Scorpions (Insects)

Scorpions are among the most recognizable groups of arthropods. Their highly segmented body plan is uniquely subdivided into a leg-bearing prosoma, a broad seven-segmented preabdomen (mesosoma), and a narrow five-segmented postabdomen (metasoma) that terminates in a bulbous stinging organ, the telson. The fossil record indicates that scorpions were the first arthropods to occupy the terrestrial environment. An important member of the class Arachnida, scorpions survived periods of mass extinctions and today occupy a prominent position in arthropod communities. This gives them the distinction of being the oldest surviving group of terrestrial arthropods.

EVOLUTION

Scorpion fossils date back to the Silurian, some 400 mya. The oldest known taxon, Palaeophonus, is strikingly similar in structure to modern forms in their extensive, characteristic segmentation, appendages, and general body form. Their body was organized into four regions: a prosoma (cephalothorax), a broad mesosoma (“prea-bdomen” containing digestive and reproductive organs), a taillike metasoma (slender “postabdomen” composed of five ringlike segments), and a terminal postanal segment, the telson. Palaeophonus differs from modern scorpions in not having pretarsal claws on the distal ends of walking legs and in having a blunt ending of the tel-son. The absorptive membranes of the topic lungs might have been eversible, forming a gill-like respiratory structure. This suggests that Palaeophonus might have existed in aquatic and land habitats.
The fossil record indicates that scorpions underwent rapid adaptive radiations that led to structural diversity, especially in the segmentation and morphology of the ventral prosoma, the development of pretarsal claws, the formation of a sharp venom-delivering aculeus on the terminus of the telson, and trichobothrial patterns.


MORPHOLOGICAL CHARACTERISTICS

Scorpions are readily distinguished by their unique morphology (Fig. 1). Their prosoma consists of six body segments that are covered dorsally by an unsegmented carapace. Each of these segments bears a pair of characteristic segmented appendages. The first pair, the cheli-cerae, is small, chelate or scissorlike, and serves as the mouthparts. The second appendages are large, conspicuous pedipalps that terminate in a strong scissorlike chela used to capture and immobilize prey. The
Dorsal and partial ventral views of scorpion showing the distinctive body segmentation, tagmosis, and morphological features.
FIGURE 1 Dorsal and partial ventral views of scorpion showing the distinctive body segmentation, tagmosis, and morphological features.
next four pairs of appendages are the walking legs, each of which terminates in a pair of distinctive pretarsal claws, the ungues. Along with a single pair of median ocelli, there are two groups of lateral ocelli located at the anterolateral corners of the carapace. Each group of lateral ocelli consists of zero to four facets, evidently derived from a primitive compound eye. The mesosoma is attached broadly to the prosoma, lacks appendages other than the sensory pair of comblike pec-tines, and houses the digestive, urinary, and reproductive organs. The four pairs of respiratory topic lungs open to the outside through stigma on mesosomal sterna two through six. Expansion of tergal-sternal and intersegmental membranes permits meal engorgement during feeding and also allows for increase in body volume to accommodate internal development of the eggs and embryos. The metasoma is composed of five narrow segments forming a freely articulating, taillike body region. A single unsegmented telson freely articulates with the metasoma and is composed of a swollen vesicle that terminates in a sharply pointed aculeus or “sting.” Paired venom glands are housed in the vesicle, and the associated aculeus injects the venom into the victim (Fig. 2 ).
A giant hairy scorpion (Hadrurus concolorous) in alert posture. Notice the pedipalps positioned forward with their chelae open. The metasoma is in flexed position, ready to strike. The body is balanced over and supported by the walking legs.
FIGURE 2 A giant hairy scorpion (Hadrurus concolorous) in alert posture. Notice the pedipalps positioned forward with their chelae open. The metasoma is in flexed position, ready to strike. The body is balanced over and supported by the walking legs.
Scorpions all over the world have the same basic body structure, which is surprising considering the antiquity of the extant families.

ANATOMICAL ADAPTATIONS

Scorpions demonstrate basically the same organ systems characteristic of most arachnids. These systems reflect adaptations that support their successful lifestyle as opportunistic ambush predators. The digestive system begins with a tiny mouth located below the base of the chelicerae. Its small diameter allows intake of food only in fluid form. Preoral enzymes are secreted by salivary glands directly into the body of the prey, and the liquefied, preorally digested tissues are ingested through a sucking action. This is facilitated by a pressing or grinding action of the gnathobases at the inner bases of the pedipalps.
Large, expansive midgut ceca facilitate rapid intake and storage of nutrients. Because particulate matter is not ingested with a meal, fecal wastes are minimal. Nitrogenous wastes are excreted by means of Malpighian tubules that deposit wastes, primarily in the form of crystalline guanine directly into the digestive tube for subsequent elimination. Some species have also been found to excrete nitrogenous wastes in the form of uric acid and xanthine. Scorpions have no tracheal system but instead use four pairs of topic lungs to respire. Air passes through four pairs of tiny stigmata, which are opened and closed by membranous opercula in the mesosomal sterna. Inspired air then enters the lamellar spaces of the membranous topic lungs. This limiting mode of oxygen uptake is correlated with the sedentary lifestyle and metabolic simplicity characteristic of most scorpions. A ventral nerve cord with highly meta-meric ganglia characterizes the nervous system. At the cephalic end is found the subesophageal ganglion and supraesophageal brain. The circulatory system consists of a long, pulsing muscular tube, called the dorsal vessel, and colorless blood composed of hemolymph and hemocytes.
The reproductive system has organs arranged in a somewhat segmented, fishnet like form within the mesosoma. The male has a paraxial organ that contains a pair of hemispermatophores that fuse to form a sperm-carrying spermatophore used for indirect insemination of a female during the courtship dance. The female system is composed of a hollow tube with an inner lumen that exits the body through a ventrally located genital aperture which is covered by a valve like genital operculum adjacent to the basal plate of the pec-tines. Multiple oocytes in different stages of maturation may be seen on the surface of the reproductive tube. Fertilization is internal, and the female carries the developing embryos until birthing as larvae.

SYSTEMATICS

Substantial numbers of undescribed species and genera are still being discovered. This largely results from the nocturnal and secretive habits characteristic of most scorpions, which make them difficult to find. The scorpion fauna from many regions are also poorly known and largely unrepresented in scientific collections. Approximately 1300 species of living scorpions are currently recognized and are assigned to 13 families and about 160 genera (Table I). The higher classification of scorpions is the subject of current reevaluation. The trend has been to recognize more families and genera, and to base the resulting classification on phylogenetic relationships as suggested by cladistic analyses. Recently, a number of impressive regional studies have been published, and most have resulted in recognizing numerous new taxa. The numbers of extant scorpion families and species are surprisingly low considering their success as a group, their broad distribution, their diverse habitats, and their antiquity.

DISTRIBUTION AND BIODIVERSITY

Scorpions are conspicuously common in tropical and subtropical regions throughout the world. They also range into the more temperate regions of both the Northern and Southern Hemispheres. In North America, they range as far north as British Columbia, Alberta, and Saskatchewan, Canada (approximately 52°N). In South America, scorpions range south into Chile and Argentina (approximately 50°S).
Scorpions show wide-ranging adaptations to different elevations. In Baja California, Mexico, scorpions are found in the intertidal and beach habitats. In the White Mountains of California, they are established at elevations up to 3170 m (10,400ft). Scorpion species show definite habitat preferences and are normally found in patchy distributions. Although they are often thought of as tropical, scorpions seem to reach their highest biotic diversity in the arid regions of more temperate latitudes. Most scorpion habitats are characterized by a modest diversity ranging from one to five sympatric species. The greatest regional diversity of scorpions is reported from Baja California, where more than 61 species and 11 genera are found. The higher latitudes and higher elevations have more limited diversity, often with only one or two species represented in a given habitat.

BIOLOGY

General Life Cycle Strategy

Scorpions often go unnoticed, even when abundant. They are mainly nocturnal, secretive animals and usually remain inactive, hidden in their retreats, except when feeding, mating, or disturbed. For many scorpions, such as Hadrurus and Vaejovis, adults are the stage usually encountered; juveniles tend to minimize their exposure in the environment. This pattern is most conspicuous in scorpions found in more temperate areas and at higher elevations. In other species, such as Centruroides spp. and some Paruroctonus and Smeringerus spp., however, juvenile instars are commonly encountered.
The scorpion life cycle is a simple one consisting of an egg, a single larva stage, 2-6 nymphal instars (depending on the species), and the adult stage. The larva differs morphologically from other instars in lacking the typical unguicular claws at the tips of the walking legs, lack of a sharp aculeus or “sting,” lack of dark pigmentation and scle-rotization of the exoskeleton, and lack of effective locomotor ability. Behaviorally, the larva is unique in that it usually remains on the mother’s back and does not travel alone or feed. The sedentary larva usually molts to the first nymph stage on the mother’s back at which time it shows increase in linear dimensions, has pretarsal claws and a sharp

TABLE I
Higher Classification of Contemporary Scorpions
Family Number of
genera
Common genera
Bothriuridae 14 Bothriurus, Brachistosternus, Brazilobothriurus, Centromachetes, Cercophonius, Lisposoma, Orobothriurus, Pachakutej, Phoniocercus, Tehuankea, Thestylus, Timogenes, Urophonius, Vachonia
Buthidae 76 Ananteris, Androctonus, Babycurus, Buthacus, Buthus, Centruroides, Compsobuthus, Hottentotta, Isometrus, Leiurus, Lychas, Mesobuthus, Orthochirus, Parabuthus, Rhopalurus, Tityus, Uroplectes
Caraboctonidae 4 Caraboctonus, Hadruroides, Hadrurus, Hoffmannihadrurus
Chactidae 11 Anuroctonus, Belisarius, Broteochactas, Brotheas, Chactas, Hadrurochactas, Neochactas, Nullibrotheas, Teuthraustes, Uroctonus, Vachoniochactas
Chaerilidae 1 Chaerilus
Euscorpiidae 11 Alloscorpiops, Chactopsis , Dasyscorpiops, Euscorpiops, Euscorpius, Megacormus, Neoscorpiops, Parascorpiops, Plesiochactas, Scorpiops, Troglocormus
Hemiscorpiidae 11 Cheloctonus, Chiromachetes, Chiromachus, Habibiella, Hadogenes, Hemiscorpius, Heteroscorpion, Iomachus, Liocheles, Opisthacanthus, Palaeocheloctonus
Iuridae 2 Calchas, Iurus
Microcharmidae 2 Microcharmus, Neoprotobuthus
Pseudochactidae 1 Pseudochactas
Scorpionidae 13 Bioculus, Cazierius, Didymocentrus, Diplocentrus, Heterometrus, Heteronebo, Nebo, Oiclus, Opistophthalmus, Pandinus, Scorpio, Tarsoporosus, Urodacus
Superstitioniidae 5 Alacran, Solanochactas, Superstitionia, Troglotayosics, Typhlochactas
Vaejovidae 16 Franckeus, Gertschius, Hoffmannius, Kochius, Paravaejovis, Paruroctonus, Pseudouroctonus, Serradigitus, Smeringurus, Stahnkeus, Syntropis, Thorellius, Uroctonites, Vaejovis, Vejovoidus, Wernerius

aculeus, and becomes increasingly physically active. At this time, the first nymphal instar normally leaves the mother’s back, disperses from the mother’s shelter, locates a new shelter, and begins its own independent life. After periods of feeding and growth, nymphs periodically molt to their successive instars, until adulthood is reached. Adults differ from the nymphs in being sexually mature, but they often show little morphological difference except in their larger size and sometimes in the proportion of body parts such as the larger pedipalp chela and elongation of the metasoma. Adults may also show distinctive sexual dimorphism in body proportions. A few species appear capable of reaching sexual maturity during the last two life cycle instars.

Feeding

Ambush predation is the main means of prey capture. For example, some burrowing species such as Anuroctonus phaiodactylus quietly wait inside their burrow entrances and ambush prey that enters or passes by their burrow. Many species of scorpions, when hungry, leave their protective shelter during nocturnal hours and take a stationary position in their environment, which may be on the substrate surface or in vegetation. Here, they will remain motionless until an unsuspecting prey ventures close, at which time it is grasped with the strong pedipalp chela and may be stung, if necessary. Other scorpions such as Centruroides spp. often range great distances from their diurnal shelters in search for prey. At such times, they may actively stalk their prey. If a scorpion is unsuccessful, it will return to its shelter and resume its predation behavior the next night, and the night after, until successful.
After successful prey capture, a scorpion draws the immobilized prey close to its oral cavity and exudes preoral digestive fluids that digest the prey’s tissues internally. During ingestion, the cheli-cerae also shred prey tissues. The resultant fluid is sucked into the oral region, and any remaining fluids are pressed from the prey remains by the gnathobases of the inner pedipalp coxae. Up to 97% of the body mass of a prey may be ingested. A satiated scorpion then returns to its shelter and will normally not be seen until hungry again, which may be from 2 weeks to a month or 2. Its effective mode of predation, habit of engorgement feeding, and low metabolic rate result in a lifestyle that requires minimal prey, minimal exposure of the scorpion, and survival when prey are not readily available. Scorpions in the laboratory may go for as long as 12 months between meals. However, in nature, one feeding every 2-6 weeks is probably more common. Almost any animal that a scorpion can catch and immobilize is a potential prey, so scorpions feed on a variety of insects and spiders. Predation on centipedes, other scorpions, lizards, and small snakes has also been observed.

Reproduction

The scorpions’ reproductive mode is one of the most ancient ones used by terrestrial arthropods. It involves sexual reproduction, with insemination of the female by the male following a ritualistic courtship dance. Internal fertilization is complicated by the lack of an intromittent organ in the male. Fertilization takes place indirectly by a spermatophore deposited by the male.
Pheromones bring potential mates together and initiate courtship. When two potential mates encounter each other, they grasp each other by their pedipalpal chelae and undergo a forward and backward “dancelike” behavior, the “promenade a deux,” which may be over in few minutes or last for hours. At the end of this dance, the male emits a sclerotized spermatophore onto the substrate and draws the female over it, at which time she takes up the sperm packet in her gonopore. The two then part, and the female normally returns to her shelter. Depending on the taxa, gestation takes from approximately 2 or 3 months to nearly a year. Development is internal and results in the birth of a dozen to over a hundred larvae, which quickly ascend to the mother’s back and remain there for the remainder of their first stadium. Parturition takes from an hour to about 3 days, depending on the species and number of offsprings. Some species seem to have several birth cycles each year, whereas others seem to have only one per year. Parthenogenesis has been reported in Tityus serrulatus and Liocheles australasiae. Field observations of Liocheles suggest that some natural populations might have parthenogenesis as their primary means of reproduction.
Scorpions have been roughly categorized as “ground” vs. “bark” dwellers. Ground dwellers construct burrows in the ground, seek shelter in rocky substrate, invade the burrows of other organisms, or occupy protective spaces under or within ground surface debris. Bark dwellers are commonly found sheltered under bark, in bromeliads, in vegetation, in residential thatch, or in other plant material. They are often climbers and may primarily occupy forest canopies. During some seasons, forest-dwelling forms may migrate to the ground to seek shelter under rocks or surface debris. This group has members that invade human habitations and can be seen climbing walls and ceilings at night. They may reside in thatching and in the mortar spaces between bricks and rocks of walls. Most ground scorpions are solitary, although some bark scorpions such as Centruroides may be found in aggregations during certain times of the year. A. phaiodac-tylus constructs a burrow as a young nymph and occupies it throughout its life, which may last for several years. It seldom ventures far from its burrow and is intolerant of other scorpions in its burrow. Many of the ground-dwelling species probably spend most of their life in their permanent burrow. At time of courtship, however, mature males abandon their burrows or other territory and become more nomadic in search of mates. Scorpions are thigmotactic, seeking closely fitting shelters. The availability of suitable shelters, or home sites, is believed to limit the population size of some species.
Scorpions are found in a variety of habitats from desert to mesic, and they thrive in highly xeric environments. They are common in tropical and subtropical regions, and many species extend into the temperate latitudes. Most scorpions prefer well-drained habitats, but some species of Centruroides, for example, thrive in riparian and tropical rain forest environments, and seem to survive drowning by climbing trees during times of flooding. Some scorpions, such as species of Lsometrus and Centruroides, are successful oceanic travelers, dispersing among islands on floating palm logs. Burrowers vary greatly in their choice of habitats and behavior: A. phaiodactylus requires soil with good drainage and adequate compaction to support permanent burrows; Vejovoidus lon-giunguis constructs burrows in unconsolidated soil of sand dunes; species of Serradigitus do not burrow but live in crevices, talus slopes, and rock fractures; Superstitionia donensis may occupy a burrow or may be found in a simple cell constructed under a rock, animal dung, or vegetation; Centruroides thorellii resides in bromeliads within tropical forests; Vaejovis littoralis is found in the intertidal wrack zone, where it continually migrates ahead of the changing tides; Centruroides exilicauda seeks shelter under bark of trees, but may also be found under surface debris or in rock crevices, and is a common invader of human habitations; and Uroctonus mordax is commonly found in wet habitats, where it may reside under mosses and rotting logs, and in soil crevices. Almost any habitat that provides adequate shelter and prey is suitable for some kind of scorpion. As a result, these arthropods are globally widely distributed.

Predators and Parasites

Although scorpions do not have many natural enemies, they are an attractive prey for a few predators. Their rich nutrient content and numerical abundance contribute to their vulnerability as prey. Predators that hunt during the nocturnal hours are particularly effective in feeding on scorpions. Most capture scorpions while they are exposed on the substrate surface, but some, such as coyotes, will dig them up. Some 124 vertebrate and 26 invertebrate predators have been reported to feed on scorpions. Elf owls (Micrathene), burrowing owls (Speotyto), barn owls (Tyto), grasshopper mice (Onychomys), coyotes, bats, desert shrews (Noteosorex), and a variety of lizards are among the most conspicuous scorpion predators. Parasitism of scorpions is not common; reported endoparasites include larvae of a tachinid fly (Spilochaetosoma californicum), mermithid nematode larvae, and the larvae of a sarcophagid fly (Sarcodexia sternodontis). About eight species of mites have been reported as ectoparasites of scorpions, but mite parasitism is not commonly observed.

VENOMS

Scorpions are universally recognized because of their venoms and conspicuous stinging apparatus. All species are venomous, but only a few are harmful to humans. The venom is produced in a pair of glands located in the vesicle of the telson and collects in the lumen of each gland, flowing through a simple duct that terminates near the sharp tip of the aculeus. The venom is delivered during a rapid thrust of the metasoma that results in the penetration of the victim’s skin by the sharp aculeus. Contraction of muscles surrounding each venom gland discharges the venom.
The venoms are complex substances composed of water, a number of low molecular weight proteins (neurotoxins), and various organic compounds such as oligopeptides, nucleotides, amino acids, mucus, and cellular debris. A common fraction of venoms function as simple irritants, often causing a sharp burning sensation. Other fractions may cause inflammatory responses resulting in edema. In highly toxic venoms, systemic components may result in neurological symptoms such as convulsion followed by death. The neurotoxic effects seem to be the result of multiple interactions of certain toxins with voltage-dependent ion channels of excitable cell membranes, such as those of neurons. Venom interactions may include membrane depolarization, repetitive firing, prolonged action potentials, and massive release of neurotransmitters, most importantly from the adrenal medulla.
The cumulative effects of a venom in envenomated animals are complex and varied. Envenomation particularly may affect skeletal muscles, the cardiovascular system, lungs, visceral smooth muscle, uterus, and glands. The severe convulsions that may be associated with the sting of more toxic species (e.g., Centruroides spp.) are of particular concern. Each scorpion venom contains a number of different toxic components, which explains the varied venom reactions observed.
Scorpion venoms are toxic to a variety of organisms, including arthropods and vertebrates, but are often nontoxic to other scorpions. The hemolymph may have the capacity for neutralizing the toxic components in the venom from other scorpions.
The number of reported human deaths from scorpion stings ranges from several hundred to several thousand annually. Stings are considered to be more serious when they happen to children, the elderly, and those in poor health. The number of reported deaths has been declining because of better treatments, development of antivenins, the reduction of human contact with scorpions in critical areas, and public health awareness. Because of unreliable reporting in developing countries, however, the reported incidence of scorpion stings and mortality is probably greatly underestimated.
Venomous scorpions of concern to humans are found throughout the world, but some of the more severe problems are in the more arid regions. Of the 1300 known species, only a dozen or so cause significant health problems for humans. The species of greatest medical concern are members of the family Buthidae and belong to the genera Androctonus, Buthacus, Buthus, Centruroides, Leiurus, Mesobuthus, Parabuthus, and Tityus. Regions with significant scorpion sting problems often have antivenins available at local pharmacies.

METHODS OF OBSERVATION AND STUDY

Sampling and Collecting

Scorpion assessments and collections were traditionally made by inspecting trees and by looking under rocks, trash piles, and other surface debris. The discovery that scorpions fluoresce under the ultraviolet wavelengths of black light blue (BLB) bulbs led to the use of the “ultraviolet method” of scorpion detection and study. Equipped with a portable ultraviolet light, an investigator can walk through a habitat at night, counting and observing scorpions undisturbed in their natural state. With such a lamp, a scorpion can be readily detected on a sand dune at a distance of about 15 m.

Care and Maintenance of Captive Specimens

Scorpions are easily kept in captivity and require minimal care. They may be housed in any closed container, such as a terrarium, plastic box, jar, or plastic bag. If soil is provided, a rock or other surface cover should be added for shelter because scorpions are thigmo-tactic and seek close-fitting shelters. A small amount of water needs to be provided periodically. Scorpions can be maintained at room temperature and should not be exposed to freezing temperatures, especially the tropical forms. Most live insects may be supplied for food, but crickets are particularly well accepted. Generally, one cricket every 2-4 weeks is adequate feeding for a moderate-sized scorpion.
As a rule, scorpions remain inactive in their shelters during the day and will show normal activity (if any) at night. Lack of physical activity is common and typical of most scorpions. Excessive activity often indicates a lack of food or water, excess light, excessive temperature, or other disturbance. Covering the rearing chamber with red cellophane readily simulates nocturnal environmental conditions. Most scorpions will live for 1-5 years in captivity.

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