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As in other arachnids, the scorpion’s body is divided into two main parts, the prosoma (cephalothorax) and the opisthosoma (abdomen). The opisthosoma is further divided into the mesosoma (preabdomen), which bears the bulk of the internal organs, and the tubular metasoma (postabdomen). The mesosoma consists of seven segments and the metasoma of five segments. Following the metasoma is the telson, which, in scorpions, is modified into a bulb-like structure to carry the paired venom glands and a curved aculeus (sting) to deliver the venom.
The prosoma is covered dorsally by a shield-like carapace, which bears the eyes (except in advanced troglobites). The typical scorpion has one pair of larger median eyes located near the front half of the carapace along the midline and two to five pairs of smaller lateral eyes positioned at the front corners of the carapace. The underside of the prosoma is made up of the bases of the appendages (coxae) that surround a small plate called the sternum. The mouth lies at the back of a small pre-oral chamber at the anterior end of the cephalothorax. This chamber, enclosed by the chelicerae above and the coxae of the pedipalps and first two pairs of legs below, is important in the feeding process because it serves as a temporary storage place for predigested food before it is sucked into the mouth. In all, the prosoma bears six pairs of appendages: the chelicerae (mouthparts), the pedipalps (pincers), and four pairs of legs.
The opisthosoma exhibits a clear segmentation pattern. The mesosoma bears a series of dorsal tergites that, except for the first two segments, lie over corresponding ventral sternites. The underside of the first abdominal segment bears the genital opercula, a pair of small semi-oval plates that conceal the genital pore. The second segment has the pectines, sensory organs that are unique to scorpions. The pectines are paired, comblike structures attached to a small plate called the basal piece; evidence suggests that the pectines function in evaluating textures of surfaces the scorpion is walking on and in detecting chemical substances (pheromones) used in sexual attraction. The first four sternites have small paired openings, or spiracles, in the posterior corners that lead into respiratory organs called book lungs. Each book lung consists of a set of leaflike membranes inside a small chamber. The membranes are vascularized and the surfaces exchange gases with the moist air in the chamber.
The tergites and sternites of the five metasomal segments are no longer visible as separate structures because they have presumably fused for strength. The inside of the metasomal segments are well supplied with musculature to provide for the rapid flexing and extensions needed while stinging, either in defense or offense. The intestine also runs through the metasoma and terminates in the anus, located in a membranous area on the underside between the end of the fifth metasomal segment and the telson.
The appendages are also segmented, although the number of segments in the chelicerae and pedipalps have been reduced by loss or fusion. The chelicerae are three-segmented, but only the last two of these are readily visible. The manus (the basal portion) is thought to represent the original tibia, and it bears a fixed finger armed with hard cusps. The movable finger, which represents the tarsus, apposes the fixed finger and also bears cusps. The fixed and movable fingers work together to “chew” the food.
The pedipalps are six-segmented appendages that terminate in enlarged, claw-like chelae. Each pedipalp is attached to the body by its coxa. The remaining articles of the pedipalp are the ring-like trochanter, the femur, the patella, and the chela (consisting of the tibia and tarsus). The femur, patella, and chela are long structures that are provided internally with numerous muscles; externally, they are provided with a variety of sensory hairs called setae and trichobothria. As in the case of the chelicera, the pedipalp chela manus and fixed finger comprise the tibia, and the movable finger the tarsus. The grasping ability of the pedipalp chela is essential in the prey-capture process and often in defense as well (especially in those species armed with massive pedipalps). The apposing edges of the fixed and movable fingers bear a series of small granules that are useful in holding onto prey once it has been captured; these granules are also useful to taxonomists who attempt to classify scorpions.
Each leg bears seven segments: the coxa, trochanter, femur, patella, tibia, basitarsus, and telotarsus. In particular, the basitarsus and telotarsus may bear spines and large bristles that are helpful when the scorpion is digging its burrow in the substrate. Sand specialists, such as members of the genus Paruroctonus, bear a row of long, bristle-like hairs on the outer edge of the tibia and basitarsi called setal combs -- these enhance locomotion on the sand. The tarsus terminates in a pair of curved ungues (tarsal claws) and a shorter dactyl (median claw). These claws enable the scorpion to climb vegetation, rocks, and other surfaces as well as to help wedge themselves into burrows when attacked by predators or while handling large prey.
Additional information on scorpion anatomy, including internal structure, may be found in Hjelle (1990).
Hjelle, J. T. Anatomy and Morphology. In G. A. Polis (Ed.), The Biology of Scorpions. Stanford University Press, Stanford, CA, 9-63.