Ricinulei is a small order ofarachnids. Like most arachnids, they are predatory; eating smallarthropods. They occur today in west-central Africa (Ricinoides) and the Americas (Cryptocellus andPseudocellus) 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 familyRicinoididae.[1] Due to their obscurity they do not have a proper common-name, though in academic literature they are occasionally referred to ashooded tickspiders.
In addition to the three living genera, Ricinulei has a fossil-record spanning over 300 million years, including fossils from the LateCarboniferous ofEuramerica and theCretaceousBurmese amber.
The Ricinulei order is considered by paleontologists and zoologists to be aLazarus taxon - that is, a taxon (plural taxa) which disappears for one or more periods from the fossil record, only to appear again either in subsequent fossil records, or as actual living organisms - often in isolated, obscure, or otherwise very specialized habitats.
The most important general account of ricinuleid anatomy remains the 1904 monograph byHans 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 Ricinulei known to ever exist was theLate CarboniferousCurculioides 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" (orcucullus) 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 (oropisthosoma) exhibits a narrowpedicel, or waist, where it attaches to theprosoma. 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, orchelicerae, are composed of two 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.
Ricinuleidpedipalps are complexappendages. They are typically used to manipulate food items, but also bear many sensory structures and are used as 'short range' sensory organs.[10] The pedipalps end in pincers that are small relative to their bodies, when compared to those of the related orders ofscorpions andpseudoscorpions. Similar pincers on the pedipalps have now been found in the extinct orderTrigonotarbida (see Relationships).
As in manyharvestmen, 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 (which are used more frequently to sense the surroundings) differ from those of legs III and IV.[11][12] 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.[13]
An older summary of ricinuleid internal anatomy was published byJacques Millot.[14] The midgut has been described,[15] while the excretory system consists ofMalpighian tubules and a pair ofcoxal glands. Female ricinuleids havespermathecae,[16] presumably to store sperm. The male genitalia, sperm cells and sperm production have also been intensively studied.[17][18]Gas exchange takes place throughtrachea, and opens through a single pair of spiracles on the prosoma.[19] At least one Brazilian species appears to have aplastron, which may help it prevent getting wet and allow it to continue to breathe, even if inundated with water.[20]
Ricinuleids inhabit theleaf litter ofrainforest floors, as well as caves, where they search for prey with their elongate sensory second leg pair.[24] Ricinulei feed on other small invertebrates, although details of their natural prey are sparse.[25] Relatively little is known about their courtship and mating habits,[26] but males have been observed using their modified third pair of legs to transfer aspermatophore to the female. The eggs are carried under the mother's hood, until the young hatch into six-legged larva, which latermolt into their eight-legged adult forms. The six-legged larva is a feature they share withAcari (seeRelationships). 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.[27] Ricinuleids are often found in large congregations, the exact purpose of which is unknown.[28]
Fifteen of the twenty species of fossil ricinuleids discovered so far originate from the lateCarboniferous (Pennsylvanian) coal measures ofEurope andNorth America. They were revised in detail in 1992 by Paul Selden,[30] who placed them in a separate suborder,Palaeoricinulei.
The fossils are divided into four families:Curculioididae,Poliocheridae,Primoricinuleidae, andSigillaricinuleidae. The poliocherids are more like modern ricinuleids in having anopisthosoma 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 theelytra 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 andH. dentata, are known from theCenomanian (~99 million years old)Burmese amber ofMyanmar;[31][32][33][34]Curculioides bohemondi, the largest of all Ricinulei, was a member of the Curculioididae.[7]Monooculricinuleus incisus andM. semiglobosus from Burmese amber were originally described as members of Ricinulei, but they might belong toOpiliones instead.[35]
Some Carboniferous genera of Palaeoricinulei exceed modern Ricinulei in size, with bodies 24 millimetres (0.94 in) 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.[36] 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.[34]
In 1665,Robert Hooke described a large crab-like mite he observed with a microscope, he published a description of it in his book;Micrographia.[38] The first living ricinuleid described usingLinnaean taxonomy was from West Africa byFélix Édouard Guérin-Méneville in 1838,[39]i.e. one year after the first fossil. This was followed by a second living example collected byHenry Walter Bates inBrazil and described byJohn Obadiah Westwood in 1874,[40] and a third fromSierra Leone byTamerlan Thorell in 1892.[41] In these early studies ricinuleids were thought to be unusualharvestmen (Opiliones), and in his 1892 paper Thorell introduced the name "Ricinulei" for these animals as a suborder of the harvestman. Ricinuleids were subsequently recognized as an arachnid order in their own right in the 1904 monograph by Hansen & Soerensen. These authors recognised a group called "Arachnida micrura", comprisingspiders,whip spiders,whip scorpions and ricinuleids, which they defined as having a rather narrow join between the prosoma and opisthosoma and a small 'tail end' to the opisthosoma.
Morphological studies of arachnid relationships have largely concluded that ricinuleids are most closely related toAcari (mites and ticks) though more recent phylogenomic studies refute this.[42][43] L. van der Hammen placed ricinuleids in a group called "Cryptognomae",[44] together with the anactinotrichid mites only. Peter Weygoldt and Hannes Paulus referred to ricinuleids and all mites as "Acarinomorpha".[45][46] Jeffrey Shultz used the name "Acaromorpha".[47][48] This hypothesis recognizes that both ricinuleids and mites hatch with a larval stage with only six legs, rather than the usual eight seen in arachnids. The additional pair of legs appears later during development. Some authors have also suggested that thegnathosoma, a separate part of the body bearing the mouthparts, is also a unique character for ricinuleids and mites,[49] but this feature is rather complex and difficult to interpret and other authors would restrict the presence of a gnathosomasensu stricto to mites only.
In 1892,Ferdinand Karsch suggested that ricinuleids were the last living descendants of the extinct arachnid orderTrigonotarbida.[50] This hypothesis was widely overlooked, but was reintroduced by Jason Dunlop in 1996.[51] Characteristics shared by ricinuleids and trigonotarbids include the division of the tergites on the opisthososma into median and lateral plates and the presence of an unusual 'locking mechanism' between the two halves of the body. A further study subsequently recognised that the tip of the pedipalp in both ricinuleids and trigonotarbids ends in a similar small claw.[52] Ricinuleids as sister group of trigonotarbids was also recovered in the 2002 study by Gonzalo Giribet and colleagues.[53]
Recent phylogenomic studies have recovered different relationships than those previously suggested. An analysis in early 2019 suggested the sister group of the ricinuleids may beXiphosura, the arthropod order containing horseshoe crabs.[42] In response to this work, a more recent study placed Ricinulei andOpiliones as sister taxa.[54]
^Kay Pittard & Robert W. Mitchell (1972). "Comparative morphology of the life stages ofCryptocellus pelaezi (Arachnida, Ricinulei)".Graduate Studies.1.Texas Tech University:3–77.
^G. Talarico, J. G. Palacios-Vargas & G. Alberti (2008). "Taste while chewing? Sensory structures in the chelicerae ofPseudocellus pearsei (Chamberlin & Ivie, 1938) (Ricinulei, Arachnida)".Revista Ibérica de Aracnología.15:47–53.
^P. M. Brignoli (1973). "On some Ricinulei of Mexico with notes on the female genital apparatus (Arachnida, Ricinulei)".Accademia Nazionale dei Lincei.171:153–174.
^Wunderlich, Jörg (2012). "Description of the first fossil Ricinulei in amber from Burma (Myanmar), the first report of this arachnid order from the Mesozoic and from Asia, with notes on the related extinct order Trigonotarbida". In Wunderlich, Jörg (ed.).Beiträge zur Araneologie. Vol. 7: Fifteen papers on extant and fossil spiders (Araneae). pp. 233–244.
^Wunderlich, Jörg (2015). "New and rare fossil Arachnida in Cretaceous Burmese amber (Amblypygi, Ricinulei and Uropygi: Thelephonida)". In Wunderlich, Jörg (ed.).Beiträge zur Araneologie. Vol. 9: Mesozoic spiders and other fossil arachnids. pp. 409–436.
^Wunderlich, Jörg (2017). "New extinct taxa of the arachnid order Ricinulei, based on new fossils preserved in mid Cretaceous Burmese amber". In Wunderlich, Jörg (ed.).Beiträge zur Araneologie. Vol. 10. pp. 48–71.
^"World Ricinulei Catalog".World Ricinulei Catalog. Natural History Museum Bern. 2022. Retrieved24 September 2022.
^Hooke, Robert (1665). "Of the crab-like insect".Micrographia, or Some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon. London: James Allestry andJohn Martyn in theRoyal Society. pp. 207–208.
^Félix Édouard Guérin-Méneville (1838). "Note sur l'Acanthodon et sur leCryptostemme, nouveaux genres d'Arachnides".Revue Zoologique par la Société Cuvierienne (in French).1:10–12.
^Tamerlan Thorell (1892). "On an apparently new arachnid belonging to the family Cryptostemmoidae, Westw".Kungliga Svenska Ventenskaps-akademiens Handlingar.17:1–18.
^E. E. Lindquist (1984). "Current theories on the evolution of major groups of Acari and on their relationships with other groups of Arachnida with consequent implications for their classification". In D. A. Griffiths; C. E. Bowman (eds.).Acarology VI, Volume 1. Chichester: Ellis Horwood Ltd. pp. 28–62.ISBN978-0-85312-603-4.
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