Ricinulei
| Ricinulei Temporal range: Late Carboniferous–Recent |
|
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Arachnida |
| Order: |
Ricinulei Thorell, 1876 |
| Family: |
Ricinoididae Ewing, 1929 |
| Genera | |
| Diversity | |
| 3 recent genera, 58 species | |
The order Ricinulei is a group of arachnids known as hooded tickspiders, though they are not true spiders. Like most arachnids, they are predatory, eating small arthropods. In older works they are sometimes referred to as Podogona.
As of December 2011[update], 58 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa (Ricinoides) and the Neotropical region ( and Pseudocellus). In addition to the three living genera, there are two families and four genera containing fossil species.
The most important general account of ricinuleid anatomy remains the 1904 monograph by Hans Jacob Hansen and William Sørensen. Useful further studies can be found in, e.g., the work of Pittard and Mitchell, Gerald Legg and L. van der Hammen.
Ricinulei are typically about 5 to 10 millimetres (0.2 to 0.4 in) long. The cuticle (or exoskeleton) of both the legs and body is remarkably thick. 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 ricinuleids 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 heavy-bodied abdomen (or opisthosoma) exhibits a narrow pedicel, or waist, where it attaches to the prosoma. Curiously, 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. The advantages of this unusual system are not well understood, and since the genital opening is located on the pedicel (another rather unusual feature) the animals have to '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, or chelicerae, are composed of two segments forming a fixed and a moveable digit. Sensory organs are also found associated with the mouthparts; presumably for tasting the food. The chelicerae can be retracted and at rest they are normally hidden beneath the cucullus.
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Wikipedia