A scorpion has a regular body plan that can be recognized even from the outside. In the traditional description, the body is divided into three parts: the prosoma at the front, the broad mesosoma in the middle and the slender metasoma at the rear. Together, the mesosoma and metasoma form the opisthosoma. This plan is not merely a descriptive convenience; species identification also relies largely on the measurements and proportions of the structures in these parts.
Scorpions are one of the oldest terrestrial groups among the arthropods, and their basic body plan has been largely preserved over a long stretch of geological time. The anatomy of living species can therefore be read as the same plan repeated with differences in size and proportion. Body length varies by species from a few centimetres to much larger sizes.
Prosoma: head and thorax combined
The prosoma is covered from above by a one-piece dorsal shield, the carapace. A pair of median eyes sits in the middle of the carapace, and pairs of lateral eyes, whose number varies by species, sit along its front edges. These eyes do not form detailed images; they serve to detect changes in light intensity. At the front are the small chelicerae used in feeding and, beside them, the pedipalps used for seizing and holding prey. The pincer at the tip of the pedipalp is called the chela. Four pairs of walking legs arise from the prosoma; this feature, which scorpions share with spiders, is one of the key differences that set them apart from insects.
Mesosoma: the section housing the vital organs
The mesosoma consists of seven segments and houses the digestive, circulatory, excretory and reproductive organs. Two structures unique to scorpions stand out on its underside. The first is a small flap called the genital operculum. The second is the comb-shaped pectines: this paired organ, which extends down to touch the ground, detects the texture of the substrate and chemical traces. It is considered to play an important role in finding mates and in hunting. Because the number of teeth on the pectines differs consistently between species, it is one of the criteria used in identification.
Respiration takes place through structures called book lungs. Made up of thin leaves stacked on top of one another, these organs open to the outside through small openings on the lower sides of the mesosoma. This design, which increases the surface area, is kept internal so as to limit water loss in arid environments.
A large part of the internal volume of the mesosoma is taken up by the digestive system and by tissues that serve as reserves. The ability of scorpions to go without food for long periods is explained partly by this storage capacity and partly by their low metabolic rate. In females, the developing young are also carried in this section.
Metasoma and telson
The metasoma is the slender, five-segmented section commonly called the tail. The telson that follows the last segment has two parts: the swollen vesicle that houses the venom glands, and the pointed aculeus that follows it. The venom glands sit as a pair inside this vesicle, and the secretion is expelled through openings near the tip of the aculeus. The thickness of the metasomal segments, their carinae and their granulation pattern are decisive in distinguishing families and genera; in members of the Buthidae the metasoma is typically robust and the pincers relatively slender. How the gland works and how the secretion is collected under controlled conditions are covered in the article on venom extraction.
| Section | Number of segments | Key structures |
|---|---|---|
| Prosoma | One-piece carapace | Eyes, chelicerae, pedipalps, four pairs of legs |
| Mesosoma | Seven | Genital operculum, pectines, book lungs, internal organs |
| Metasoma | Five, plus the telson | Vesicle, venom glands, aculeus |
The segmented structure of the metasoma gives it great freedom of movement. When the animal perceives a threat, it curls its tail over its back and brings the telson forward; this posture is both a warning and a sign of readiness. The way the tail is carried, even at rest, is a behavioural trait that differs between species.
Exoskeleton and senses
The cuticle that encases the body is a hard exoskeleton made of chitin-based layers. It provides mechanical protection and also reduces water loss. Because it is not possible to grow inside a rigid casing, scorpions moult at intervals; the period until the new exoskeleton hardens is when the animal is at its most vulnerable. Another interesting property of the cuticle is that it gives off a greenish glow under ultraviolet light; the reasons for this are discussed in the article on UV fluorescence.
On the sensory side, the most striking structures are specialized hairs called trichobothria. Located at particular points on the pedipalps and legs, these hairs detect the slightest movement of air and help to locate prey and threats in the dark. Because the number and arrangement of trichobothria differ consistently between species, they are a criterion frequently used in systematics. Each of these hairs is connected to a sensory cell at its base and responds only to air movement coming from certain directions; the arrangement is therefore not random but functional.
The limited ability to see increases the importance of the other senses. Vibrations in the ground are detected by specialized receptors in the legs, and the direction of prey can be determined with surprising accuracy. For a predator that hunts in the dark, this sensory equipment is a far more reliable guide than the eyes. The use of the chelicerae and pincers is also guided by this sensory information.
Which details are examined in identification?
Morphological identification relies not on a single feature but on a set of criteria: the granulation of the carapace, the length-to-width ratios of the metasomal segments, carina patterns, the number of pectinal teeth, the arrangement of trichobothria and the shape of the pincers are assessed together. The general features of the family can be found in the article on the Buthidae, and the profile of our facility's source species in the article on Androctonus turkiyensis.
