Classification

Scorpions form an order of their own (Scorpiones) in the class Arachnida (arachnids). At present there are over 1,800 known species (Rein: "The Scorpion Files"). Their distribution ranges from Canada to the southern tip of South America in the New World and from southern Europe and central Asia to South Africa and Australia in the Old World. Some introduced species seem to be establishing themselves in southeast England and New Zealand.

In more recent classifications, 15–18 families are distinguished (Fet and Soleglad 2005 or Prendini and Wheeler 2005). The classification of Fet and Soleglad (2005) (shown in Rein: "The Scorpion Files") is listed below:

• Akravidae: until now only dead, troglomorphic specimens found in Israel (see below)
• Bothriuridae: 15 genera, 138 species. South America, southern Africa, India and Australia
• Buthidae: largest scorpion family with 85 genera and 892 species. Widespread around the world (not Antarctica and New Zealand)      
• Caraboctonidae: previously included in the family Iuridae. 5 genera, 18 species. North, Central and South America (including the Galapagos Islands)
• Chactidae: 11 genera, 167 species. North, Central and South America
• Chaerilidae: 1 genus, 28 species. South and Southeast Asia
• Euscorpiidae: 11 genera, 83 species. Central and Southern Europe, Mediterranean coast of Africa, Asia, Central and South America
• Hemiscorpiidae: previously = Ischnuridae. 12 genera, 84 species. Cosmopolitan in tropical and subtropical regions, except North America
• Iuridae: 2 genera, 4 species. Turkey, Iraq (and Syria?), Greece
• Pseudochactidae: 2 genera, 2 species. Asia (Uzbekistan, Tajikistan, Laos)
• Scorpionidae: includes former family of Diplocentridae. 17 genera, 248 species. Africa, Asia, Australia, North, Central and South America   
• Superstitioniidae: 1 genus, 1 species. Southwest USA
• Troglotayosicidae: 2 genera, 3 species. Troglomorphic. Europe (Spain and France) and South America (Ecuador and Colombia) 
  
• Typhlochactidae: 4 genera, 10 species. Troglomorphic. Eastern Mexico   
• Vaejovidae: 16 genera, 162 species. North and Central America.

Almost all the medically important species belong to the family Buthidae. With almost 900 species, this is the largest and most widely distributed family. Dangerous genera are Androctonus, Buthacus, Buthus, Buthotus, Centruroides, Leiurus, Mesobuthus, Parabuthus and Tityus. Many Buthidae species are yellowish, fawn or brownish in colour, but black species do exist. Among the other families there are only a few members that are known to cause serious envenoming: Nebo (Diplocentridae), Hemiscorpius and Heterometrus (Scorpionidae), to a lesser extent also Bothriurus (Bothriuridae).

Although exact taxonomic identification is based on characteristics that are difficult for laypersons to ascertain and identify, it is possible in most cases to distinguish between the medically important Buthidae and the other families on the basis of some simple criteria (Fig. 4.33c).

a			b
c	Shape of the sternum			Shape of the pincers
	triangular	pentagonal (sides parallel)	wide or divided	slender and smooth	massive, sometimes angular, sometimes rough surface
Buthidae	X	(only rarely)		X
Bothriuridae		X			X
Chactidae		X			X
Chaerilidae		X			X
Euscorpiidae		X			X
Iuridae & Caboctonidae		(only rarely)	X		X
Hemiscorpiidae		X			X
Scorpionidae		X			X
Vaejovidae		X			X

Fig. 4.33:  Morphology of scorpions (a, b adapted from Keegan 1980).
a Appearance from the dorsal view.
b Ventral view. Note in particular the sternum, which is bounded by the tops of the legs and the operculum.
c Relative characteristics of the most important families.

Morphological characteristics (Fig. 4.33)

The body is surrounded by a segmented chitin-containing shell and can be divided into 2 main parts, the prosoma (cephalothorax) and the opisthosoma (abdomen).

The prosoma is covered dorsally by a plate (carapace). A median pair of eyes is located on top of the carapace and is usually accompanied by lateral eyes on the sides of the carapace, in an anterior position. At the front of the prosoma there is a pair of chelicerae (mouth parts) and a pair of pedipalps, the terminal segments of which are in the form of prehensile pincers. Buthidae usually have slender pincers, in contrast to most species in the other families, which have more massive pincers (Fig 4.33c). Ventrally there are 4 pairs of legs as well as a pair of so-called combs (pectins) that act as sensory organs. The tops of the legs surround the sternum, the shape of which, together with other criteria, is a simple characteristic that can be used to differentiate between Buthidae and other families (Fig. 4.33c).

The opisthosoma consists of the mesosoma and metasoma ("tail"). Dorsally, the mesosoma is divided into 7 plates (tergites). The metasoma is composed of 5 segments and the terminal telson (sting). 

The largest species belong to the Scorpionidae family and can reach a length of 15–20 cm and a weight of up to 32 grams (Heterometrus swammerdami and Pandinus imperator). Buthidae species are known to be around 2–12 cm; the body lengths of most of the medically important species are between 5 and 9 cm.

Venom apparatus

The telson is bulbous at the tip and ends in a curved barb. The bulbous part contains a pair of venom glands. They are surrounded by musculature, contraction of which allows venom to be squeezed out at will. The two venom ducts exit through separate but closely located openings on the convex side of the sting tip. When threatened, the scorpion uses its flexible metasoma to bend the telson in an arch towards the region of the “head”. From this position it can carry out defensive strikes and is able to sting larger or resistant prey that cannot be overcome with the pincers alone.

In a recent publication Nisani & Hayes (2011) showed that scorpions can adjust the quantities of venom they deliver with a sting according to the level of threat. Under low threat conditions they tend to inject no venom (dry stings) or significantly lower quantities than under high threat or persistant threat conditions. Addittionally they seem to have the possibility to inject a less toxic "prevenom" or a more potent protein rich and toxic venom depending on the persistance of threat. The control of quantity and quality of venom injection seems to help to save the high costs in venom production.

Way of life

Scorpions are nocturnal animals that spend the majority of their lives in a state of inactivity in their hiding places. These microhabitats are usually narrow recesses, into which the scorpion literally squeezes itself, in order to be in contact with the substrate both above and below (thigmotaxis).
Of the ground-dwelling species of scorpions, some are burrowing animals that dig themselves tunnels in loose soil or take over tunnels from another animal. The non-burrowing species live in crevices or under stones and in similar places. Of the medically significant species, Androctonus spp., Leiurus quinquestrieatus, Buthus occitanus, Mesobuthus tamulus and some Parabuthus species belong to this group. So-called “bark scorpions” are found in vegetation and usually live above the ground, under loose bark, in hollow trees, leaf axils and so on. Several genera of the Buthidae, such as Centruroides, lead an epiphytic existence. Due to this way of life, and like the non-burrowing scorpions, they often find shelter in human habitations.
The range of habitats in which scorpions are found is wide. They inhabit deserts, savannas, grasslands, forests in temperate zones, rainforests, intertidal zones and even snow-covered mountains. Some species are found in or near caves. They show troglomorphic adaptations (reduced pigmentation and/or missing eyes). The variety of species increases steadily from the equator and the poles towards the subtropical regions. There it is the arid zones in particular that display a large number of species. Among them are found most of the dangerous species.

Epidemiology

In terms of the number of fatalities they cause, scorpions are the most important venomous animals after snakes and perhaps also hymenopterans. Bücherl (1971a) estimated over 5,000 fatal scorpion stings annually throughout the world. Newer estimates of the global epidemiology of scorpion stings assume an annual incidence of over 1.2 million stings, with over 3,250 fatalities (0.27%)  (Chippaux and Goyffon 2008). In Morocco the annual incidence of stings is estimated to be 40,000 (Ghalim et al. 2000). Regionally, the case fatality rate in Morocco is around 0.42–0.7% (Charrab et al. 2009, El Oufir et al. 2008), and in southern Algeria as high as 1.9% was determined (Balozet 1964). In Saudi Arabia the case fatality rate dramatically dropped parallel to better medical treatment with antivenoms (Mahaba 1997). In Mexico, up until the end of the 1950s, well over 1,000 people died each year from scorpion stings (Mazzotti and Bravo-Becherelle 1963). The number of fatalities in Mexico has now been dramatically reduced, but the incidence of stings is as high as before, with over 250,000 cases of scorpion envenoming each year (Alagon et al. 1999). With regard to other regions of the New World, numbers are available for Brazil, where there is an incidence of 8,000–21,000 stings per year (Alagon 1999).
The regions where scorpions are known to be a medical problem are Mexico (Centruroides sp.), Brazil and Trinidad (Tityus sp.), North Africa and the Middle East (Androctonus sp., Leiurus quinquestriatus, Buthus occitanus) as well as the Indian Subcontinent (Mesobuthus tamulus). To a lesser extent there are also dangerous species in the southern USA (Centruroides exilicauda), in some South American countries (Tityus sp.) and in southern Africa (Parabuthus sp.) that can cause serious envenoming.
In North Africa, Saudi Arabia and India the incidence of scorpion stings is highest during the hot summer months. The medically important species are often found close to the living and working spaces of humans, and, in some countries, such as Mexico, scorpion stings are also not uncommon in urban areas. In Saudi Arabia, the ratio of men to women stung by scorpions is 1.9:1 (Jahan et al. 2007). Infants and children up to the age of 15 represent the main risk group. Not only are they at greater risk of being stung while crawling on the ground or playing in and around the house, but envenoming is also generally more severe than in adults, due to their comparably low body weight. Mortality in children is many times higher than in adults.

Prevention

Prevention in regions with a high incidence of stings is primarily aimed at measures that can be taken in and around the house. Garbage, woodpiles or unrendered walls offer ideal hiding places for scorpions. Because of the simple methods of construction in warmer, poor countries, windows and doors, if they are present, are often not properly sealed, which allows scorpions and other vermin to enter without difficulty. Major structural changes are usually not possible, for economic and other reasons, but there is a simple method of preventing or at least making it difficult for scorpions to enter a house (Junqua and Vachon 1968). This involves mounting smooth glazed tiles in a seamless line around the external walls of the house, in such a way that the tiles extend from the ground up to the height of all the windows and doors. Scorpions are unable to climb up such smooth vertical surfaces. However, it is necessary that the tiles be cleaned regularly and that broken tiles are replaced.

An important principle within the house is never to leave clothes or shoes lying on the floor. They should always be shaken out before being put on. Before going to bed the bedclothes should be checked for hidden scorpions. During sleep, mosquito nets also provide good protection against scorpions.

Scorpions have various natural enemies, in particular birds. Chickens can be useful in reducing scorpion populations. Using insecticide to control scorpions can lead to temporary containment, but it must be insecticide that is specifically effective against arachnids. Large-scale culling efforts, such as take place every year in Sfax, Tunisia, also only have a temporary effect. There the local scorpion populations seem to recover within one year (Jeddi et al. 1988).

A clever means of finding nocturnal scorpions is the use of ultraviolet light. If scorpions are exposed to UV light, they phosphoresce a glowing yellow in the darkness. It is possible to buy torches with UV lamps.

Literature

General reviews: Brownell and Polis 2001, Fet et al. 2000, Fet and Soleglad 2005, Prendini and Wheeler 2005, Keegan 1980, Polis 1990, Stahnke 1984, Stockman and Ythier 2010

The Scorpion Files

The Scorpion Fauna

Arachnodata

Regional reviews: Baerg 1961, Braunwalder 2005, Franke and Stockwell 1987, Kaltsas et al. 2008, Levi and Amitai 1980, Lucas and Meier 1995, Najera 1975, Newlands 1978, Probst 1973, Stahnke and Calos 1977, Vachon 1948a and b, 1949