scorpion

Size range and diversity of structure

Scorpions are relatively large among terrestrial arthropods, with an average size of about 6 cm (2.5 inches). Scorpions exhibit few sexual differences, although males usually are more slender and have longer tails than females. Giants among scorpions include the black emperor scorpion (Pandinus imperator), an African species found in Guinea, which attains a body length of about 18 cm (7 inches) and a mass of 60 grams (more than 2 ounces). The longest scorpion in the world is the rock scorpion (Hadogenes troglodytes) of South Africa; females attain a length of 21 cm (8.3 inches). The length of the smallest scorpions, the Caribbean Microtityus fundorai, is 12 mm (0.5 inch). A few precursors of modern scorpions were comparative giants. Fossils of two species (Gigantoscorpio willsi and Brontoscorpio anglicus) measure from 35 cm (14 inches) to a metre (3.3 feet) or more, and an undescribed species is estimated to have been 90 cm (35.5 inches). Most species from deserts and other arid regions are yellowish or light brown in colour; those found in moist or mountain habitats, however, are brown or black.

Distribution and abundance

In addition to desert habitats, scorpions have adapted to temperate, subtropical, and tropical environments such as grasslands, savannas, and forests. They live on all major landmasses except Greenland and Antarctica. Their range extends from Canada and central Europe to the southern tips of South America (Tierra del Fuego) and Africa, and they have been accidently introduced into New Zealand and England. Scorpions have been found at elevations from sea level to 5,000 metres (more than 16,000 feet) in mountains of Europe and North and South America. A few species live as far north as southern Canada, southern Germany, and Russia.

Reproduction and life cycle

Breeding is seasonal and generally occurs during the warm months, ranging from late spring through early fall. Males may travel hundreds of metres to find receptive females. It appears that males find females by localizing a pheromone that the female emits from the end of her abdomen. Mating in scorpions is preceded by a complicated and characteristic courtship initiated by the male. He first faces and grasps the female, using his pincers (pedipalps). Then the pair, directed by the male, moves sideways and backward in a dancelike motion called promenade à deux. These actions result from the efforts of the pair to find a smooth surface on which the male can extrude a glandular secretion that forms a stalk to which the spermatophore (sperm-containing structure) is attached. He then maneuvers the female so that her genital opening contacts the spermatophore. Once she is positioned over the spermatophore, physical contact with it causes spermatozoa to be ejected into the genital opening (gonopore) of the female. Males that remain near females after mating are sometimes killed and eaten.

In general, females mate multiple times. In some species, mating must occur after each clutch of offspring is produced in order to fertilize another clutch of eggs. In others, multiple clutches of offspring can be produced from the sperm stored from a single mating. There are at least two species known that can produce offspring without mating at all (parthenogenesis).

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The mother invests a great amount of time and energy in her offspring. Unlike most nonmammalian animals, scorpions are viviparous, giving birth to live young instead of laying eggs. Once fertilized, the eggs are retained in the female’s body, where the embryos are nourished in utero for periods varying from several months to a year. The birth process itself may last from several hours to several days. Temperate species usually give birth in spring and summer, whereas tropical species give birth throughout the year. Litter size averages 25, with a range of 1 to more than 100.

At birth a young scorpion is white and enveloped in a membrane, or chorion. After freeing itself, the immature scorpion crawls onto the mother’s back, where it remains for a period ranging from 1 to 50 days. During this time the young scorpions are defenseless and utilize food reserves in their bodies while receiving water transpired through the mother’s cuticle and taken up through their own. The young molt their soft embryonic cuticle for one that is fully functional when they assume independence. This early mother-young association is obligatory for newborns. Without it, they do not molt successfully and usually die. The young generally leave the mother soon after this first molt.

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As in all arthropods, growth is accompanied by molting (ecdysis). Scorpions molt an average of five times (the range is four to nine) before reaching maturity. The number of molts in some species is variable. In some Centruroides, for example, small males mature after four molts and large males after five. There are no reports of molting’s occurring after reproductive maturity has been reached.

Food and feeding

Scorpions are opportunistic predators that eat any small animal they can capture. Common prey includes insects as well as spiders and other arachnids, including other scorpions. Less-common but regular prey includes pill bugs, snails, and small vertebrates such as lizards, snakes, and rodents. The only known specialist scorpion is the Australian spiral burrow, or spider-hunting, scorpion (Isometroides vescus), which feeds solely on burrowing spiders.

Most scorpions are sit-and-wait predators that remain motionless until a suitable victim has moved into an ambush zone. Scorpions can sense tiny ground vibrations, and some can detect airborne vibrations of flying insects. These behaviours are sophisticated to the extent that scorpions can determine the precise distance and direction of their prey. Once the prey has been detected, the scorpion turns, runs to the prey, and seizes it. The prey is stung if it is relatively large, aggressive, or active. Otherwise it is simply held by the pedipalps as it is eaten. Many of the thick-tailed scorpions (family Buthidae), however, actively search for prey. These species usually have long, slender bodies and pincers (chelae). Many have powerful venoms to compensate for their small pincers.

Scorpions lack conventional jaws, and their feeding habits are unusual. An additional pair of pincerlike appendages (chelicerae) are toothed, and, with these tools as well as the sharp edges of adjacent jawlike structures (maxillae and coxae), the scorpion chews the prey as quantities of digestive fluids secreted from the midgut pour over it. The victim’s soft parts are broken down, liquefied, and sucked into the scorpion’s stomach by a pumping action. The victim is gradually reduced to a ball of indigestible material, which is cast aside. Eating is a slow process, often taking many hours.