd to 6th abdominal segments. Undigested remains are discharged via the anus. They also have a type of kidney (Malpighian tubules) via which excess nitrogen is discharged in the form of guanine. There are excretory coxal glands at the base of the first pair of legs.

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Some scorpion species are long (Hadogenes troglodytes up to 21 cm), others are heavy (Pandinus imperator, also called the Emperor scorpion; Heterometrus sp.), while others are small (Microtityus waeringi, adult 12 mm). All scorpions are exclusively carnivorous. While they wait for their prey, the tips of the movable fingers of the pedipalps as well as the tips of the comb (chemo- and mechanoreceptors) rest on the ground. A scorpion first grasps its prey (generally insects) with the pedipalps. If the prey is not immediately overpowered, they sting it by bending the tail forwards over the body. The venom is actively injected. The scorpion releases gastrointestinal juices over the prey in order to liquify it and later suck it up. They consume only the body fluids and liquidified tissues of their prey. A meal can last several hours. Some species are cannibalistic.

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Because many scorpions live in dry environments, they have become adapted to minimise loss of water. This is made possible in part by a watertight cuticle based on chitin. This has unusual optical characteristics. Scorpions fluoresce with a greenish colour under long-wave UV light. This makes them easy to spot at night with the aid of a UV lamp. The reason for this fluorescence is unclear. Recently moulted scorpions do not fluoresce. The fluorescence is caused by a substance in a thin layer of the cuticle, the so-called hyaline layer. The fluorescent substance might be beta-carboline (tryptophan metabolite), a by-product of the tanning process as the new cuticle hardens. If this layer is preserved in fossils, it also fluoresces. When the animals are preserved in alcohol, after some time the alcohol can also fluoresce.