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Description
Opiliones (formerly Phalangida) are an order of arachnids commonly known as harvestmen. As of December 2011, over 6,500 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000. The order Opiliones includes four suborders: Cyphophthalmi, Eupnoi, Dyspnoi, and Laniatores. Well-preserved fossils have been found in the 400-million-year-old Rhynie cherts of Scotland, which look surprisingly modern, indicating that the basic structure of the harvestmen has not changed much since then. Their phylogenetic position is disputed: their closest relatives may be the mites (Acari) or the Novogenuata (the Scorpiones, Pseudoscorpiones and Solifugae).
Although they belong to the class Arachnida, harvestmen are in the order Opiliones; therefore they are not spiders, which are members of the order Araneae.
In some places, Opiliones are known by the name "daddy longlegs" or "granddaddy longlegs", but this name is also used for two other unrelated arthropods: the crane fly (Tipulidae) and the cellar spider (Pholcidae).

Physical description

Opiliones are known for their exceptionally long legs relative to their body size; however some species are short-legged. As in the Araneae, the body in the Opiliones has two tagmata, the anterior cephalothorax or prosoma, and the posterior ten-segmented abdomen or opisthosoma. The most obvious difference between harvestmen and spiders is that in harvestmen the connection between the cephalothorax and abdomen is broad, so that the body appears to be a single oval structure. Other differences are that Opiliones have no venom glands in their chelicerae. They also have no silk glands and therefore they do not build webs. In some highly derived species the first five abdominal segments are fused into a dorsal shield called the scutum, which in most such species is fused with the carapace. Some such Opiliones only have this shield in the males. In some species the two posterior abdominal segments are reduced. Some of them divided medially on the surface to form two plates beside each other. The second pair of legs are longer than the others and they function as antennae or feelers. In short-legged species this may not be obvious.
The feeding apparatus (stomotheca) differs from most arachnids in that Opiliones can swallow chunks of solid food, not only liquids. The stomotheca is formed by extensions from the pedipalps and the first pair of legs.
Opiliones have a single pair of eyes in the middle of their heads, oriented sideways. However, there are eyeless species, such as the Brazilian Caecobunus termitarum (Grassatores) from termite nests, Giupponia chagasi (Gonyleptidae) from caves, and all species of Guasiniidae.

Harvestmen have a pair of prosomatic defensive scent glands (ozopores) that secrete a peculiar smelling fluid when disturbed. In some species the fluid contains noxious quinones. Harvestmen do not have silk glands. They also do not possess venom glands and accordingly pose absolutely no danger to humans (see below). They do not have book lungs, and breathe through tracheae. Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located, one opening on each side. In more active species, spiracles are also found upon the tibia of the legs. They have a gonopore on the ventral cephalothorax, and the copulation is direct as the male has a penis (while the female has an ovipositor). All species lay eggs.
The legs continue to twitch after they are detached. This is because there are 'pacemakers' located in the ends of the first long segment (femur) of their legs. These pacemakers send signals via the nerves to the muscles to extend the leg and then the leg relaxes between signals. While some harvestman's legs will twitch for a minute, other kinds have been recorded to twitch for up to an hour. The twitching has been hypothesized as a means to keep the attention of a predator while the harvestman escapes.
Typical body length does not exceed 7 millimetres (0.28 in), and some species are smaller than one mm, although the largest known species Trogulus torosus (Trogulidae) grow as long as 22 millimetres (0.87 in). The leg span of many species is much greater than the body length and sometimes exceeds 160 millimetres (6.3 in). Most species live for a year.

Behavior

Many species are omnivorous, eating primarily small insects and all kinds of plant material and fungi; some are scavengers, feeding upon dead organisms, bird dung and other fecal material. This broad range is quite unusual in arachnids, which are usually pure predators. Most hunting harvestmen ambush their prey, although active hunting is also found. Because their eyes cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making them vulnerable to internal parasites such as gregarines.
Although parthenogenetic species do occur, most harvestmen reproduce sexually. Mating involves direct copulation, rather than the deposition of a spermatophore. The males of some species offer a secretion from their chelicerae to the female before copulation. Sometimes the male guards the female after copulation and, in many species, the males defend territories. The females lay eggs shortly after mating or anytime up to several months later. Some species build nests for this purpose. A unique feature of harvestmen is that in some species the male is solely responsible for guarding the eggs resulting from multiple partners, often against egg-eating females, and subjecting the eggs to regular cleaning. Depending on circumstances such as temperature, the eggs may hatch at any time after the first 20 days, up to about half a year after being laid. Harvestmen pass through four to eight nymphal instars to reach maturity. Most known species have six instars.
Most species are nocturnal and colored in hues of brown, although there are a number of diurnal species, some of which have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
To deal with predators such as birds, mammals, amphibians and spiders, some species glue debris onto their body, while many play dead when disturbed. Many species practice autotomy; they detach their legs, which keep on moving, to confuse predators. Especially long-legged species vibrate their body ("bobbing"), probably also to confuse predators. This is similar to the behavior of the similar looking but unrelated cellar spider, which vibrates wildly in its web when touched. Scent glands emit substances that can deter larger predators, but are also effective against ants.
Many species of harvestmen easily tolerate members of their own species, with aggregations of many individuals often found at protected sites near water. These aggregations may number 200 animals in the Laniatores, but more than 70,000 in certain Eupnoi. This behavior is likely a strategy against climatic odds, but also against predators, combining the effect of scent secretions, and reducing the probability of any particular individual of being eaten.

Endangered status

All troglobitic species (of all animal taxa) are considered to be at least threatened in Brazil. There are four species of Opiliones in the Brazilian National List for endangered species, all of them cave-dwelling species. Giupponia chagasi, Iandumoema uai, Pachylospeleus strinatii and Spaeleoleptes spaeleus.
Several Opiliones in Argentina appear to be vulnerable, if not endangered. These include Pachyloidellus fulvigranulatus, which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) and Pachyloides borellii is in rainforest patches in North West Argentina which are in an area being dramatically destroyed by humans. The cave living Picunchenops spelaeus is apparently endangered through human action. So far no harvestman has been included in any kind of a Red List in Argentina and therefore they receive no protection.
Maiorerus randoi has only been found in one cave in the Canary Islands. It is included in the Catálogo Nacional de especies amenazadas (National catalog of threatened species) from the Spanish government.
Texella reddelli and Texella reyesi are listed as endangered species in the USA. Both are from caves in central Texas. Texella cokendolpheri from a cave in central Texas and Calicina minor, Microcina edgewoodensis, Microcina homi, Microcina jungi, Microcina leei, Microcina lumi, and Microcina tiburona from around springs and other restricted habitats of central California are being considered for listing as endangered species, but as yet receive no protection.

Misconception

An urban legend claims that the harvestman is the most venomous animal in the world, but possesses fangs too short or a mouth too round and small to bite a human and therefore is not dangerous (the same myth applies to Pholcus phalangioides and the cranefly, which are both also called a 'daddy longlegs'). This is untrue on several counts. None of the known species of harvestmen have venom glands; their chelicerae are not hollowed fangs but grasping claws that are typically very small and definitely not strong enough to break human skin.

Research

Harvestmen are a scientifically neglected group. Description of new taxa has always been dependent on the activity of a few dedicated taxonomists. Carl Friedrich Roewer described about a third (2,260) of today's known species from the 1910s to the 1950s, and published the landmark systematic work Die Weberknechte der Erde (Harvestmen of the World) in 1923, with descriptions of all species known to that time. Other important taxonomists in this field include: Pierre Latreille (18th century) Carl Ludwig Koch, Maximilian Perty (1830s-1850s) L. Koch, Tord Tamerlan Teodor Thorell (1860s-1870s) Eugène Simon, William Sørensen (1880s-1890s) James C. Cokendolpher, Raymond Forster, Jürgen Gruber, Reginald Frederick Lawrence, Jochen Martens, Cândido Firmino de Mello-Leitão (20th century) Gonzalo Giribet, Adriano Brilhante Kury (21st century).
Since the 1990s, study of the biology and ecology of harvestmen has intensified, especially in South America.

Phylogeny

Harvestmen are very old arachnids. Fossils from the Devonian Rhynie chert, 410 million years ago, already show characteristics like tracheae and sexual organs, proving that the group has lived on land since that time. They are probably closely related to the scorpions, pseudoscorpions and solifuges; these four orders form the clade Dromopoda. The Opiliones have remained almost unchanged morphologically over a long period. Indeed, one species discovered in China, fossilized by fine grained volcanic ash around 165 million years ago, is hardly discernible from its modern day descendant and belongs to the extant family of harvestman Sclerosomatidae.

Etymology

The Swedish naturalist and arachnologist Carl Jakob Sundevall (1801–1875) honored the naturalist Martin Lister (1638–1712) by adopting his term Opiliones for this order; Lister characterized three species from England, United Kingdom (although not formally describing them, being a pre-Linnean work).

Fossil record

Despite their long history, few harvestman fossils are known. This is mainly due to their delicate body structure and terrestrial habitat, making it unlikely to be found in sediments. As a consequence, most known fossils have been preserved as amber.
The oldest known harvestman, from the 400 million years old Devonian Rhynie chert, already has almost all the characteristics of modern species, placing the origin of harvestmen in the Silurian, or even earlier.
Interestingly, no fossils of Cyphophthalmi or Laniatores much older than 50 million years are known, despite the former presenting a basal clade, and the latter having probably diverged from the Dyspnoi more than 300 million years ago.
Naturally, most finds are from comparatively recent times. More than 20 fossil species are known from the Cenozoic, three from the Mesozoic, and at least seven from the Paleozoic.

Paleozoic

The 400 million years old Eophalangium sheari is known from two specimens, one a female, the other a male. The female bears an ovipositor and is about 10 millimetres (0.39 in) long, the male penis can be discerned too. It is not definitely known whether both sexes belong to the same species. They have long legs, tracheae, and no median eyes.
Brigantibunum listoni from East Kirkton near Edinburgh in Scotland is almost 340 million years old. Its placement is rather uncertain, apart from it being a harvestman.
From about 300 million years ago (mya) there are several finds from the Coal Measures of North America and Europe. While the two described Nemastomoides species are currently grouped as Dyspnoi, they look more like Eupnoi.
Kustarachne tenuipes was shown in 2004 to be a harvestman, after residing for almost hundred years in its own arachnid order, the "Kustarachnida".
There are some fossils from the Permian that are possibly harvestmen, but these are not well preserved.

Mesozoic

No fossil harvestmen are known from the Triassic. They are also so far absent from the Lower Cretaceous Crato Formation of Brazil, which has yielded many other terrestrial arachnids. An unnamed long-legged harvestman was reported from the Early Cretaceous of Koonwarra, Victoria, Australia, which may be a Eupnoi.
Halitherses grimaldii from Burmese amber (c. 100 million years ago) is a long-legged Dyspnoi with large eyes, which may be related to the Ortholasmatinae (Nemastomatidae).

Description
Opiliones (formerly Phalangida) are an order of arachnids commonly known as harvestmen. As of December 2011, over 6,500 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000. The order Opiliones includes four suborders: Cyphophthalmi, Eupnoi, Dyspnoi, and Laniatores. Well-preserved fossils have been found in the 400-million-year-old Rhynie cherts of Scotland, which look surprisingly modern, indicating that the basic structure of the harvestmen has not changed much since then. Their phylogenetic position is disputed: their closest relatives may be the mites (Acari) or the Novogenuata (the Scorpiones, Pseudoscorpiones and Solifugae).
Although they belong to the class Arachnida, harvestmen are in the order Opiliones; therefore they are not spiders, which are members of the order Araneae.
In some places, Opiliones are known by the name "daddy longlegs" or "granddaddy longlegs", but this name is also used for two other unrelated arthropods: the crane fly (Tipulidae) and the cellar spider (Pholcidae).

Physical description

Opiliones are known for their exceptionally long legs relative to their body size; however some species are short-legged. As in the Araneae, the body in the Opiliones has two tagmata, the anterior cephalothorax or prosoma, and the posterior ten-segmented abdomen or opisthosoma. The most obvious difference between harvestmen and spiders is that in harvestmen the connection between the cephalothorax and abdomen is broad, so that the body appears to be a single oval structure. Other differences are that Opiliones have no venom glands in their chelicerae. They also have no silk glands and therefore they do not build webs. In some highly derived species the first five abdominal segments are fused into a dorsal shield called the scutum, which in most such species is fused with the carapace. Some such Opiliones only have this shield in the males. In some species the two posterior abdominal segments are reduced. Some of them divided medially on the surface to form two plates beside each other. The second pair of legs are longer than the others and they function as antennae or feelers. In short-legged species this may not be obvious.
The feeding apparatus (stomotheca) differs from most arachnids in that Opiliones can swallow chunks of solid food, not only liquids. The stomotheca is formed by extensions from the pedipalps and the first pair of legs.
Opiliones have a single pair of eyes in the middle of their heads, oriented sideways. However, there are eyeless species, such as the Brazilian Caecobunus termitarum (Grassatores) from termite nests, Giupponia chagasi (Gonyleptidae) from caves, and all species of Guasiniidae.

Harvestmen have a pair of prosomatic defensive scent glands (ozopores) that secrete a peculiar smelling fluid when disturbed. In some species the fluid contains noxious quinones. Harvestmen do not have silk glands. They also do not possess venom glands and accordingly pose absolutely no danger to humans (see below). They do not have book lungs, and breathe through tracheae. Between the base of the fourth pair of legs and the abdomen a pair of spiracles are located, one opening on each side. In more active species, spiracles are also found upon the tibia of the legs. They have a gonopore on the ventral cephalothorax, and the copulation is direct as the male has a penis (while the female has an ovipositor). All species lay eggs.
The legs continue to twitch after they are detached. This is because there are 'pacemakers' located in the ends of the first long segment (femur) of their legs. These pacemakers send signals via the nerves to the muscles to extend the leg and then the leg relaxes between signals. While some harvestman's legs will twitch for a minute, other kinds have been recorded to twitch for up to an hour. The twitching has been hypothesized as a means to keep the attention of a predator while the harvestman escapes.
Typical body length does not exceed 7 millimetres (0.28 in), and some species are smaller than one mm, although the largest known species Trogulus torosus (Trogulidae) grow as long as 22 millimetres (0.87 in). The leg span of many species is much greater than the body length and sometimes exceeds 160 millimetres (6.3 in). Most species live for a year.

Behavior

Many species are omnivorous, eating primarily small insects and all kinds of plant material and fungi; some are scavengers, feeding upon dead organisms, bird dung and other fecal material. This broad range is quite unusual in arachnids, which are usually pure predators. Most hunting harvestmen ambush their prey, although active hunting is also found. Because their eyes cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making them vulnerable to internal parasites such as gregarines.
Although parthenogenetic species do occur, most harvestmen reproduce sexually. Mating involves direct copulation, rather than the deposition of a spermatophore. The males of some species offer a secretion from their chelicerae to the female before copulation. Sometimes the male guards the female after copulation and, in many species, the males defend territories. The females lay eggs shortly after mating or anytime up to several months later. Some species build nests for this purpose. A unique feature of harvestmen is that in some species the male is solely responsible for guarding the eggs resulting from multiple partners, often against egg-eating females, and subjecting the eggs to regular cleaning. Depending on circumstances such as temperature, the eggs may hatch at any time after the first 20 days, up to about half a year after being laid. Harvestmen pass through four to eight nymphal instars to reach maturity. Most known species have six instars.
Most species are nocturnal and colored in hues of brown, although there are a number of diurnal species, some of which have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
To deal with predators such as birds, mammals, amphibians and spiders, some species glue debris onto their body, while many play dead when disturbed. Many species practice autotomy; they detach their legs, which keep on moving, to confuse predators. Especially long-legged species vibrate their body ("bobbing"), probably also to confuse predators. This is similar to the behavior of the similar looking but unrelated cellar spider, which vibrates wildly in its web when touched. Scent glands emit substances that can deter larger predators, but are also effective against ants.
Many species of harvestmen easily tolerate members of their own species, with aggregations of many individuals often found at protected sites near water. These aggregations may number 200 animals in the Laniatores, but more than 70,000 in certain Eupnoi. This behavior is likely a strategy against climatic odds, but also against predators, combining the effect of scent secretions, and reducing the probability of any particular individual of being eaten.

Endangered status

All troglobitic species (of all animal taxa) are considered to be at least threatened in Brazil. There are four species of Opiliones in the Brazilian National List for endangered species, all of them cave-dwelling species. Giupponia chagasi, Iandumoema uai, Pachylospeleus strinatii and Spaeleoleptes spaeleus.
Several Opiliones in Argentina appear to be vulnerable, if not endangered. These include Pachyloidellus fulvigranulatus, which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) and Pachyloides borellii is in rainforest patches in North West Argentina which are in an area being dramatically destroyed by humans. The cave living Picunchenops spelaeus is apparently endangered through human action. So far no harvestman has been included in any kind of a Red List in Argentina and therefore they receive no protection.
Maiorerus randoi has only been found in one cave in the Canary Islands. It is included in the Catálogo Nacional de especies amenazadas (National catalog of threatened species) from the Spanish government.
Texella reddelli and Texella reyesi are listed as endangered species in the USA. Both are from caves in central Texas. Texella cokendolpheri from a cave in central Texas and Calicina minor, Microcina edgewoodensis, Microcina homi, Microcina jungi, Microcina leei, Microcina lumi, and Microcina tiburona from around springs and other restricted habitats of central California are being considered for listing as endangered species, but as yet receive no protection.

Misconception

An urban legend claims that the harvestman is the most venomous animal in the world, but possesses fangs too short or a mouth too round and small to bite a human and therefore is not dangerous (the same myth applies to Pholcus phalangioides and the cranefly, which are both also called a 'daddy longlegs'). This is untrue on several counts. None of the known species of harvestmen have venom glands; their chelicerae are not hollowed fangs but grasping claws that are typically very small and definitely not strong enough to break human skin.

Research

Harvestmen are a scientifically neglected group. Description of new taxa has always been dependent on the activity of a few dedicated taxonomists. Carl Friedrich Roewer described about a third (2,260) of today's known species from the 1910s to the 1950s, and published the landmark systematic work Die Weberknechte der Erde (Harvestmen of the World) in 1923, with descriptions of all species known to that time. Other important taxonomists in this field include: Pierre Latreille (18th century) Carl Ludwig Koch, Maximilian Perty (1830s-1850s) L. Koch, Tord Tamerlan Teodor Thorell (1860s-1870s) Eugène Simon, William Sørensen (1880s-1890s) James C. Cokendolpher, Raymond Forster, Jürgen Gruber, Reginald Frederick Lawrence, Jochen Martens, Cândido Firmino de Mello-Leitão (20th century) Gonzalo Giribet, Adriano Brilhante Kury (21st century).
Since the 1990s, study of the biology and ecology of harvestmen has intensified, especially in South America.

Phylogeny

Harvestmen are very old arachnids. Fossils from the Devonian Rhynie chert, 410 million years ago, already show characteristics like tracheae and sexual organs, proving that the group has lived on land since that time. They are probably closely related to the scorpions, pseudoscorpions and solifuges; these four orders form the clade Dromopoda. The Opiliones have remained almost unchanged morphologically over a long period. Indeed, one species discovered in China, fossilized by fine grained volcanic ash around 165 million years ago, is hardly discernible from its modern day descendant and belongs to the extant family of harvestman Sclerosomatidae.

Etymology

The Swedish naturalist and arachnologist Carl Jakob Sundevall (1801–1875) honored the naturalist Martin Lister (1638–1712) by adopting his term Opiliones for this order; Lister characterized three species from England, United Kingdom (although not formally describing them, being a pre-Linnean work).

Fossil record

Despite their long history, few harvestman fossils are known. This is mainly due to their delicate body structure and terrestrial habitat, making it unlikely to be found in sediments. As a consequence, most known fossils have been preserved as amber.
The oldest known harvestman, from the 400 million years old Devonian Rhynie chert, already has almost all the characteristics of modern species, placing the origin of harvestmen in the Silurian, or even earlier.
Interestingly, no fossils of Cyphophthalmi or Laniatores much older than 50 million years are known, despite the former presenting a basal clade, and the latter having probably diverged from the Dyspnoi more than 300 million years ago.
Naturally, most finds are from comparatively recent times. More than 20 fossil species are known from the Cenozoic, three from the Mesozoic, and at least seven from the Paleozoic.

Paleozoic

The 400 million years old Eophalangium sheari is known from two specimens, one a female, the other a male. The female bears an ovipositor and is about 10 millimetres (0.39 in) long, the male penis can be discerned too. It is not definitely known whether both sexes belong to the same species. They have long legs, tracheae, and no median eyes.
Brigantibunum listoni from East Kirkton near Edinburgh in Scotland is almost 340 million years old. Its placement is rather uncertain, apart from it being a harvestman.
From about 300 million years ago (mya) there are several finds from the Coal Measures of North America and Europe. While the two described Nemastomoides species are currently grouped as Dyspnoi, they look more like Eupnoi.
Kustarachne tenuipes was shown in 2004 to be a harvestman, after residing for almost hundred years in its own arachnid order, the "Kustarachnida".
There are some fossils from the Permian that are possibly harvestmen, but these are not well preserved.

Mesozoic

No fossil harvestmen are known from the Triassic. They are also so far absent from the Lower Cretaceous Crato Formation of Brazil, which has yielded many other terrestrial arachnids. An unnamed long-legged harvestman was reported from the Early Cretaceous of Koonwarra, Victoria, Australia, which may be a Eupnoi.
Halitherses grimaldii from Burmese amber (c. 100 million years ago) is a long-legged Dyspnoi with large eyes, which may be related to the Ortholasmatinae (Nemastomatidae).