Ricinulei
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.
, 58 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa and the Neotropical region. In addition to the three living genera, there are fossil species from the Carboniferous of Euramerica and the Cretaceous Burmese Amber.
Description
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.Body
Ricinulei are typically about long. The cuticle of both the legs and body is remarkably thick. 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. 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 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 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.
Appendages
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.Ricinuleid 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 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 extinct order Trigonotarbida.
As in many harvestmen, the second pair of legs is longest in ricinuleids 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 differ from those of legs III and IV. In male ricinuleids, the third pair of legs are uniquely modified to form copulatory organs. The shape of these organs is very important for taxonomy and can be used to tell males of different species apart.
Internal anatomy
An older summary of ricinuleid internal anatomy was published by Jacques Millot. The midgut has been described, while the excretory system consists of Malpighian tubules and a pair of coxal glands. Female ricinuleids have spermathecae, presumably to store sperm. The male genitalia, sperm cells and sperm production have also been intensively studied. Gas exchange takes place through trachea, and opens through a single pair of spiracles on the prosoma. At least one Brazilian species appears to have a plastron, which may help it prevent getting wet and allow it to continue to breathe, even if inundated with water.Biology
Ricinulei are predators feeding on other small arthropods, although details of their natural prey are sparse. Relatively little is known about their courtship and mating habits, but males have been observed using their modified third pair of legs 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 six-legged larva is a feature they share with Acari.Habitat
Ecological studies are rather infrequent, but ricinuleids are typically found in leaf mold in tropical rainforests or in caves. They seem to need dampness to survive.Fossil record
Ricinulei are unique among arachnids in that the first one to be discovered was a fossil, described in 1837 by the noted English geologist William Buckland; albeit misinterpreted as a beetle. Further fossil species were added in subsequent years by, among others, Samuel Hubbard Scudder, Reginald Innes Pocock and Alexander Petrunkevitch.Fifteen of the twenty species of fossil ricinuleids discovered so far originate from the late Carboniferous Coal Measures of Europe and North America. Five species: ?Poliochera cretacea, Primoricinuleus pugio, Hirsutisoma acutiformis, H. bruckschi and H. dentata, are known from the Cenomanian aged Burmese Amber of Myanmar; Monooculricinuleus incisus and M. semiglobosus from the Cretaceous of Asia were originally described as members of Ricinulei, but they might belong to Opiliones instead. They were revised in detail in 1992 by Paul Selden, who placed them in a separate suborder, Palaeoricinulei. The fossils are divided into two families: Curculioididae with eleven 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, by contrast, have an opisthosoma without obvious tergites, but with a single median sulcus; a dividing line running down the middle of the back. This superficially resembles the elytra of a beetle and explains why Buckland originally misidentified the first fossil species.
Relationships
| Cladogram showing Ricinulei among other arachnids, after Giribet et al.. |