Ricinulei

Ricinulei is a small order of arachnids. Like most arachnids, they are predatory, eating small arthropods. They occur today in west-central Africa (Ricinoides) and the Neotropics (Cryptocellus and Pseudocellus) as far north as Texas. As of 2021, 91 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae.^[1] In older works they are sometimes referred to as Podogona. Due to their obscurity they do not have a proper common name, though in academic literature they are occasionally referred to as hooded tickspiders.

In addition to the three living genera, there are fossil species from the upper Carboniferous of Euramerica and the Cretaceous Burmese amber.

The most important general account of ricinuleid anatomy remains the 1904 monograph by Hans Jacob Hansen and William Sørensen.^[2] Useful further studies can be found in, e.g., the work of Pittard and Mitchell,^[3] Gerald Legg^[4][5] and L. van der Hammen.^[6]

Ricinulei are typically about 5 to 10 millimetres (0.2 to 0.4 in) long. The largest of ever existed Ricinulei was the Late Carboniferous Curculioides bohemondi with a body length of 21.77 mm (0.857 in).^[7] The cuticle (or exoskeleton) of both the legs and body is remarkably thick.^[8] 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 2 segments forming a fixed and a moveable digit. Sensory organs are also found associated with the mouthparts;^[9] presumably for tasting the food. The chelicerae can be retracted and at rest they are normally hidden beneath the cucullus.

Ricinoides atewa nymph from Ghana

Male Ricinoides karschii from Campo Reserve, Cameroon

Male Pseudocellus pearsei from Grutas Tzabnah, Yucatán, Mexico