Ricinulei

{{Short description|Order of obscure arachnids}} {{Automatic taxobox | image = Cryptocellus goodnighti.jpg | image_caption = ''Cryptocellus goodnighti'' | fossil_range = {{Fossil range|Late Carboniferous|Recent}} | parent_authority = Thorell, 1876 | taxon = Ricinoididae | authority = Ewing, 1929 | display_parents = 2 | diversity_link = List of Ricinulei species | range_map = Ricinulei distribution map.png | range_map_caption = Distribution map of extant species. | diversity = | subdivision_ranks = Exant genera | subdivision = *''Cryptocellus'' *''Pseudocellus'' *''Ricinoides'' For fossil genera, see text }} '''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 Americas (''Cryptocellus'' and ''Pseudocellus'') 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'''.<ref>{{Cite journal |last1=Valdez-Mondragón |first1=Alejandro |last2=Juárez-Sánchez |first2=Alma R. |date=2021-02-23 |title=A new epigean species of ricinuleid of the genus Pseudocellus (Arachnida: Ricinulei: Ricinoididae) from a tropical sub-deciduous forest in Oaxaca, Mexico |url=https://bioone.org/journals/the-journal-of-arachnology/volume-48/issue-3/JoA-S-20-014/A-new-epigean-species-of-ricinuleid-of-the-genus-Pseudocellus/10.1636/JoA-S-20-014.full |journal=The Journal of Arachnology |volume=48 |issue=3 |doi=10.1636/JoA-S-20-014 |s2cid=232021020 |issn=0161-8202|url-access=subscription }}</ref> 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.<ref>{{cite book |author=Hans Jacob Hansen & William Sørensen |year=1904 |title=On two orders of Arachnida |url=https://archive.org/details/ontwoordersofara00hans |publisher=Cambridge University Press |pages=[https://archive.org/details/ontwoordersofara00hans/page/1 1] 182}}</ref> Useful further studies can be found in, e.g., the work of Pittard and Mitchell,<ref>{{cite journal |author1=Kay Pittard |author2=Robert W. Mitchell |name-list-style=amp |year=1972 |title=Comparative morphology of the life stages of ''Cryptocellus pelaezi'' (Arachnida, Ricinulei) |journal=Graduate Studies |publisher=Texas Tech University |volume=1 |pages=3–77}}</ref> Gerald Legg<ref>{{cite journal |author=Gerald Legg |year=1976 |title=The external morphology of a new species of ricinuleid (Arachnida) from Sierra Leone |journal=Journal of Zoology |volume=59 |issue=1 |pages=1–58 |doi=10.1111/j.1096-3642.1976.tb01007.x}}</ref><ref>{{cite journal |author=Gerald Legg |year=1976 |title=The external morphology of immature stages of ''Ricinoides karschi'' (Arachnida: Ricinulei) |journal=Bulletin of the British Arachnological Society |volume=3 |pages=243–248}}</ref> and L. van der Hammen.<ref>{{cite journal |author=L. van der Hammen |year=1979 |title=Comparative studies in Chelicerata I. The Cryptognomae (Ricinulei, Architarbi and Anactinotrichida) |journal=Zoologische Verhandelingen |volume=174 |issue=1 |pages=1–62 |url=http://www.repository.naturalis.nl/record/317785}}</ref> [[File:Ricinulei from Fernandez & Giribet, nymph of Ricinoides atewa (2015) (cropped).jpg|left|thumb|''Ricinoides atewa'' nymph from Ghana]]

=== Body === Ricinulei are typically about {{convert|5|to|10|mm|in|1}} long. The largest Ricinulei known to ever exist was the Late Carboniferous ''Curculioides bohemondi'' with a body length of {{cvt|21.77|mm}}.<ref name=Whalen2021>Niall Whalen, Paul Selden. "[https://www.researchgate.net/publication/347895037_A_new_giant_ricinuleid_Arachnida_Ricinulei_from_the_Pennsylvanian_of_Illinois_and_the_identification_of_a_new_ontogenetically_stable_diagnostic_character A new, giant ricinuleid (Arachnida, Ricinulei), from the Pennsylvanian of Illinois, and the identification of a new, ontogenetically stable, diagnostic character]". Journal of Paleontology, Volume 95, Issue 3, May 2021, pp. 601 - 612 DOI: https://doi.org/10.1017/jpa.2020.104</ref> The cuticle (or exoskeleton) of both the legs and body is remarkably thick.<ref>{{cite journal |author=J. H. Kennaugh |year=1968 |title=An examination of the cuticle of three species of Ricinulei (Arachnida) |journal=Journal of Zoology |volume=156 |issue=3 |pages=393–404 |doi=10.1111/j.1469-7998.1968.tb04361.x}}</ref> 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. [[File:Ricinulei from Fernandez & Giribet, male Ricinoides karschii from Campo Reserve, Cameroon (2015) (cropped).jpg|left|thumb|Male ''Ricinoides karschii'' from Campo Reserve, Cameroon]]

=== 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;<ref>{{Cite journal |author=G. Talarico, J. G. Palacios-Vargas & G. Alberti |year=2008 |title=Taste while chewing? Sensory structures in the chelicerae of ''Pseudocellus pearsei'' (Chamberlin & Ivie, 1938) (Ricinulei, Arachnida) |journal=Revista Ibérica de Aracnología |volume=15 |pages=47–53}}</ref> 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.<ref>{{Cite journal |author=G. Talarico, J. G. Palacios-Vargas & G. Alberti |year=2008 |title=The pedipalp of ''Pseudocellus pearsei'' (Ricinulei, Arachnida) – ultrastructure of a multifunctional organ |journal=Arthropod Structure & Development |volume=37 |issue=6 |pages=511–521 |pmid=18502688 |doi=10.1016/j.asd.2008.02.001|bibcode=2008ArtSD..37..511T }}</ref> 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 (see Relationships).

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 (which are used more frequently to sense the surroundings) differ from those of legs III and IV.<ref>{{cite journal |author=Giovanni Talarico, Jose G. Palacios-Vargas, Mariano Fuentes Silva & Gerd Alberti |year=2005 |title=First ultrastructural observations on the tarsal pore organ of ''Pseudocellus pearsei'' and ''P. boneti'' (Arachnida, Ricinulei) |journal=Journal of Arachnology |volume=33 |issue=2 |pages=604–612 |doi=10.1636/04-110.1 |jstor=4129861|s2cid=86221977 |url=https://www.biodiversitylibrary.org/part/228872 }}</ref><ref>{{cite journal |author=Giovanni Talarico, José G. Palacios-Vargas, Mariano Fuentes Silva & Gerd Alberti |year=2008 |title=Ultrastructure of tarsal sensilla and other integument structures of two ''Pseudocellus'' species (Ricinulei, Arachnida) |journal=Journal of Morphology |volume=267 |issue=4 |pages=441–463 |pmid=16425267 |doi=10.1002/jmor.10415|s2cid=25580911 |doi-access=free }}</ref> 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.<ref>{{cite journal |author=S. L. Tuxen |year=1974 |title=The African genus ''Ricinoides'' (Arachnida, Ricinulei) |journal=Journal of Arachnology |volume=1 |pages=85–106 |url=http://www.americanarachnology.org/JoA_free/JoA_v1_n2/JoA_v1_p85.pdf |archive-date=2012-03-06 |access-date=2010-11-11 |archive-url=https://web.archive.org/web/20120306040416/http://www.americanarachnology.org/JoA_free/JoA_v1_n2/JoA_v1_p85.pdf |url-status=dead }}</ref>

=== Internal anatomy === An older summary of ricinuleid internal anatomy was published by Jacques Millot.<ref>{{cite journal |author=Jacques Millot |year=1945 |title=L'anatomie interne des Ricinulei |language=fr |journal=Annales des Sciences Naturelles, Zoologie |volume=7 |pages=1–29}}</ref> The midgut has been described,<ref>{{cite journal |author1=Mario Ludwig |author2=José G. Palacios-Vargas |author3=Gerd Alberti |year=1994 |title=Cellular details of the midgut of ''Cryptocellus boneti'' (Arachnida: Ricinulei) |journal=Journal of Morphology |volume=220 |issue=3 |pages=263–270 |doi=10.1002/jmor.1052200305|pmid=29865385 |s2cid=46930292 }}</ref> while the excretory system consists of Malpighian tubules and a pair of coxal glands. Female ricinuleids have spermathecae,<ref>{{cite journal |author=P. M. Brignoli |year=1973 |title=On some Ricinulei of Mexico with notes on the female genital apparatus (Arachnida, Ricinulei) |journal=Accademia Nazionale dei Lincei |volume=171 |pages=153–174}}</ref> presumably to store sperm. The male genitalia, sperm cells and sperm production have also been intensively studied.<ref>{{cite journal |author1=Gerd Alberti |author2=José G. Palacios-Vargas |name-list-style=amp |year=1984 |title=Fine structure of spermatogenesis and mature spermatozoa in ''Cryptocellus boneti'' Bolivar y Pieltain, 1941 (Arachnida, Ricinulei) |journal=Journal of Ultrastructure Research |volume=87 |issue=1 |pages=1–12 |doi=10.1016/S0022-5320(84)90111-4}}</ref><ref>{{cite journal |author=G. Talarico, L. F. García Hernández & P. Michalik |year=2008 |title=The male genital system of the New World Ricinulei (Arachnida): ultrastructure of spermatozoa and spermiogenesis with special emphasis on its phylogenetic implications |journal=Arthropod Structure & Development |volume=37 |issue=5 |pages=396–409 |pmid=18539528 |doi=10.1016/j.asd.2008.01.006|bibcode=2008ArtSD..37..396T }}</ref> Gas exchange takes place through trachea, and opens through a single pair of spiracles on the prosoma.<ref>{{cite book | chapter-url=https://link.springer.com/chapter/10.1007%2F978-3-662-10396-8_36 | doi=10.1007/978-3-662-10396-8_36 | chapter=Ricinulei — Acari | title=Multicellular Animals | date=2000 | last1=Ax | first1=Peter | pages=130–135 | isbn=978-3-642-08681-6 }}</ref> 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.<ref>{{cite journal |author=Joachim Adis, Benjamin Messner & Norman Platnick |year=1999 |title=Morphological structures and vertical distribution in the soil indicate facultative plastron respiration in ''Cryptocellus adisi'' (Arachnida, Ricinulei) from Central Amazonia |journal=Studies on Neotropical Fauna and Environment |volume=34 |issue=1 |pages=1–9 |doi=10.1076/snfe.34.1.1.8915|bibcode=1999SNFE...34....1A }}</ref>

== Distribution == Of the three living genera, ''Ricinoides'' inhabits tropical West Africa from The Gambia and Senegal, eastwards to the Congo and Gabon,<ref>{{Cite journal |last1=Murienne |first1=Jérôme |last2=Benavides |first2=Ligia R. |last3=Prendini |first3=Lorenzo |last4=Hormiga |first4=Gustavo |last5=Giribet |first5=Gonzalo |date=2013-02-23 |title=Forest refugia in Western and Central Africa as 'museums' of Mesozoic biodiversity |journal=Biology Letters |language=en |volume=9 |issue=1 |article-number=20120932 |doi=10.1098/rsbl.2012.0932 |issn=1744-9561 |pmc=3565512 |pmid=23193047}}</ref><ref>{{Cite journal |last1=Botero-Trujillo |first1=Ricardo |last2=Sain |first2=Colby E. |last3=Prendini |first3=Lorenzo |date=2021-04-05 |title=Systematics of the "Giant" Ricinulei (Ricinoididae: Ricinoides) of West Africa, with Descriptions of Five New Species and Comparative Morphology of the Male Copulatory Apparatus |url=https://bioone.org/journals/bulletin-of-the-american-museum-of-natural-history/volume-448/issue-1/0003-0090.448.1.1/Systematics-of-the-Giant-Ricinulei-Ricinoididae--Ricinoides-of-West/10.1206/0003-0090.448.1.1.full |journal=Bulletin of the American Museum of Natural History |volume=448 |issue=1 |doi=10.1206/0003-0090.448.1.1 |issn=0003-0090}}</ref> ''Pseudocellus'' inhabits North and Central America from Panama to Southern Texas, as well as the Caribbean (including Cuba), while ''Cryptocellus'' ranges from Rondônia in western Brazil northwards to Honduras in Central America.<ref>{{Cite journal |last1=Botero-Trujillo |first1=Ricardo |last2=Carvalho |first2=Leonardo S. |last3=Florez D. |first3=Eduardo |last4=Prendini |first4=Lorenzo |date=2021-08-25 |title=Four New Species of "Hooded Tick-Spiders" (Ricinulei, Ricinoididae) from South and Central America, with Clarification of the Identity of Cryptocellus leleupi Cooreman, 1976 |url=https://bioone.org/journals/american-museum-novitates/volume-2021/issue-3976/3976.1/Four-New-Species-of-Hooded-Tick-Spiders-Ricinulei-Ricinoididae-from/10.1206/3976.1.full |journal=American Museum Novitates |issue=3976 |doi=10.1206/3976.1 |issn=0003-0082}}</ref>

==Behavior and life history== [[File:Ricinulei from Fernandez & Giribet, male Pseudocellus pearsei from Grutas Tzabnah, Yucatán, Mexico (2015) (cropped).jpg|left|thumb|Male ''Pseudocellus pearsei'' from Grutas Tzabnah, Yucatán, Mexico]] Ricinuleids inhabit the leaf litter of rainforest floors, as well as caves, where they search for prey with their elongate sensory second leg pair.<ref>{{cite journal|author=Joachim U. Adis, Norman I. Platnick, José W. de Morais & José M. Gomes Rodrigues|year=1989|title=On the abundance and ecology of Ricinulei (Arachnida) from Central Amazonia, Brazil|journal=Journal of the New York Entomological Society|volume=97|issue=2|pages=133–140|jstor=25009750}}</ref> Ricinulei feed on other small invertebrates, although details of their natural prey are sparse.<ref>{{cite journal |author=J. A. L. Cooke |year=1967 |title=Observations on the biology of Ricinulei (Arachnida) with descriptions of two new species of ''Cryptocellus'' |journal=Journal of Zoology |volume=151 |issue=1 |pages=31–42 |doi=10.1111/j.1469-7998.1967.tb02864.x}}</ref> Relatively little is known about their courtship and mating habits,<ref>{{cite journal |author=Gerald Legg |year=1977 |title=Sperm transfer and mating in ''Ricinoides hanseni'' (Ricinulei: Arachnida) |journal=Journal of Zoology |volume=182 |issue=1 |pages=51–61 |doi=10.1111/j.1469-7998.1977.tb04140.x}}</ref> 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 (see Relationships). 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.<ref>{{Cite journal|last1=García|first1=L. F.|last2=Torrado-León|first2=E.|last3=Talarico|first3=G.|last4=Peretti|first4=A. V.|date=2015-07-01|title=First Characterization of the Behavioral Repertory in a Ricinuleid: Cryptocellus narino Platnick & Paz 1979 (Arachnida, Ricinulei, Ricinoididae)|journal=Journal of Insect Behavior|language=en|volume=28|issue=4|pages=447–459|doi=10.1007/s10905-015-9517-1|bibcode=2015JIBeh..28..447G |s2cid=2409069|issn=1572-8889|hdl=11336/7938|hdl-access=free}}</ref> Ricinuleids are often found in large congregations, the exact purpose of which is unknown.<ref>{{Cite journal|last1=Whalen|first1=Niall|last2=Selden|first2=Paul|date=May 2021|title=A new, giant ricinuleid (Arachnida, Ricinulei), from the Pennsylvanian of Illinois, and the identification of a new, ontogenetically stable, diagnostic character|url=https://www.cambridge.org/core/product/identifier/S0022336020001043/type/journal_article|journal=Journal of Paleontology|language=en|volume=95|issue=3|pages=601–612|doi=10.1017/jpa.2020.104|bibcode=2021JPal...95..601W |s2cid=233301949 |issn=0022-3360|url-access=subscription}}</ref>

==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;<ref>{{cite book |first=William |last=Buckland |author-link=William Buckland |year=1837 |series=The Bridgewater treatises on the power, wisdom and goodness of God as manifested in the creation |volume=Treatise IV |title=Geology and mineralogy with reference to natural theology |edition=2nd |publisher=William Pickering |location=London, UK }}</ref> 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 (Pennsylvanian) coal measures of Europe and North America. They were revised in detail in 1992 by Paul Selden,<ref>{{cite journal |first=P.A. |last=Selden |year=1992 |title=Revision of the fossil ricinuleids |journal=Transactions of the Royal Society of Edinburgh |series=Earth Sciences |volume=83 |issue=4 |pages=595–634 |doi=10.1017/s0263593300003333 |s2cid=85945454 }}</ref> 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 (~99 million years old) Burmese amber of Myanmar;<ref>{{cite book |first=Jörg |last=Wunderlich |year=2012 |chapter=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 |editor=Wunderlich, Jörg |title=Beiträge zur Araneologie |volume=7: Fifteen papers on extant and fossil spiders (Araneae) |pages=233–244 }}</ref><ref>{{cite book |first=Jörg |last=Wunderlich |year=2015 |chapter=New and rare fossil Arachnida in Cretaceous Burmese amber (Amblypygi, Ricinulei and Uropygi: Thelephonida) |editor=Wunderlich, Jörg |title=Beiträge zur Araneologie |volume=9: Mesozoic spiders and other fossil arachnids |pages=409–436 }}</ref><ref name=BzA10Ricinulei>{{cite book |first=Jörg |last=Wunderlich |year=2017 |chapter=New extinct taxa of the arachnid order Ricinulei, based on new fossils preserved in mid Cretaceous Burmese amber |editor=Wunderlich, Jörg |title=Beiträge zur Araneologie |volume=10 |pages=48–71 }}</ref><ref name=":0">{{cite journal |last1=Botero-Trujillo |first1=Ricardo |last2=Davis |first2=Steven R. |last3=Michalik |first3=Peter |last4=Prendini |first4=Lorenzo |date=22 September 2022 |title=Hirsutisoma grimaldii sp. nov., a ca. 99 million-year-old ricinuleid (Primoricinulei, Hirsutisomidae) from Cretaceous Burmese amber with a corticolous, scansorial lifestyle |journal=Palaeoentomology |volume=5 |issue=5 |doi=10.11646/palaeoentomology.5.5.11 |issn=2624-2834 |url=https://mapress.com/pe/article/view/palaeoentomology.5.5.11 |url-access=subscription }}</ref> ''Curculioides bohemondi'', the largest of all Ricinulei, was a member of the Curculioididae.<ref name=Whalen2021/> ''Monooculricinuleus incisus'' and ''M. semiglobosus'' from Burmese amber were originally described as members of Ricinulei, but they might belong to Opiliones instead.<ref>{{cite journal |first1=Paul A. |last1=Selden |first2=Dong |last2=Ren |year=2017 |title=A review of Burmese amber arachnids |journal=Journal of Arachnology |volume=45 |issue=3 |pages=324–343 |doi=10.1636/JoA-S-17-029 |s2cid=90983791 |url=https://www.biodiversitylibrary.org/part/289917 }}</ref>

Some Carboniferous genera of Palaeoricinulei exceed modern Ricinulei in size, with bodies {{convert|24|mm|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.<ref>{{cite journal |last1=Whalen |first1=Niall |last2=Selden |first2=Paul |date=May 2021 |title=A new, giant ricinuleid (Arachnida, Ricinulei), from the Pennsylvanian of Illinois, and the identification of a new, ontogenetically stable, diagnostic character |url=https://www.cambridge.org/core/product/identifier/S0022336020001043/type/journal_article |journal=Journal of Paleontology |language=en |volume=95 |issue=3 |pages=601–612 |doi=10.1017/jpa.2020.104 |bibcode=2021JPal...95..601W |issn=0022-3360 |url-access=subscription }}</ref> 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.<ref name=":0" />

== Genera == {{as of|2022|09}}, the World Ricinulei Catalog accepts the following genera:<ref name="NMBE">{{cite web |date=2022 |title=World Ricinulei Catalog |url=https://wac.nmbe.ch/order/ricinulei/4 |access-date=24 September 2022 |website=World Ricinulei Catalog |publisher=Natural History Museum Bern}}</ref> *'''Ricinoididae''' Ewing, 1929 (103+ species) **''Cryptocellus'' Westwood, 1874 **''Pseudocellus'' Platnick, 1980 **''Ricinoides'' Ewing, 1929 *† '''Curculioididae''' Cockerell, 1916 (12 species, Carboniferous) **† ''Amarixys'' Selden, 1992 **† ''Curculioides'' Buckland, 1837 *† '''Hirsutisomidae''' Wunderlich, 2017 (4 species, Burmese amber) **† ''Hirsutisoma'' Wunderlich, 2017 *† '''Poliocheridae''' Scudder, 1884 (5 species, Carboniferous, ?Burmese amber) **† ''Poliochera'' Scudder, 1884 **† ''Terpsicroton'' Selden, 1992 *† '''Primoricinuleidae''' Wunderlich, 2015 (1 species, Burmese amber) **† ''Primoricinuleus'' Wunderlich, 2015 *† '''Sigillaricinuleidae''' Wunderlich, 2022 (1 species, Burmese amber) **† ''Sigillaricinuleus'' Wunderlich, 2022

==Relationships==

===Early work=== 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''.<ref>{{Cite book |last=Hooke |first=Robert |title=Micrographia, or Some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon |publisher=James Allestry and John Martyn in the Royal Society |year=1665 |location=London |pages=207–208 |chapter=Of the crab-like insect |author-link=Robert Hooke}}</ref> The first living ricinuleid described using Linnaean taxonomy was from West Africa by Félix Édouard Guérin-Méneville in 1838,<ref>{{cite journal |author=Félix Édouard Guérin-Méneville |year=1838 |title=Note sur l{{'}}''Acanthodon'' et sur le ''Cryptostemme'', nouveaux genres d'Arachnides |language=fr |journal=Revue Zoologique par la Société Cuvierienne |volume=1 |pages=10–12|author-link=Félix Édouard Guérin-Méneville }}</ref> ''i.e.'' one year after the first fossil. This was followed by a second living example collected by Henry Walter Bates in Brazil and described by John Obadiah Westwood in 1874,<ref>{{cite book |author=John Obadiah Westwood |year=1874 |title=Thesaurus Entomologicus Oxoniensis |location=Oxford |publisher=Clarendon Press |chapter=Class Arachnida |pages=200–202 |chapter-url=https://archive.org/details/thesaurusentomol00west|author-link=John Obadiah Westwood }}</ref> and a third from Sierra Leone by Tamerlan Thorell in 1892.<ref>{{cite journal |author=Tamerlan Thorell |year=1892 |title=On an apparently new arachnid belonging to the family Cryptostemmoidae, Westw. |journal=Kungliga Svenska Ventenskaps-akademiens Handlingar |volume=17 |pages=1–18 |author-link=Tamerlan Thorell}}</ref> In these early studies ricinuleids were thought to be unusual harvestmen (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 "{{lang|la|Arachnida micrura}}", comprising spiders, 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.

===Ricinuleids and mites=== Morphological studies of arachnid relationships have largely concluded that ricinuleids are most closely related to Acari (mites and ticks) though more recent phylogenomic studies refute this.<ref name="ReferenceA">{{Cite journal|last1=Ballesteros|first1=Jesús A.|last2=Sharma|first2=Prashant P.|title=A Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error|journal=Systematic Biology|volume=68|issue=6|pages=896–917|language=en|doi=10.1093/sysbio/syz011|year=2019|pmid=30917194|doi-access=free}}</ref><ref>{{Cite journal|last1=Lozano-Fernandez|first1=L.| display-authors=etal | title=Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida|journal=Nature Communications|volume=10|issue=2295|doi=10.1038/s41467-019-10244-7|pmc=6534568 |year=2019|pages=459–462 |pmid=6534568|bibcode=2019NatCo..10.2295L |doi-access=free}}</ref> L. van der Hammen placed ricinuleids in a group called "Cryptognomae",<ref>{{cite journal |author=L. van der Hammen |year=1977 |title=A new classification of Chelicerata |journal=Zoologische Mededelingen |volume=51 |issue=20 |pages=307–319 |url=http://www.repository.naturalis.nl/record/319210}}</ref> together with the anactinotrichid mites only. Peter Weygoldt and Hannes Paulus referred to ricinuleids and all mites as "Acarinomorpha".<ref>{{cite journal |author1=Peter Weygoldt |author2=Hannes Paulus |name-list-style=amp |year=1979 |title=Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. I. Morphologische Untersuchungen |language=de |journal=Zeitschrift für zoologische Systematik und Evolutionsforschung |volume=17 |issue=3 |pages=85–116 |doi=10.1111/j.1439-0469.1979.tb00694.x}}</ref><ref>{{cite journal |author1=Peter Weygoldt |author2=Hannes Paulus |name-list-style=amp |year=1979 |title=Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. II. Cladogramme und die Entfaltung der Chelicerata |language=de |journal=Zeitschrift für zoologische Systematik und Evolutionsforschung |volume=17 |issue=3 |pages=177–200 |doi=10.1111/j.1439-0469.1979.tb00699.x|doi-access=free }}</ref> Jeffrey Shultz used the name "Acaromorpha".<ref>{{cite journal |author=Jeffrey W. Shultz |year=1990 |title=Evolutionary morphology and phylogeny of Arachnida |journal=Cladistics |volume=6 |issue=1 |pages=1–38 |doi=10.1111/j.1096-0031.1990.tb00523.x|pmid=34933471 |s2cid=85410687 |doi-access=free }}</ref><ref name="Shultz2007">{{cite journal |author=Jeffrey W. Shultz |year=2007 |title=A phylogenetic analysis of the arachnid orders based on morphological characters |journal=Zoological Journal of the Linnean Society |volume=150 |issue=2 |pages=221–265 |doi=10.1111/j.1096-3642.2007.00284.x|doi-access=free }}</ref> 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 the gnathosoma, a separate part of the body bearing the mouthparts, is also a unique character for ricinuleids and mites,<ref>{{cite book |author=E. E. Lindquist |year=1984 |chapter=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 |editor1=D. A. Griffiths |editor2=C. E. Bowman |title=Acarology VI, Volume 1 |publisher=Ellis Horwood Ltd. |location=Chichester |pages=28–62 |isbn=978-0-85312-603-4}}</ref> but this feature is rather complex and difficult to interpret and other authors would restrict the presence of a gnathosoma ''sensu stricto'' to mites only.

===Ricinuleids and trigonotarbids=== In 1892, Ferdinand Karsch suggested that ricinuleids were the last living descendants of the extinct arachnid order Trigonotarbida.<ref>{{cite journal |author=Ferdinand Karsch |year=1892 |title=Ueber ''Cryptostemma'' Guèr. als einziger recenter Ausläufer der fossilen Arachnoideen-Ordnung Meridogastra Thor. |journal=Berliner Entomologische Zeitschrift |volume=37 |issue=1 |pages=25–32 |doi=10.1002/mmnd.18920370108 |language=de|author-link=Ferdinand Karsch |url=https://zenodo.org/record/1427707 }}</ref> This hypothesis was widely overlooked, but was reintroduced by Jason Dunlop in 1996.<ref>{{cite journal |author=Jason A. Dunlop |year=1996 |title=Evidence for a sister group relationship between Ricinulei and Trigonotarbida |journal=Bulletin of the British Arachnological Society |volume=10 |issue=6 |pages=193–204 |url=http://insects.tamu.edu/research/collection/hallan/acari/Palpigradi127.pdf |access-date=2010-11-11 |archive-url=https://web.archive.org/web/20110613054155/http://insects.tamu.edu/research/collection/hallan/acari/Palpigradi127.pdf |archive-date=2011-06-13 }}</ref> 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.<ref>{{Cite journal |author=Jason A. Dunlop, Carsten Kamenz and Giovanni Talarico |year=2009 |title=A fossil trigonotarbid arachnid with a ricinuleid-like pedipalpal claw |journal=Zoomorphology |volume=128 |issue=4 |pages=305–313 |doi=10.1007/s00435-009-0090-z|s2cid=6769463 }}</ref> Ricinuleids as sister group of trigonotarbids was also recovered in the 2002 study by Gonzalo Giribet and colleagues.<ref name="Giribet">{{cite journal |author=Gonzalo Giribet, Gregory D. Edgecombe, Ward C. Wheeler & Courtney Babbitt |year=2002 |title=Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data |journal=Cladistics |volume=18 |issue=1 |pages=5–70 |pmid=14552352 |url=http://research.amnh.org/scicomp/pdfs/wheeler/Giribet_etal2002.pdf |doi=10.1111/j.1096-0031.2002.tb00140.x|s2cid=16833833 }}</ref>

=== Phylogenomic studies === Recent phylogenomic studies have recovered different relationships than those previously suggested. An analysis in early 2019 suggested the sister group of the ricinuleids may be Xiphosura, the arthropod order containing horseshoe crabs.<ref name="ReferenceA"/> In response to this work, a more recent study placed Ricinulei and Opiliones as sister taxa.<ref>{{Cite journal|last1=Lozano-Fernandez|first1=L.| display-authors=etal | title=Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida|journal=Nature Communications|volume=10|issue=2295|doi=10.1038/s41467-019-10244-7|pmc=6534568|year=2019|pages=459–462 |pmid=6534568|bibcode=2019NatCo..10.2295L |doi-access=free}}</ref>

==References== {{Reflist|25em}}

==Further reading== {{Portal|Arthropods}} *{{Cite journal |author=Mark S. Harvey |year=2002 |title=The neglected cousins: what do we know about the smaller arachnid orders? |journal=Journal of Arachnology |volume=30 |issue=2 |pages=357–372 |url=http://www.americanarachnology.org/JoA_Congress/JoA_v30_n2/arac-30-02-357.pdf |doi=10.1636/0161-8202(2002)030[0357:TNCWDW]2.0.CO;2 |s2cid=59047074 |archive-url=https://web.archive.org/web/20120207165733/http://www.americanarachnology.org/JoA_Congress/JoA_v30_n2/arac-30-02-357.pdf |archive-date=2012-02-07 }}

==External links== {{Commons category}} *{{cite web |url=http://www.americanarachnology.org/gallery_ricinulei.html |title=Photos of Ricinulei |date=November 27, 2009 |publisher=American Arachnological Society |access-date=2008-04-07 |archive-url=https://web.archive.org/web/20080412144716/http://www.americanarachnology.org/gallery_ricinulei.html |archive-date=2008-04-12 }}

Category:Ricinulei Category:Arachnid orders Category:Monotypic animal orders Category:Extant Pennsylvanian first appearances