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Hooded tickspiders (Order Ricinulei)

Hooded tickspiders belong to the Order Ricinulei, but were referred to as Podogona. There are about 58 living species, all in the family Ricinoididae. They occur in west-central Africa (Ricinoides) and South America (Cryptocellus and Pseudocellus). There are two families and four genera of fossil species.

Ricinulei are typically about 5-10 mm long. The cuticle (or exoskeleton) of the legs and body is very thick. A "hood" (cucullus) can be raised and lowered over the head. When lowered, it covers the mouth and chelicerae. Living ricinuleids lack eyes, but there are two pairs of lateral eyes in fossils and living species retain light-sensitive areas of cuticle in this position.The heavy-bodied abdomen (opisthosoma) shows a narrow pedicel (waist), where it attaches to the prosoma. There is a complex coupling mechanism between the prosoma and opisthosoma. The front margin of the opisthosoma tucks into a corresponding fold at the back of the carapace. As the genital opening is on the pedicel, the animals must 'unlock' themselves in order to mate. The abdomen is divided dorsally into a series of large plates or tergites, each of which is subdivided into a median and lateral plate. The mouthparts (chelicerae) are composed of two segments forming a fixed and a moveable digit. Sensory organs are also associated with the mouthparts, presumably to taste the food. The chelicerae can be retracted and at rest are normally hidden beneath the cucullus.The pedipalps are complex appendages. They are typically used to manipulate food items, but also bear many sensory structures and are used as 'short range' sensory organs. The pedipalps end in pincers that are small relative to the body. There are similar pincers on the pedipalps of the extinct order Trigonotarbida. As in many harvestmen, the second pair of legs is longest and these limbs are used to feel ahead of the animal, almost like antennae. If the pedipalps are 'short range' sensory organs, the second pair of legs are the corresponding 'long range' ones. Sensilla on the tarsi at the ends of legs I and II (which are used more often to sense the surroundings) differ from those of legs III and IV. In males, the third pair of legs forms copulatory organs. The excretory system consists of Malpighian tubules and a pair of coxal glands. Females have spermathecae, presumably to store sperm. Unlike many other arachnids, ricinuleids have no book lungs and gas exchange takes place through trachea. At least one Brazilian species seems to have a plastron that may help it prevent getting wet and allow it to continue to breathe even if inundated with water.[1] Ricinuleids are predators that eat other small arthropods. Males may use their modified third leg to transfer a spermatophore to the female. The eggs are carried under the mother's hood until the young hatch into six-legged larva, as in the Acari. The larvae later molt into eight-legged adult forms. Ricinuleids are typically found in leaf mould in tropical rainforests or in caves. They seem to need dampness to survive. The first member of the Ricinulei to be discovered was a fossil, described in 1837 by William Buckland. who misinterpreted as a beetle. 15 of the 16 species of fossil ricinuleids originate from the late Carboniferous (Pennsylvanian) Coal Measures of Europe and North America; one species, Poliochera cretacea, is known from the Cretaceous of Asia.[2] They were revised in 1992 by Paul Selden, who placed them in a separate suborder, Palaeoricinulei. The fossils are divided into two families: Curculioididae with 11 fossil species in two genera, and Poliocheridae with four species in two genera. The poliocherids are more like modern ricinuleids in having an opisthosoma with a series of three large, divided tergites. Curculioidids have an opisthosoma without obvious tergites, but with a single median sulcus - a dividing line running down the middle of the back. This resembles the elytra of a beetle. The first living ricinuleid was described from West Africa by Félix Édouard Guérin-Méneville in 1838. In early studies, ricinuleids were thought to be unusual harvestmen (Opiliones). In his 1892 paper Thorell introduced the name "Ricinulei" for these animals as a suborder of the Opiliones. Ricinuleids were recognized as an arachnid order in the 1904 monograph by Hansen & Soerensen. These authors recognised a group called "Arachnida micrura", comprising spiders, whip spiders, whip scorpions and ricinuleids, which they defined as having a rather narrow join between the prosoma and opisthosoma and a small 'tail end' to the opisthosoma. Ricinuleids seem to be most closely related to the Acari (mites and ticks). L. van der Hammen placed ricinuleids in a group called "Cryptognomae",[3] with the anactinotrichid mites. Peter Weygoldt and Hannes Paulus called ricinuleids and all mites "Acarinomorpha".[4] Jeffrey Shultz used the name "Acaromorpha".[5][6] This recognises that ricinuleids and mites hatch with a larval stage with six pairs of legs, rather than the usual eight of arachnids. The additional pair of legs appears later during development. Some authors suggest that the gnathosoma, a separate part of the body bearing the mouthparts, is a unique character for ricinuleids and mites,[7] but this feature is complex and hard to interpret and other authors restrict the presence of a gnathosoma to mites only. In 1892, Ferdinand Karsch suggested that ricinuleids were the last living descendants of the extinct arachnid order Trigonotarbida.[8] This hypothesis was reintroduced by Jason Dunlop in 1996.[9] Characters shared by ricinuleids and trigonotarbids include the division of the tergites on the opisthososma into median and lateral plates and the presence of a 'locking mechanism' between the two halves of the body. The tip of the pedipalp in ricinuleids and trigonotarbids ends in a similar small claw.[10] The idea of ricinuleids as the sister group of trigonotarbids was recovered in the 2002 study by Gonzalo Giribet and colleagues.[11]

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