Temporal range: Late Carboniferous–recent
|Solifugid from Arizona|
The Solifugae are an order of animals in the class Arachnida known variously as camel spiders, wind scorpions, sun spiders, or solifuges. The order includes more than 1,000 described species in about 153 genera. Despite the common names, they are neither true scorpions (order Scorpiones) nor true spiders (order Araneae) – though they are more closely related to scorpions than to spiders. Much like a spider, the body of a solifugid has two tagmata: an opisthosoma (abdomen) behind the prosoma (that is, in effect, a combined head and thorax). At the front end, the prosoma bears two chelicerae that, in most species, are conspicuously large. The chelicerae serve as jaws and in many species also are used for stridulation. Unlike scorpions, solifugids do not have a third tagma that forms a "tail". Most species of Solifugae live in dry climates and feed opportunistically on ground-dwelling arthropods and other small animals. The largest species grow to a length of 12–15 cm (5–6 in), including legs. A number of urban legends exaggerate the size and speed of the Solifugae, and their potential danger to humans, which is negligible.
Solifugae are moderately small to large arachnids (a few millimeters to several centimeters in body length), with the larger species reaching 12–15 cm (5–6 in) in length, including legs. In practice, the respective lengths of the legs of various species differ drastically, so the resulting figures are often misleading. More practical measurements refer primarily to the body length, quoting leg lengths separately, if at all. The body length is up to 7 cm (3 in). Most species are closer to 5 cm (2 in) long, and some small species are under 1 cm (0.4 in) in head-plus-body length when mature.
Like that of the spider order, the Araneae, the body plan of the Solifugae has two main tagmata: the prosoma, or cephalothorax, is the anterior tagma, and the 10-segmented abdomen, or opisthosoma, is the posterior tagma. As shown in the illustrations, the solifugid prosoma and opisthosoma are not separated by nearly as clear a constriction and connecting tube or "pedicel" as occurs in "true spiders", the order Araneae. The lack of the pedicel reflects another difference between Solifugae and spiders, namely that Solifugids lack both spinnerets and silk, and do not spin webs. Spiders need considerable mobility of their abdomens in their spinning activities, and the Solifugae have no need for any such adaptation.
The prosoma comprises the head, the mouthparts, and the somites that bear the legs and the pedipalps. The alternative name "cephalothorax" reflects the fact that the prosoma includes the parts that in insects form the head plus the thorax. Though it is not split into two clear tagmata, the prosoma does have a large, relatively well-defined anterior carapace, bearing the animal's eyes and chelicerae, while a smaller posterior section bears the legs.
Among the most distinctive features of the Solifugae are their large chelicerae, which in many species are longer than the prosoma. Each of the two chelicerae has two articles (segments, parts connected by a joint), forming a powerful pincer, much like that of a crab; each article bears a variable number of teeth, largely depending on the species. The chelicerae of many species are surprisingly strong; they are capable of shearing hair or feathers from vertebrate prey or carrion, and of cutting through skin and thin bones such as those of small birds. Many Solifugae stridulate with their chelicerae, producing a rattling noise.
Legs and pedipalps
Like most other arachnids, although Solifugae appear to have five pairs of legs, only the hind four pairs actually are "true" legs. Each true leg has seven segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus.
The first, or anterior, of the five pairs of leg-like appendages are not "actual" legs, but pedipalps and they have only five segments each. The pedipalps of Solifugae function partly as sense organs similar to insects' antennae, and partly in locomotion, feeding and fighting. In normal locomotion, they do not quite touch the ground, but are held out to detect obstacles and prey; in that attitude, they look particularly like an extra pair of legs or perhaps arms. Reflecting the great dependence of Solifugae on their tactile senses, their anterior true legs commonly are smaller and thinner than the posterior three pairs. That smaller anterior pair acts largely in a sensory role as a supplement to the pedipalps, and in many species they accordingly lack tarsi. At the tips of their pedipalps, Solifugae bear eversible adhesive organs, which they may use to capture flying prey, and which at least some species certainly use for climbing smooth surfaces.
For the most part, only the posterior three pairs of legs are used for running. On the undersides of the coxae and trochanters of the last pair of legs, Solifugae have fan-shaped sensory organs called malleoli or racquet (or racket) organs. Sometimes, the blades of the malleoli are directed forward, sometimes not. They have been suspected to be sensory organs for the detection of vibrations in the soil, perhaps to detect threats and potential prey or mates. These structures may be chemoreceptors.
Males are usually smaller than females, with relatively longer legs. Unlike females, the males bear a pair of flagella, one on each chelicera. In the accompanying photograph of a male solifugid, one flagellum is just visible near the tip of each chelicera. The flagella, which bend back over the chelicerae, are sometimes called horns and are believed to have some sexual connection, but their function has not yet been clearly explained.
Some species have very large central eyes. They look like simple eyes or ocelli, but they are in fact surprisingly sophisticated. They can recognise forms, and are used in hunting and avoiding enemies. These eyes are remarkable in their internal anatomy; they may represent the last step in the integration of the aggregate of simple ocelli into a compound eye, and of further integration of a compound eye into a simple eye. In contrast, lateral eyes are absent in many species, and where they are present at all, they are only rudimentary.
The Solifugae are an order of their own, though are sometimes confused with spiders, which form a completely distinct order, the Araneae. The order comprises over 1000 described species in 153 genera assigned to the following 12 families::213
The family Protosolpugidae is only known from one fossil species from the Pennsylvanian.
Although the Solifugae are considered to be endemic indicators of desert biomes,:1 they occur widely in semidesert and scrub. Some species also live in grassland or forest habitats. Solifugae generally inhabit warm and arid habitats, including virtually all warm deserts and scrublands in all continents except Antarctica and Australia.
Solifugae are carnivorous or omnivorous, with most species feeding on termites, darkling beetles, and other small, ground-dwelling arthropods. Solifuges are aggressive hunters and voracious opportunistic feeders and have been recorded as feeding on snakes, small lizards, and rodents. Prey is located with the pedipalps and killed and cut into pieces by the chelicerae. The prey is then liquefied and the liquid ingested through the pharynx. Although they do not normally attack humans, their chelicerae can penetrate human skin, and painful bites have been reported.
Solifugae are typically univoltine.:8 Reproduction can involve direct or indirect sperm transfer; when indirect, the male emits a spermatophore on the ground and then inserts it with his chelicerae in the female's genital pore. To do this, he flings the female on her back.
The female then digs a burrow, into which she lays 50 to 200 eggs - some species then guard them until they hatch. Because the female does not feed during this time, she will try to fatten herself beforehand, and a species of 5 cm (2.0 in) has been observed to eat more than 100 flies during that time in the laboratory. Solifugae undergo a number of stages including, egg, postembryo, 9-10 nymphal instars, and adults.
The name Solifugae derives from Latin, and means "those that flee from the sun". The order is also known by the names Solpugida, Solpugides, Solpugae, Galeodea, and Mycetophorae. Their common names include camel spider, wind scorpion, jerrymunglum, sun scorpion, and sun spider. In southern Africa, they are known by a host of names, including red romans, haarskeerders ("hair cutters") and baardskeerders ("beard cutters"), the latter two relating to the belief they use their formidable jaws to clip hair from humans and animals to line their subterranean nests.
Solifugids and humans
Solifugids have been recognised as distinct taxa from ancient times. The Greeks recognized that they were distinct from spiders; spiders were called ἀράχνη (arachne) while Solifugae were named φαλάγγιον (phalangion). In Aelian's De natura animalium, they are mistakenly mentioned, along with scorpions, as responsible for the abandoning of a country in Ethiopia. Anton August Heinrich Lichtenstein theorised in 1797 that the "mice" which plagued the Philistines in the Old Testament were Solifugae. During World War I, troops stationed in Abū Qīr, Egypt would stage fights between captive "jerrymanders," as they referred to them, and placed bets on the outcome. Similarly, British troops stationed in Libya in World War II would stage fights between solifugids and scorpions.:2–3
Solifugae are the subject of many urban legends and exaggerations about their size, speed, behaviour, appetite, and lethality. They are not especially large, the biggest having a leg span of about 12 cm (4.7 in). They are fast on land compared to other invertebrates, with their top speed estimated to be 16 km/h (10 mph), close to one-third as fast as the fastest human sprinter.
The Solifugae apparently have neither venom glands nor any venom-delivery apparatus such as the fangs of spiders, stings of wasps, or venomous setae of the caterpillars of Lonomia species. One 1978 study is frequently quoted, in which the authors report detection of an exception to this rule in India, in that Rhagodes nigrocinctus had venom glands, and that injection of the secretion into mice was frequently fatal. However, no supporting studies have confirmed either the facts of the matter, such as by independent detection of the glands as claimed, or the relevance of the observations if correct. Even the authors of the original account denied having found any means of delivery of the putative venom by the animal, and the only means of administering the material to the mice was by parenteral injection. Given that many non-venoms such as saliva, blood and glandular secretions can be lethal if injected, and that no venomous function was even speculated upon in this study, there is no evidence as yet for even one venomous species of solifugid.
Because of their unfamiliar spider-like appearance and rapid movements, Solifugae have startled or even frightened many people. This fear was sufficient to drive a family from their home when one was discovered in a soldier's house in Colchester, England, and caused the family to blame the solifugid for the death of their pet dog. Though they are not venomous, the powerful chelicerae of a large specimen may inflict a painful nip, but nothing medically significant.
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- M. Aruchami & G. Sundara Rajulu (1978). "An investigation on the poison glands and the nature of the venom of Rhagodes nigrocinctus (Solifugae: Arachnida)". Nat. Acad. Sci. Letters (India) 1: 191–192.
- Klann, Anja Elisabeth. Histology and ultrastructure of solifuges comparative studies of organ systems of solifuges (Arachnida, Solifugae) with special focus on functional analyses and phylogenetic interpretations Dissertation: Greifswald, Univ., Diss., 2009 Edition/Format:Thesis/dissertation Manuscript: eBook Archival Material: English View all editions and formats Database:WorldCat. 
- "Stowaway Afghan spider kills family dog". CNN. August 28, 2008. Retrieved January 8, 2011.
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- Mikkelson, Barbara & David P. "Camel Spiders" at Snopes.com: Urban Legends Reference Pages.
- "Camel Spiders: Behind an E-Mail Sensation From Iraq". National Geographic. June 29, 2004.
- "The Arachnid Order Solifugae".