Temporal range: Late Carboniferous–Recent
|3 recent genera, 58 species|
As of December 2011[update], 58 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa (Ricinoides) and the Neotropical region (Cryptocellus and Pseudocellus). In addition to the three living genera, there are two families and four genera containing fossil species.
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.
Ricinulei are typically about 5 to 10 millimetres (0.2 to 0.4 in) long. The cuticle (or exoskeleton) of both the legs and body is remarkably thick. 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 unique 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 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 (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 (whch are used more frequently to sense the surroundings) 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.
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. Unlike many other arachnids, ricinuleids have no book lungs, and gas exchange takes place through trachea. Interestingly, 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.
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 leg 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).
Ecological studies are rather infrequent, but ricinuleids are typically found in leaf mould in tropical rainforests or in caves. They seem to need dampness to survive.
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 sixteen species of fossil ricinuleids discovered so far originate from the late Carboniferous (Pennsylvanian) Coal Measures of Europe and North America; one species, ?Poliochera cretacea, is known from the Cretaceous of Asia. 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.
|After Giribet et al. (2002)|
The first living ricinuleid was described from West Africa by Félix Édouard Guérin-Méneville in 1838, 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, and a third from Sierra Leone by Tamerlan Thorell in 1892. 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 "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
Recent studies of arachnid relationships have largely concluded that ricinuleids are most closely related to Acari (mites and ticks). L. van der Hammen placed ricinuleids in a group called "Cryptognomae", together with the anactinotrichid mites only. Peter Weygoldt and Hannes Paulus referred to ricinuleids and all mites as "Acarinomorpha". Jeffrey Shultz used the name "Acaromorpha". This hypothesis recognies that both ricinuleids and mites hatch with a laval stage with only six pairs of 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, 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. This hypothesis was widely overlooked, but was reintroduced by Jason Dunlop in 1996. Characters 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. Ricinuleids as sister group of trigonotarbids was also recovered in the 2002 study by Gonzalo Giribet and colleagues.
- Lorenzo Prendini (2011). Order Ricinulei Thorell, 1876 (PDF). In Z.-Q. Zhang. "Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness". Zootaxa 4138: 122.
- Hans Jacob Hansen & William Sørensen (1904). On two orders of Arachnida. Cambridge University Press. pp. 1 182.
- Kay Pittard & Robert W. Mitchell (1972). "Comparative morphology of the life stages of Cryptocellus pelaezi (Arachnida, Ricinulei)". Graduate Studies (Texas Tech University) 1: 3–77.
- Gerald Legg (1976). "The external morphology of a new species of ricinuleid (Arachnida) from Sierra Leone". Journal of Zoology 59 (1): 1–58. doi:10.1111/j.1096-3642.1976.tb01007.x.
- Gerald Legg (1976). "The external morphology of immature stages of Ricinoides karschi (Arachnida: Ricinulei)". Bulletin of the British Arachnological Society 3: 243–248.
- L. van der Hammen (1979). "Comparative studies in Chelicerata I. The Cryptognomae (Ricinulei, Architarbi and Anactinotrichida)". Zoologische Verhandelingen 174 (1): 1–62.
- J. H. Kennaugh (1968). "An examination of the cuticle of three species of Ricinulei (Arachnida)". Journal of Zoology 156 (3): 393–404. doi:10.1111/j.1469-7998.1968.tb04361.x.
- G. Talarico, J. G. Palacios-Vargas & G. Alberti (2008). "Taste while chewing? Sensory structures in the chelicerae of Pseudocellus pearsei (Chamberlin & Ivie, 1938) (Ricinulei, Arachnida)". Revista Ibérica de Arachnología 15: 47–53.
- G. Talarico, J. G. Palacios-Vargas & G. Alberti (2008). "The pedipalp of Pseudocellus pearsei (Ricinulei, Arachnida) – ultrastructure of a multifunctional organ". Arthropod Structure & Development 37 (6): 511–521. doi:10.1016/j.asd.2008.02.001. PMID 18502688.
- Giovanni Talarico, Jose G. Palacios-Vargas, Mariano Fuentes Silva & Gerd Alberti (2005). "First ultrastructural observations on the tarsal pore organ of Pseudocellus pearsei and P. boneti (Arachnida, Ricinulei)". Journal of Arachnology 33 (2): 604–612. doi:10.1636/04-110.1. JSTOR 4129861.
- Giovanni Talarico, José G. Palacios-Vargas, Mariano Fuentes Silva & Gerd Alberti (2008). "Ultrastructure of tarsal sensilla and other integument structures of two Pseudocellus species (Ricinulei, Arachnida)". Journal of Morphology 267 (4): 441–463. doi:10.1002/jmor.10415. PMID 16425267.
- S. L. Tuxen (1974). "The African genus Ricinoides (Arachnida, Ricinulei)" (PDF). Journal of Arachnology 1: 85–106.
- Jacques Millot (1945). "L'anatomie interne des Ricinulei". Annales des Sciences Naturelles, Zoologie (in French) 7: 1–29.
- Mario Ludwig, José G. Palacios-Vargas, Gerd Alberti, (1994). "Cellular details of the midgut of Cryptocellus boneti (Arachnida: Ricinulei)". Journal of Morphology 220 (3): 263–270. doi:10.1002/jmor.1052200305.
- 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.
- Gerd Alberti & José G. Palacios-Vargas (1984). "Fine structure of spermatogenesis and mature spermatozoa in Cryptocellus boneti Bolivar y Pieltain, 1941 (Arachnida, Ricinulei)". Journal of Ultrastructural Research 87 (1): 1–12. doi:10.1016/S0022-5320(84)90111-4.
- G. Talarico, L. F. García Hernández & P. Michalik (2008). "The male genital system of the New World Ricinulei (Arachnida): ultrastructure of spermatozoa and spermiogenesis with special emphasis on its phylogenetic implications". Arthropod Structure & Development 37 (5): 396–409. doi:10.1016/j.asd.2008.01.006. PMID 18539528.
- Joachim Adis, Benjamin Messner & Norman Platnick (1999). "Morphological structures and vertical distribution in the soil indicate facultative plastron respiration in Cryptocellus adisi (Arachnida, Ricinulei) from Central Amazonia". Studies in Neotropical Fauna and Environment 34 (1): 1–9. doi:10.1076/snfe.22.214.171.12415.
- J. A. L. Cooke (1967). "Observations on the biology of Ricinulei (Arachnida) with descriptions of two new species of Cryptocellus". Journal of Zoology 151 (1): 31–42. doi:10.1111/j.1469-7998.1967.tb02864.x.
- Gerald Legg (1977). "Sperm transfer and mating in Ricinoides hanseni (Ricinulei: Arachnida)". Journal of Zoology 182 (1): 51–61. doi:10.1111/j.1469-7998.1977.tb04140.x.
- Joachim U. Adis, Norman I. Platnick, José W. de Morais & José M. Gomes Rodrigues (1989). "On the abundance and ecology of Ricinulei (Arachnida) from Central Amazonia, Brazil". Journal of the New York Entomological Society 97 (2): 133–140. JSTOR 25009750.
- William Buckland (1837). Treatise IV. Geology and mineralogy with reference to natural theology. The Bridgewater treatises on the power, wisdom and goodness of God as manifested in the creation. (2nd ed.). London: William Pickering.
- Jörg Wunderlich (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 Jörg Wunderlich (ed). Beiträge zur Araneologie, 7: Fifteen papers on extant and fossil spiders (Araneae). pp. 233–244.
- P. A. Selden (1992). "Revision of the fossil ricinuleids". Transactions of the Royal Society of Edinburgh. Earth Sciences 83: 595–634. doi:10.1017/s0263593300003333.
- Gonzalo Giribet, Gregory D. Edgecombe, Ward C. Wheeler & Courtney Babbitt (2002). "Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data" (PDF). Cladistics 18 (1): 5–70. doi:10.1111/j.1096-0031.2002.tb00140.x. PMID 14552352.
- Félix Édouard Guérin-Méneville (1838). "Note sur l'Acanthodon et sur le Cryptostemme, nouveaux genres d'Arachnides". Revue zoologique par le Société Cuvierienne (in French) 1: 10–12.
- John Obadiah Westwood (1874). "Class Arachnida". Thesaurus Entomologicus Oxoniensis. Oxford: Clarendon Press. pp. 200–202.
- Tamerlan Thorell (1892). "On an apparently new arachnid belonging to the family Cryptostemmoidae, Westw.". Kungliga Svenska Ventenskaps-akademiens Handlingar 17: 1–18.
- L. van der Hammen (1977). "A new classification of Chelicerata". Zoologische Mededelingen 51 (20): 307–319.
- Peter Weygoldt & Hannes Paulus (1979). "Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. I. Morphologische Untersuchungen". Zeitschrift für zoologische Systematik und Evolutionsforschung (in German) 17 (3): 85–116. doi:10.1111/j.1439-0469.1979.tb00694.x.
- Peter Weygoldt & Hannes Paulus (1979). "Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. II. Cladogramme und die Entfaltung der Chelicerata". Zeitschrift für zoologische Systematik und Evolutionsforschung (in German) 17 (3): 177–200. doi:10.1111/j.1439-0469.1979.tb00699.x.
- Jeffrey W. Shultz (1990). "Evolutionary morphology and phylogeny of Arachnida". Cladistics 6 (1): 1–38. doi:10.1111/j.1096-0031.1990.tb00523.x.
- Jeffrey W. Shultz (2007). "A phylogenetic analysis of the arachnid orders based on morphological characters". Zoological Journal of the Linnean Society 150 (2): 221–265. doi:10.1111/j.1096-3642.2007.00284.x.
- 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. Acarology VI, Volume 1. Chichester: Ellis Horwood Ltd. pp. 28–62. ISBN 978-0-85312-603-4.
- Ferdinand Karsch (1892). "Ueber Cryptostemma Guèr. als einziger recenter Ausläufer der fossilen Arachnoideen-Ordnung Meridogastra Thor.". Berliner Entomologische Zeitschrift (in German) 37 (1): 25–32. doi:10.1002/mmnd.18920370108.
- Jason A. Dunlop (1996). "Evidence for a sister group relationship between Ricinulei and Trigonotarbida" (PDF). Bulletin of the British Arachnological Society 10 (6): 193–204.
- Jason A. Dunlop, Carsten Kamenz and Giovanni Talarico (2009). "A fossil trigonotarbid arachnid with a ricinuleid-like pedipalpal claw". Zoomorphology 128 (4): 305–313. doi:10.1007/s00435-009-0090-z.
- Mark S. Harvey (2002). "The neglected cousins: what do we know about the smaller arachnid orders?" (PDF). Journal of Arachnology 30 (2): 357–372. doi:10.1636/0161-8202(2002)030[0357:TNCWDW]2.0.CO;2.
- "Photos of Ricinulei". American Arachnological Society. November 27, 2009.
- Joel Hallan. "Family Ricinoididae Ewing, 1929:586". Biology Catalog. Texas A&M University.