Ricinulei
Encyclopedia
The Order
Ricinulei is a group of arachnid
s known as hooded tickspiders. In older works they are sometimes referred to as Podogona.
, 60 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.
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.
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.
, are composed of two segments forming a fixed and a moveable digit. Sensory organs are also found associated with the mouthparts; presumbaly for tasting the food. The chelicerae can be retracted and at rest they are normally hidden beneath the cucullus.
Ricinuleid pedipalp
s are complex appendage
s. 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 scorpion
s and pseudoscorpion
s. 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.
s. 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 lung
s, 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.
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).
, 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
.
All of the fifteen 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, 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.
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 "", comprising spider
s, 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.
, 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.
. 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.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Ricinulei is a group of arachnid
Arachnid
Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...
s known as hooded tickspiders. In older works they are sometimes referred to as Podogona.
, 60 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa (Ricinoides
Ricinoides
Ricinoides is a genus in the family Ricinoididae.-Species :*Ricinoides westermannii *Ricinoides afzelii *Ricinoides atewa Naskrecki, 2008*Ricinoides crassipalpe...
) and the Neotropical region (Cryptocellus and Pseudocellus). In addition to the three living genera, there are two families and four genera containing fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
species.
Physical description
The most important general account of ricinuleid anatomy remains the 1904 monograph by Hans Jacob HansenHans Jacob Hansen
-References:*.-Literature:...
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 5 to 10 mm (0.196850393700787 to 0.393700787401575 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
Opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...
) 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.
Appendages
The mouthparts, or cheliceraeChelicerae
The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...
, are composed of two segments forming a fixed and a moveable digit. Sensory organs are also found associated with the mouthparts; presumbaly for tasting the food. The chelicerae can be retracted and at rest they are normally hidden beneath the cucullus.
Ricinuleid pedipalp
Pedipalp
Pedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s are complex appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s. 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 scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...
s and pseudoscorpion
Pseudoscorpion
A pseudoscorpion, , is an arachnid belonging to the order Pseudoscorpionida, also known as Pseudoscorpiones or Chelonethida....
s. Similar pincers on the pedipalps have now been found in the extinct order Trigonotarbida
Trigonotarbida
The Order Trigonotarbida is an extinct group of 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...
(see Relationships).
As in many harvestmen
Opiliones
Opiliones are an order of arachnids commonly known as harvestmen. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000. The order Opiliones can be divided into four suborders: Cyphophthalmi, Eupnoi, Dyspnoi and Laniatores...
, 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.
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 glandCoxal gland
The coxal gland is a gland found in some arthropods, for collecting and excreting urine. They are found in all arachnids , and in other chelicerates, such as horseshoe crabs. The coxal gland is thought to be homologous with the antennal gland of crustaceans...
s. 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 lung
Book lung
A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...
s, and gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
takes place through trachea
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
. 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.
Biology
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 spermatophoreSpermatophore
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
to the female. The eggs are carried under the mother's hood, until the young hatch into six-legged larva, which later molt
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
into their eight-legged adult forms. The six-legged larva is a feature they share with Acari (see Relationships).
Habitat
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.Fossil record
Ricinulei are unique among arachnids in that the first one to be discovered was a fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
, described in 1837 by the noted English geologist William Buckland
William Buckland
The Very Rev. Dr William Buckland DD FRS was an English geologist, palaeontologist and Dean of Westminster, who wrote the first full account of a fossil dinosaur, which he named Megalosaurus...
; albeit misinterpreted as a beetle
Beetle
Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...
. Further fossil species were added in subsequent years by, among others, Samuel Hubbard Scudder
Samuel Hubbard Scudder
Samuel Hubbard Scudder was an American entomologist and palaeontologist.Born in Boston, Massachusetts, Scudder may be most widely known for his essay on the importance of first-hand, careful observation in the natural sciences...
, Reginald Innes Pocock
Reginald Innes Pocock
Reginald Innes Pocock F.R.S. was a British zoologist.Pocock was born in Clifton, Bristol, the fourth son of Rev. Nicholas Pocock and Edith Prichard. He began showing interest in natural history at St. Edward's School, Oxford. He received tutoring in zoology from Sir Edward Poulton, and was allowed...
and Alexander Petrunkevitch
Alexander Petrunkevitch
Alexander Ivanovitch Petrunkevitch was an eminent arachnologist of his time. From 1910 to 1939 he described over 130 spider species.-Biography:...
.
All of the fifteen species of fossil ricinuleids discovered so far originate from the late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
(Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
) Coal Measures of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. 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
Opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...
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.
Relationships
After Giribet et al. (2002)Early work
The first living ricinuleid was described from West Africa by Félix Édouard Guérin-MénevilleFélix Édouard Guérin-Méneville
Félix Édouard Guérin-Méneville was a French entomologist.Guérin-Méneville changed his surname from Guérin in 1836. He was the author of the illustrated work Iconographie du Règne Animal de G. Cuvier 1829–1844, a complement to the work of Georges Cuvier and Pierre André Latreille, which lacked...
in 1838, i.e. one year after the first fossil. This was followed by a second living example collected by Henry Walter Bates
Henry Walter Bates
Henry Walter Bates FRS FLS FGS was an English naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the Amazon with Alfred Russel Wallace in 1848. Wallace returned in 1852, but lost his collection in a shipwreck...
in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
and described by John Obadiah Westwood
John Obadiah Westwood
John Obadiah Westwood was an English entomologist and archaeologist also noted for his artistic talents.Born in Sheffield, he studied to be a lawyer but abandoned that for his scientific interests....
in 1874, and a third from Sierra Leone
Sierra Leone
Sierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea to the north and east, Liberia to the southeast, and the Atlantic Ocean to the west and southwest. Sierra Leone covers a total area of and has an estimated population between 5.4 and 6.4...
by Tamerlan Thorell
Tamerlan Thorell
Tord Tamerlan Teodor Thorell was a Swedish arachnologist.Thorell studied spiders with Giacomo Doria at the Museo Civico di Storia Naturale de Genoa...
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 "", comprising spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, 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 gnathosomaGnathosoma
The gnathosoma is the part of the body of the Acari comprising the mouth and feeding parts. These are the hypostome, the chelicerae and the pedipalps. It is also called the capitulum...
, 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 TrigonotarbidaTrigonotarbida
The Order Trigonotarbida is an extinct group of 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...
. 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.