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Ricinulei

    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. 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.

    Physical description

    Ricinulei are 5–10 mm long. Their most notable feature is a "hood" which can be raised and lowered over the head; when lowered, it covers the mouth and the chelicerae. Ricinulei have no eyes. The pedipalps end in pincers that are small relative to their bodies, when compared to those of the related orders of scorpions and pseudoscorpions. 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 the 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

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    The first 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 studyGiribet et al. 2002 considers them a sister group to the fossil Trigonotarbida, and these two rather closely related to spiders. (2007): Phylogeny and Biogeography. In: Pinto-da-Rocha et al. 2007.

    Footnotes

    References

    • 's Biology Catalog: Ricinoididae
    • (2002): Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5-70.
    • (eds.) (2007): Harvestmen - The Biology of Opiliones. Harvard University Press ISBN 0-674-02343-9

    Further reading

    • (2002): The neglected cousins: what do we know about the smaller arachnid orders? The Journal of Arachnology 30(2): 357-372. PDF

    External links