Ricinulei
Ricinulei is a small order of arachnids. Like most arachnids, they are predatory; eating small arthropods. They occur today in west-central Africa and the Americas from Brazil to southern Texas, where they inhabit either leaf-litter or caves. As of 2022, 103 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. 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, Ricinulei has a fossil-record spanning over 300 million years, including fossils from the Late Carboniferous of Euramerica and the Cretaceous Burmese amber.
Anatomy and physiology
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 largest Ricinulei known to ever exist was the Late Carboniferous Curculioides bohemondi with a body length of. 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.Distribution
Of the three living genera, Ricinoides inhabits tropical West Africa from The Gambia and Senegal, eastwards to the Congo and Gabon, Pseudocellus inhabits North and Central America from Panama to Southern Texas, as well as the Caribbean, while Cryptocellus ranges from Rondônia in western Brazil northwards to Honduras in Central America.Behavior and life history
Ricinuleids inhabit the leaf litter of rainforest floors, as well as caves, where they search for prey with their elongate sensory second leg pair. Ricinulei feed on other small invertebrates, 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. Despite the scarce number of studies about the biology of this group, recent studies have reported nocturnal habits, as well as novel behaviors for this group, which include interactions between individuals different than mating. Ricinuleids are often found in large congregations, the exact purpose of which is unknown.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. They were revised in detail in 1992 by Paul Selden, who placed them in a separate suborder, Palaeoricinulei.
The fossils are divided into four families: Curculioididae, Poliocheridae, Primoricinuleidae, and Sigillaricinuleidae. 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. Five species: ?Poliochera cretacea, Primoricinuleus pugio, Hirsutisoma acutiformis, H. bruckschi, H. grimaldii and H. dentata, are known from the Cenomanian Burmese amber of Myanmar; Curculioides bohemondi, the largest of all Ricinulei, was a member of the Curculioididae. Monooculricinuleus incisus and M. semiglobosus from Burmese amber were originally described as members of Ricinulei, but they might belong to Opiliones instead.
Some Carboniferous genera of Palaeoricinulei exceed modern Ricinulei in size, with bodies in length, and many appear to have had eyes, unlike modern representatives which are completely blind. It is likely they had a surface dwelling ecology, unlike that of modern Ricinulei. The fossil genera from the Cretaceous Burmese amber are referred to the extinct order Primoricinulei, and are thought to have had a different ecology than modern species as tree-dwelling predators that crawled on bark.
Genera
, the World Ricinulei Catalog accepts the following genera:- Ricinoididae Ewing, 1929
- *Cryptocellus Westwood, 1874
- *Pseudocellus Platnick, 1980
- *Ricinoides Ewing, 1929
- † Curculioididae Cockerell, 1916
- *† Amarixys Selden, 1992
- *† Curculioides Buckland, 1837
- † Hirsutisomidae Wunderlich, 2017
- *† Hirsutisoma Wunderlich, 2017
- † Poliocheridae Scudder, 1884
- *† Poliochera Scudder, 1884
- *† Terpsicroton Selden, 1992
- † Primoricinuleidae Wunderlich, 2015
- *† Primoricinuleus Wunderlich, 2015
- † Sigillaricinuleidae Wunderlich, 2022
- *† Sigillaricinuleus Wunderlich, 2022
Relationships