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| Ricinulei | ||||||||||||
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| 3 recent genera, c. 60 species | ||||||||||||
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The Order Ricinulei is a group of arachnids known as hooded tickspiders.
As of 2007, approximately 60 species of ricinuleids have been described worldwide, all in a single family, Ricinoididae, which contains 3 genera. They occur today in Africa and in South and Central America. There are also two families that only contain fossil species, the Curculioididae with eleven fossil species in two genera, and the Poliocheridae with five species in two genera. These fossil species occur in the late Carboniferous (Pennsylvanian) of Europe and North America.
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Physical description
Ricinulei are 5–10 mm long. Their most notable feature is a "hood" (or cucullus) which can be raised and lowered over the head; when lowered, it covers the mouth and the chelicerae. Living Ricinulei have no eyes, although two pairs of lateral eyes can be seen in fossils and even living species retain light-sensitive areas of cuticle in this position. The pedipalps end in pincers that are small relative to their bodies, when compared to those of the related orders of scorpions and pseudoscorpions. Similar pincers on the pedipalps have now been found in the extinxt order Trigonotarbida. The heavy-bodied abdomen forms a narrow pedicel, or waist, where it attaches to the prosoma. In males, the third pair of legs are modified to form copulatory organs. Malpighian tubules and a pair of coxal glands make up the excretory system. They have no book lungs, as gas exchange takes place through trachea.
Behavior
Ricinulei are predators, feeding on other small arthropods. Little is known about their mating habits; the males have been observed using 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, which later molt into their eight-legged adult forms. The increase of a pair of legs is a feature they share with the Acari.
Habitat
Ricinulei require dampness to survive.
Relationships
after Giribet et al. 2002 |
Ricinulei are unique among arachinds in that a fossil species were discovered in 1837 (albeit misinterpreted as a beetle), before the fist living example.
The first living species was described in 1838, followed by a second in 1874, and a third in 1892. This year, Thorell erected the suborder Ricinulei within the arachnid order Opiliones (harvestmen). About ten years later it was recognized as an arachnid order in its own right. Ricinulei have sometimes been considered the sister group of Acari, but a recent study1 considers them a sister group to the fossil Trigonotarbida, and these two rather closely related to spiders. 2
Footnotes
References
- Joel Hallan's Biology Catalog: Ricinoididae
- Giribet, G., Edgecombe, G.D., Wheeler, W.C. & Babbitt, C. (2002): Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5-70.
- Pinto-da-Rocha, R., Machado, G. & Giribet, G. (eds.) (2007): Harvestmen - The Biology of Opiliones. Harvard University Press ISBN 0-674-02343-9
Further reading
- Harvey, Mark S. (2002): The neglected cousins: what do we know about the smaller arachnid orders? The Journal of Arachnology 30(2): 357-372. PDF
External links
Wikipedia content modification information:
- This page was last modified on 14 November 2008, at 19:58.
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