Scorpion anatomy (morphology)

The scorpion's body, like that of the spider, is divided into two main sections, the prosoma (cephalothorax) and the opisthosoma (abdomen).

The prosoma is the anterior (front) section covered dorsally (on top) by a hard sclerotized plate called the carapace. The 8 eyes (varies from 0 - 10, depending on family, genus and species) are situated, in 3 groups, on the carapace; a median pair and two lateral groups of three. 

The appendages are attached to the prosoma, namely:

1) The chelicerae, jaws, are used for feeding, grooming and digging (certain genera only, e.g. Opisthophthalmus) and in the genus, Opisthophthalmus, to stridulate (rub together to make a warning hissing sound). 

2) The pedipalps are chelate (crab-like) and consist of 6 segments are called the coxa, trochanter, femur, patella, tibia and the tarsus. The tibia is referred to as the manus or hand, with the tarsus forming the movable finger. Combined they form the chela (plural = chelae). The chela is equipped with trichobothria and functions as a sensory organ and is used for holding prey, defense, prey capture and in certain scorpions for digging burrows and crushing prey.

3) The four pairs of legs each comprise 7 segments, namely the coxa, trochanter, femur, patella, tibia, basitarsus and telotarsus. At the end of the  tarsus is the apotele, which contains the ungues (claws). The coxa of the legs are enlarged protecting the ventral (underside) surface of the prosoma.

The opisthosoma is divided into the anterior mesosoma and a narrow caudal (tail) section known as the metosoma. A sclerotized plate called the tergum covers each mesosomal segment dorsally and the genital opercula are situated ventrally in the first segment (nearest the legs). The second segment contains the pectines (comb-like sensory organs) while the sterna, segments 3 7, have spiracles which, with the exception of the seventh, open into booklungs.

The metosoma (tail) has 5 segments plus the distal (end) telson (stinger). The telson consists of a round vesicle, which contains the venom gland and a pair of muscles, and the aculeus (sting). Dorsally the metosoma has ridges and keels with the fifth segment always the longest and containing sensory setaeous bristles. The anus is situated ventrally just below the point it meets the telson. 

The pectines situated ventrally on the second mesosoma, are comb-like organs found only in scorpions. It has a sensory function, the purpose of which is still unclear, but it appears that it is used to detect pheromones or to determine the correct substrate to deposit the spermatophore during mating (i.e. chemo- and mechanoreceptors).

Unique to scorpions is that the cuticle fluoresces under ultraviolet light aiding in the collection and observation of these interesting animals at night.

Scorpions are sexually dimorphic. The females are usually larger and more robust than males while the male usually has a longer tail. The pectines are smaller with shorter and straighter teeth in the females. The chela is often longer and more slender in the male.

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Text by Norman Larsen .


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