Scorpion

Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognized by the pair of grasping pedipalps and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9 mm / 0.3 in. (Typhlochactas mitchelli) to 23 cm / 9 in. (Heterometrus swammerdami).
The evolutionary history of scorpions goes back to the Silurian
 period 430 million years ago. They have adapted to a wide range of
environmental conditions, and they can now be found on all continents
except Antarctica. Scorpions number about 1,750 described species, with 13 extant (living) families recognised to date. The taxonomy has undergone changes and is likely to change further, as genetic studies are bringing forth new information.
All scorpions have a venomous sting,
 but the vast majority of the species do not represent a serious threat
to humans, and in most cases, healthy adults do not need any medical
treatment after being stung. Only about 25 species are known to 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.

Although scorpions are usually not found in large numbers in densely
populated urban areas, they do regularly occur in and near human
habitation in all tropical parts of the world. The lack of predators,
readily available shelter, and abundance of insect prey such as
crickets, cockroaches, silverfishes, and earwigs have caused scorpions
to thrive in industrial and residential areas. In Brazil, the number of people stung by scorpions increased from 12,000 in 2000 to 140,000 in 2018.
Sting and venom
All
 known scorpion species possess venom and use it primarily to kill or
paralyze their prey so that it can be eaten. The venom consists of a
collection of peptides.
In
 general, the venom is fast-acting, allowing for effective prey capture;
 however, as a general rule, scorpions kill their prey with brute force
if they can, as opposed to using venom, which is also used as a defense
against predators. The venom is a mixture of compounds (neurotoxins, enzyme inhibitors,
 etc.), each not only causing a different effect, but possibly also
targeting a specific animal. Each compound is made and stored in a pair
of glandular sacs and is released in a quantity regulated by the
scorpion itself. Of the more than one thousand known species of
scorpions, only 25 have venom that is deadly to humans; most of those
belong to the family Buthidae (including Leiurus quinquestriatus, Hottentotta spp., Centruroides spp., and Androctonus spp.).
Prevention
According to the United States National Institute for Occupational Safety and Health, these steps should be taken to prevent scorpion stings:
• Wear long sleeves and trousers.

The word "scorpion" is thought to have originated in Middle English between 1175 and 1225 AD from Old French scorpion, or from Italian scorpione, both derived from the Latin scorpius, which is the romanization of the Greek word σκορπίος – skorpíos.

Scorpions are found on all major land masses except Antarctica and New
Zealand. Scorpions did not occur naturally in Great Britain, Ireland,
Japan, South Korea, and some of the islands in Oceania,
 but now have been accidentally introduced in some of these places by
human trade and commerce. The greatest diversity of scorpions in the Northern Hemisphere is to be found in regions between the latitudes 23 and 38°N.
 Above these latitudes, the diversity decreases with the northernmost
natural occurrence of scorpions being the northern scorpion Paruroctonus boreus at Medicine Hat, Alberta, Canada 50°N. Five colonies of scorpions (Euscorpius flavicaudis) have established themselves in Sheerness on the Isle of Sheppey
 in the United Kingdom. This small population has been resident since
the 1860s, having probably arrived with imported fruit from Africa. This
 scorpion species is small and completely harmless to humans. At just
over 51°N, this marks the northernmost limit where scorpions live in the
 wild.
Today, scorpions are found in virtually every terrestrial habitat including: high-elevation mountains, caves, and intertidal zones, with the exception of boreal ecosystems such as: the tundra, high-altitude taiga, and the permanently snow-clad tops of some mountains. As regards microhabitats, scorpions may be ground-dwelling, tree-living, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and are found in every type of habitat in Baja California, while others occupy specialized niches such as Euscorpius carpathicus, which is endemic to the littoral zone of rivers in Romania.

Thirteen families and about 1,750 described species and subspecies of
scorpions are known. In addition, 111 described taxa of scorpions are
extinct.
This classification is based on that of Soleglad and Fet
(2003), which replaced the older, unpublished classification of
Stockwell. Additional taxonomic changes are from papers by Soleglad et
al. (2005).
Systematics
This classification covers extant taxa to the rank of family:
Order Scorpiones
• Infraorder Orthosterni Pocock, 1911

Scorpion remains have been found in many fossil records, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber.
 The oldest known scorpions lived around 430 million years ago in the
Silurian period. Though once believed to have lived on the bottom of
shallow tropical seas, early scorpions are now believed to have been
terrestrial and to have washed into marine settings together with plant
matter. These first scorpions were believed to have had gills instead
of the present forms' book lungs, though this has subsequently been refuted. The oldest Gondwanan scorpions (Gondwanascorpio)
 comprise the earliest known terrestrial animals from Gondwana.
Currently, 111 fossil species of scorpion are known. Unusually for
arachnids, there are more species of Palaeozoic scorpion than Mesozoic or Cenozoic ones.
Ancestral scorpions had compound eyes, but as they adapted to a nocturnal lifestyle, they became simplified.
The eurypterids, commonly called "sea scorpions", were aquatic creatures that lived during the Palaeozoic
 era that share several physical traits with scorpions and may be
closely related to them. Various species of Eurypterida could grow to be
 anywhere from 10 centimetres (3.9 in) to 2.5 metres (8.2 ft) in length.
 However, they exhibit anatomical
 differences marking them off as a group distinct from their
Carboniferous and Recent relatives. Despite this, they are commonly
referred to as "sea scorpions". Their legs are thought to have been
short, thick, tapering and to have ended in a single strong claw. It
appears that they were well-adapted for maintaining a secure hold upon
rocks or seaweed against the wash of waves, like the legs of a shore crab. Cladistic analyses have supported the idea that the eurypterids are a distinct group from the scorpions.

The body of a scorpion is divided into two parts (tagmata): the head (cephalothorax) and the abdomen (opisthosoma), which is subdivided into a broad anterior (mesosoma), or preabdomen, and a narrow tail-like posterior (metasoma), or postabdomen.
Cephalothorax
The cephalothorax, also called the prosoma, comprises the carapace, eyes, chelicerae (mouth parts), pedipalps (the pedipalps of scorpions have chelae, commonly called claws or pincers) and four pairs of walking legs.
 The scorpion's exoskeleton is thick and durable, providing good
protection from predators. 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 star light to navigate at night. Some species also have light
receptions in their tail. The position of the eyes on the cephalothorax
 depends in part on the hardness or softness of the soil upon which they
 spend their lives.
The pedipalp is a segmented, chelate (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 (humerus), 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, which are useful
taxonomically.
Mesosoma
The mesosoma is the broad part of the opisthosoma. Sometimes it is loosely called the abdomen.

Scorpions are also known to glow a vibrant blue-green when exposed to certain wavelengths of ultraviolet light such as that produced by a black light, due to the presence of fluorescent chemicals in the cuticle. One fluorescent component is now known to be beta-carboline.
 A hand-held UV lamp has long been a standard tool for nocturnal field
surveys of these animals. Fluorescence occurs as a result of sclerotisation and increases in intensity with each successive instar. This fluorescence may have an active role in scorpion light detection.