Ricinulei Temporal range: | |
---|---|
Cryptocellus goodnighti | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Subphylum: | Chelicerata |
Class: | Arachnida |
Order: | Ricinulei Thorell, 1876 |
Family: | Ricinoididae Ewing, 1929 |
Exant genera | |
For fossil genera, see text | |
Distribution map of extant species. |
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 South America to as far north as Texas, where they either inhabit leaf-litter or caves. As of 2022, 103 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. [1] In older works they are sometimes referred to as Podogona. 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.
The most important general account of ricinuleid anatomy remains the 1904 monograph by Hans 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 the Late Carboniferous Curculioides 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" (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.
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; [9] 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. [10] 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. [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 by Jacques Millot. [14] The midgut has been described, [15] while the excretory system consists of Malpighian tubules and a pair of coxal glands. Female ricinuleids have spermathecae, [16] presumably to store sperm. The male genitalia, sperm cells and sperm production have also been intensively studied. [17] [18] Gas exchange takes place through trachea, and opens through a single pair of spiracles on the prosoma. [19] 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. [20]
Ricinuleids inhabit the leaf litter of rainforest floors, as well as caves, where they search for prey with their elongate sensory second leg pair. [21] Ricinulei feed on other small invertebrates, although details of their natural prey are sparse. [22] Relatively little is known about their courtship and mating habits, [23] 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. [24] Ricinuleids are often found in large congregations, the exact purpose of which is unknown. [25]
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; [26] 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, [27] 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; [28] [29] [30] [31] Curculioides bohemondi, the largest of all Ricinulei, was a member of the Curculioididae. [7] Monooculricinuleus incisus and M. semiglobosus from Burmese amber were originally described as members of Ricinulei, but they might belong to Opiliones instead. [32]
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. [33] 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. [31]
As of September 2022 [update] , the World Ricinulei Catalog accepts the following genera: [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 . [35] The first living ricinuleid described using Linnaean taxonomy was from West Africa by Félix Édouard Guérin-Méneville in 1838, [36] 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, [37] and a third from Sierra Leone by Tamerlan Thorell in 1892. [38] 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.
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. [39] [40] L. van der Hammen placed ricinuleids in a group called "Cryptognomae", [41] together with the anactinotrichid mites only. Peter Weygoldt and Hannes Paulus referred to ricinuleids and all mites as "Acarinomorpha". [42] [43] Jeffrey Shultz used the name "Acaromorpha". [44] [45] 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, [46] 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.
In 1892, Ferdinand Karsch suggested that ricinuleids were the last living descendants of the extinct arachnid order Trigonotarbida. [47] This hypothesis was widely overlooked, but was reintroduced by Jason Dunlop in 1996. [48] 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. [49] Ricinuleids as sister group of trigonotarbids was also recovered in the 2002 study by Gonzalo Giribet and colleagues. [50]
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. [39] In response to this work, a more recent study placed Ricinulei and Opiliones as sister taxa. [51]
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.
Arachnids are arthropods in the class Arachnida of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.
Pseudoscorpions, also known as false scorpions or book scorpions, are small, scorpion-like arachnids belonging to the order Pseudoscorpiones, also known as Pseudoscorpionida or Chelonethida.
Amblypygi is an order of arachnids also known as whip spiders or tailless whip scorpions, not to be confused with whip scorpions or vinegaroons that belong to the related order Thelyphonida. The name "amblypygid" means "blunt tail", a reference to a lack of the flagellum that is otherwise seen in whip scorpions. Amblypygids possess no silk glands or venom. They rarely bite if threatened but can grab fingers with their pedipalps, resulting in thornlike puncture injuries.
The Opiliones are an order of arachnids, colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs. As of July 2024, over 6,650 species of harvestmen have been discovered worldwide, although the total number of extant species may exceed 10,000. The order Opiliones includes five suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi, which were named in 2014.
Mites are small arachnids. Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as each other's closest relative within Arachnida, rendering the group non-monophyletic. Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. The small size of most species makes them easily overlooked; some species live in water, many live in soil as decomposers, others live on plants, sometimes creating galls, while others are predators or parasites. This last type includes the commercially destructive Varroa parasite of honey bees, as well as scabies mites of humans. Most species are harmless to humans, but a few are associated with allergies or may transmit diseases.
Xiphosura is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs. They first appeared in the Hirnantian. Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.
Opilioacaridae is the sole family of mites in the order Opilioacarida, made up of about 13 genera. The mites of this family are rare, large mites, and are widely considered primitive, as they retain six pairs of eyes, and abdominal segmentation. They have historically been considered separate from other mites belonging to Acariformes and Parasitiformes, but are now generally considered a subgroup of Parasitiformes based on molecular phylogenetics.
The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian. These animals are known from several localities in Europe and North America, as well as a single record from Argentina. Trigonotarbids can be envisaged as spider-like arachnids, but without silk-producing spinnerets. They ranged in size from a few millimetres to a few centimetres in body length and had segmented abdomens (opisthosoma), with the dorsal exoskeleton (tergites) across the backs of the animals' abdomens, which were characteristically divided into three or five separate plates. Probably living as predators on other arthropods, some later trigonotarbid species were quite heavily armoured and protected themselves with spines and tubercles. About seventy species are currently known, with most fossils originating from the Carboniferous coal measures.
Spiders have been evolving for at least 380 million years. The group's origins lie within an arachnid sub-group defined by the presence of book lungs ; the arachnids as a whole evolved from aquatic chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. There may be more than 120,000 species. Fossil diversity rates make up a larger proportion than extant diversity would suggest with 1,593 arachnid species described out of 1,952 recognized chelicerates. Both extant and fossil species are described annually by researchers in the field. Major developments in spider evolution include the development of spinnerets and silk secretion.
Ricinoides atewa is a ricinuleid of the Ricinoididae family found in Ghana. It is known to inhabit evergreen forests in eastern Ghana, including the Atewa Range Forest Reserve from which it takes its specific epithet. It is a dark reddish brown arachnid densely covered in short, translucent setae with a broad cucullus. At 9.63 mm long, Ricinoides atewa is one of the world's largest ricinuleids, part of a group of nine West African species referred to as the 'giant' ricinuleids.
This list of fossil arthropods described in 2015 is a list of new taxa of trilobites, fossil insects, crustaceans, arachnids and other fossil arthropods of every kind that have been described during the year 2015. The list only includes taxa at the level of genus or species.
Pseudocellus is an arachnid genus in the order Ricinulei, first described by Norman Platnick in 1980. It is native to the Neotropics.
Pseudocellus pearsei is an arachnid species in the order Ricinulei. It occurs in caves in Yucatan, Mexico.
Uraraneida is an extinct order of Paleozoic arachnids related to modern spiders. Two genera of fossils have been definitively placed in this order: Attercopus from the Devonian of United States and Permarachne from the Permian of Russia. Like spiders, they are known to have produced silk, but lack the characteristic spinnerets of modern spiders, and retain elongate telsons.
This list of fossil arthropods described in 2018 is a list of new taxa of trilobites, fossil insects, crustaceans, arachnids, and other fossil arthropods of every kind that were described during the year 2018, as well as other significant discoveries, and events related to arthropod paleontology that are scheduled to occur in the year 2018.
The subcapitulum, also known as infracapitulum, hypognathum or hipognatum, refers to the ventral part of the gnathosoma or the fusion of the palpal coxae and the labrum complex present in some arthropods on which the mouth, pedipalps, mouthparts and pharynx are generally located. It is delimited by the subcapitular apodeme, which separates it from the cheliceral frame.
Burmese amber is fossil resin dating to the early Late Cretaceous Cenomanian age recovered from deposits in the Hukawng Valley of northern Myanmar. It is known for being one of the most diverse Cretaceous age amber paleobiotas, containing rich arthropod fossils, along with uncommon vertebrate fossils and even rare marine inclusions. A mostly complete list of all taxa described up to the end of 2023 can be found in Ross (2024).
Paracharontidae is an arachnid family within the order Amblypygi. Paracharontidae and the extinct Weygoldtinidae from the Carboniferous form the suborder Paleoamblypygi, the sister group to the remaining Amblypygi. The family contains two genera: Paracharon, containing the single species Paracharon caecus Hansen, 1921 from Guinea-Bissau in West Africa, and Jorottui with the single species Jorottui ipuanai from Colombia in northern South America. Paracharonopsis from the Eocene (Ypresian) aged Cambay amber of India was initially assigned to this family but this was later questioned and it has since been reassigned to Euamblypygi. Both living species are troglobites, having no eyes, with P. caecus living in termite nests, while J. ipuanai inhabits caves.
Douglassarachne is an extinct genus of arachnid from the Late Carboniferous (Moscovian), known from single species D. acanthopoda. It is known exclusively from one specimen recovered from the Mazon Creek Lagerstätte, Illinois, US.
{{cite journal}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: multiple names: authors list (link){{cite journal}}
: CS1 maint: multiple names: authors list (link)