- Physical descriptionEdit. 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.
- The Journal of Arachnology. There is no online version at this time. The PDF is only available to people who have bought the paper or have a subscription.
From Wikipedia, the free encyclopedia
Opiliones Fossil range: 400–0 MaDevonian - Recent | |
---|---|
Hadrobunus grandis | |
Scientific classification | |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Arachnida |
Subclass: | Dromopoda |
Order: | Opiliones Sundevall, 1833 |
Suborders | |
Cyphophthalmi Eupnoi Dyspnoi Laniatores | |
Diversity | |
4 suborders, > 6,400 species |
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Harvestmen are eight-legged invertebrate animals belonging to the order Opiliones (formerly Phalangida) in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. As of 2006, over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000.[1] The order Opiliones can be divided in four suborders: Cyphophthalmi (Simon, 1879), Eupnoi (Hansen & Sørensen, 1904), Dyspnoi (Hansen & Sørensen, 1904) and Laniatores (Thorell, 1876). 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. Phylogenetic position is disputed: their closest relatives may be the mites (Acari) or the Novogenuata (the Scorpiones, Pseudoscorpiones and Solifugae).[2]
Although they belong to the class of arachnids, harvestmen are not spiders, which are of the order Araneae rather than the order Opiliones.
In some places, harvestmen 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).
Because they are a ubiquitous order, but species are often restricted to small regions due to their low dispersal rate,[citation needed] they are good models for biogeographic studies.
Contents
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Physical description
A male Phalangium opilio, showing the long legs.
These arachnids are known for their exceptionally long walking legs, compared to body size, although there are also short-legged species. The difference between harvestmen and spiders is that in harvestmen the two main body sections (the abdomen with ten segments and cephalothorax, or prosoma and opisthosoma) are broadly joined, so that they appear to be one oval structure; they also have no venom or silk glands. In more advanced species, the first five abdominal segments are often fused into a dorsal shield called the scutum, which is normally fused with the carapace. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates lying next to each other. The second pair of legs are longer than the others and work as antennae. This can be hard to see in short-legged species.
The feeding apparatus (stomotheca) differs from other arachnids in that ingestion is not restricted to liquid, but chunks of food can be taken in. The stomotheca is formed by extensions from the pedipalps and the first pair of legs.
They 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[3].
A harvestman (a male Phalangium opilio), showing the almost fused arrangement of abdomen and cephalothorax that distinguishes these arachnids from spiders.
Harvestmen have a pair of prosomatic defensive scent glands (ozopores) that secrete a peculiar smelling fluid when disturbed, confirmed in some species to contain noxious quinones. Harvestmen do not have silk glands and do not possess venom glands, posing absolutely no danger to humans (see below). They do not have book lungs, and breathe through tracheae only. 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.[4]
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Typical body length does not exceed 7 mm (about 5/16 inch), with some species smaller than one mm, although the largest species Trogulus torosus (Trogulidae) can reach a length of 22 mm[1]. However, leg span is much larger and can exceed 160 mm (over 6 inches). Most species live for a year.
Behavior
Harvestman eating a skink tail
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.[5]
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. The eggs can hatch anytime after the first 20 days, up to almost half a year after being laid. Harvestmen need from four to eight nymphal stages to reach maturity, with six the most common.[6]
They are mostly nocturnal and colored in hues of brown, although there are a number of diurnal species 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 can 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.[7]
Mites parasitising a harvestman
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 can count up to 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 each individual of being eaten.[8]
Endangered status
Some troglobitic (cave dwelling) Opiliones are considered endangered if their home caves are in or near cities where pollution and development of the land can alter the cave habitat.[citation needed] Other species are threatened by the invasion of non-native fire ants.[citation needed]
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 chagasiPérez & Kury, 2002, Iandumoema uai[9], Pachylospeleus strinatii[10] and Spaeleoleptes spaeleus[11].
Several Opiliones in Argentina appear to be vulnerable, if not endangered. These include Pachyloidellus fulvigranulatus[12], which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) and Pachyloides borellii[13] is in rainforest patches in North West Argentina which are in an area being dramatically destroyed by humans. The cave living Picunchenops spelaeus[14] 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[15] 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[16] and Texella reyesi[17] are listed as endangered species in the USA. Both are from caves in central Texas. Texella cokendolpheri[18] from a cave in central Texas and Calicina minor[19], Microcina edgewoodensis[20], Microcina homi[21], Microcina jungi[22], Microcina leei Briggs & Ubick 1989, Microcina lumi[23], and Microcina tiburona[24] 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
Uncate (tong-like) chelicerae typical of harvestmen (200x magnification); these appendages are equivalent to a spider's fangs.
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 the cellar spider and cranefly, which are both also called a 'daddy longlegs').[25] This is untrue on several counts. None of the known species 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. This myth is so pervasive that it was debunked by two popular television shows, MythBusters and 'Bill Nye The Science Guy'.
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 Eugène Simon, Tord Tamerlan Teodor Thorell, William Sørensen and Zac Jewell around the turn of the 20th century, and later Cândido Firmino de Mello-Leitão and Reginald Frederick Lawrence. Since 1980, study of the biology and ecology of harvestmen has intensified, especially in South America.[1]
Phylogeny
Main article: Harvestman phylogeny
Harvestmen are very old arachnids. Fossils from the DevonianRhynie 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.[26] 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 an existing family of harvestman.[27]
Etymology
The Swedish naturalist and arachnologist C.J. Sundevall (1801-1875) honored the naturalist Martin Lister (1638-1712) by adopting his term Opiliones for this order; Lister taxonomically described three species from England, United Kingdom. [28]
Systematics
- CyphophthalmiSimon 1879 (c. 100 species)
- Tropicophthalmi Shear 1980
- StylocelloideaHansen & Sørensen 1904
- StylocellidaeHansen & Sørensen 1904
- Ogoveoidea Shear 1980
- OgoveidaeShear 1980
- NeogoveidaeShear 1980
- StylocelloideaHansen & Sørensen 1904
- Temperophthalmi Shear 1980
- Sironoidea Simon 1879
- PettalidaeShear 1980
- SironidaeSimon 1879
- TroglosironidaeShear 1993
- Sironoidea Simon 1879
- Tropicophthalmi Shear 1980
- EupnoiHansen & Sørensen 1904 (c. 1,800 species)
- CaddoideaBanks 1892
- CaddidaeBanks 1892
- PhalangioideaLatreille 1802
- MonoscutidaeForster 1948
- NeopilionidaeLawrence 1931
- SclerosomatidaeSimon 1879
- PhalangiidaeLatreille 1802
- CaddoideaBanks 1892
- DyspnoiHansen & Sørensen 1904 (c. 320 species)
- Ischyropsalidoidea Simon 1879
- CeratolasmatidaeShear 1986
- IschyropsalididaeSimon 1879
- SabaconidaeDresco 1970
- Nemastomatoidea Simon, 1872
- DicranolasmatidaeSimon 1879
- NemastomatidaeSimon 1872
- † NemastomoididaePetrunkevitch 1955 (fossil: Carboniferous)
- NipponopsalididaeMartens 1976
- Troguloidea Sundevall 1833
- † Eotrogulidae (fossil: Carboniferous)
- TrogulidaeSundevall 1833
- Ischyropsalidoidea Simon 1879
- LaniatoresThorell, 1876 (c. 4,000 species)
- Insidiatores Loman, 1900
- Travunioidea Absolon & Kratochvil 1932
- CladonychiidaeHadzi, 1935
- PentanychidaeBriggs 1971
- TravuniidaeAbsolon & Kratochvil 1932
- Triaenonychoidea Sørensen, 1886
- TriaenonychidaeSørensen, 1886
- SynthetonychiidaeForster 1954
- Travunioidea Absolon & Kratochvil 1932
- GrassatoresKury, 2002
- SamooideaSørensen, 1886
- BiantidaeThorell, 1889
- EscadabiidaeKury & Pérez, 2003
- KimulidaePérez González, Kury & Alonso-Zarazaga, 2007 (= Minuidae Sørensen, 1932)
- PodoctidaeRoewer, 1912
- SamoidaeSørensen, 1886
- StygnommatidaeRoewer, 1923
- EpedanoideaSørensen, 1886
- EpedanidaeSørensen, 1886
- GonyleptoideaSundevall, 1833
- AgoristenidaeŠilhavý, 1973
- AssamiidaeSørensen, 1884
- CosmetidaeKoch, 1839
- CranaidaeRoewer, 1913
- GonyleptidaeSundevall, 1833
- ManaosbiidaeRoewer, 1943
- StygnidaeSimon, 1879
- StygnopsidaeSørensen, 1932
- PhalangodoideaSimon, 1879
- OncopodidaeThorell, 1876 — possibly misplaced
- PhalangodidaeSimon, 1879
- Zalmoxoidea Sørensen, 1886
- FissiphalliidaeMartens, 1988
- GuasiniidaeGonzalez-Sponga, 1997
- IcaleptidaeKury & Pérez, 2002
- ZalmoxidaeSørensen, 1886
- SamooideaSørensen, 1886
- Insidiatores Loman, 1900
The family Stygophalangiidae (1 species, Stygophalangium karamani) from underground waters in Macedonia is sometimes misplaced in the Phalangioidea. It is not a harvestman.
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 DevonianRhynie 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, but it is interesting that while there are more than 20 known species from the Cenozoic, and at least seven from the Paleozoic, only one species from the Mesozoic has yet been found.[29]
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 mm long, the male penis can be discerned too. It is not definitely known if 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.
Described species
- Eophalangium sheari (Eupnoi) — Early Devonian (Rhynie, Scotland)
- Brigantibunum listoni (Eupnoi?)— Early Carboniferous (East Kirkton, Scotland)
- Eotrogulus fayoliThevenin, 1901 (Dyspnoi: † Eotrogulidae) — Upper Carboniferous (Commentry, France)
- Nemastomoides elaverisThevenin, 1901 (Dyspnoi: † Nemastomoididae) — Upper Carboniferous (Commentary, France)
- Nemastomoides longipesPetrunkevitch — Upper Carboniferous (Mazon Creek, USA)
- Kustarachne tenuipesScudder, 1890 (Eupnoi?) — Upper Carboniferous (Mazon Creek, USA)
- Echinopustulus samuelnelsoniDunlop, 2004 (Dyspnoi?) — Upper Carboniferous (Western Missouri, USA)
Mesozoic
No fossil harvestmen are known from the Triassic. They are also so far absent from the Lower CretaceousCrato 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 mya) is a long-legged Dyspnoi with large eyes, which may be related to the Ortholasmatinae (Nemastomatidae).[30]
Cenozoic
Unless otherwise noted, all species are from the Eocene.
- Trogulus longipesHaupt, 1956 (Dyspnoi: Trogulidae) — Geiseltal, Germany
- Philacarus hispaniolensis (Laniatores: Samoidae?) — Dominican amber
- Kimula species (Laniatores: Kimulidae) — Dominican amber
- Hummelinckiolus silhavyiCokendolpher & Poinar, 1998 (Laniatores: Samoidae) — Dominican amber
- Caddo dentipalpis (Eupnoi: Caddidae) — Baltic amber
- Dicranopalpus ramiger(Koch & Berendt, 1854) (Eupnoi: Phalangiidae) — Baltic amber
- Opilio ovalis (Eupnoi: Phalangiidae?) — Baltic amber
- Cheiromachus coriaceusMenge, 1854 (Eupnoi: Phalangiidae?) — Baltic amber
- Leiobunum longipes (Eupnoi: Sclerosomatidae) — Baltic amber
- Histricostoma tuberculatum (Dyspnoi: Nemastomatidae) — Baltic amber
- Mitostoma denticulatum (Dyspnoi: Nemastomatidae) — Baltic amber
- Nemastoma incertum (Dyspnoi: Nemastomatidae) — Baltic amber
- Sabacon claviger (Dyspnoi: Sabaconidae) — Baltic amber
- Petrunkevitchiana oculata(Petrunkevitch, 1922) (Eupnoi: Phalangioidea) — Florissant, USA (Oligocene)
- Proholoscotolemon nemastomoides (Laniatores: Cladonychiidae) — Baltic amber
- Siro platypedibus (Cyphophthalmi: Sironidae) — Bitterfeld amber
- Amauropilio atavus(Cockerell, 1907) (Eupnoi: Sclerosomatidae) — Florissant, USA (Oligocene)
- Amauropilio lacoei (A. lawei?) (Petrunkevitch, 1922) — Florissant, USA (Oligocene)
- Pellobunus proavusCokendolpher, 1987 (Laniatores: Samoidae) — Dominican amber
- Phalangium species (Eupnoi: Phalangiidae) — near Rome, Italy (Quaternary)
Footnotes
- ^ abc Pinto-da-Rocha et al. 2007: 5
- ^Shultz, J.W. (1990): Evolutionary morphology and phylogeny of Arachnida. Cladistics6: 1-38.
- ^ Pinto-da-Rocha & Kury 2003: 397
- ^ Pinto-da-Rocha et al. 2007
- ^ Pinto-da-Rocha et al. 2007:9
- ^ Pinto-da-Rocha et al. 2007: 8, 11
- ^ Pinto-da-Rocha et al. 2007: 9-10
- ^ Pinto-da-Rocha et al. 2007: 11
- ^Pinto-da-Rocha, 1996
- ^Šilhavý, 1974
- ^H. Soares, 1966
- ^(Mello-Leitão, 1930)
- ^(Roewer, 1925)
- ^Maury, 1988
- ^Rambla, 1993
- ^Goodnight & Goodnight, 1967
- ^Ubick & Briggs, 1992
- ^Ubick & Briggs, 1992
- ^(Briggs & Hom, 1966)
- ^Briggs & Ubick, 1989
- ^Briggs & Ubick, 1989
- ^Briggs & Ubick, 1989
- ^Briggs & Ubick, 1989
- ^(Briggs & Hom, 1966)
- ^ The Spider Myths Site: Daddy-Longlegs
- ^ Pinto-da-Rocha et al. 2007: 4
- ^ Perkins, Science News: http://www.sciencenews.org/view/generic/id/44918/title/Long-lasting_daddy_longlegs
- ^ Martin Lister's English Spiders 1678, ed. John Parker and Basil Hartley (Colchester, Essex: Harley Books, 1992), p. 26.
- ^Dunlop, Jason A. (2007): Paleontology. In: Pinto-da-Rocha et al. 2007
- ^ Giribet & Dunlop 2005
References
- Joel Hallan's Biology Catalog (2005)
- Giribet, Gonzalo & Dunlop, Jason A. (2005): First identifiable Mesozoic harvestman (Opiliones: Dyspnoi) from Cretaceous Burmese amber. Proc Biol Sci.272(1567): 1007-1013. full HTML
- Pinto-da-Rocha, R., Machado, G. & Giribet, G. (eds.) (2007): Harvestmen - The Biology of Opiliones. Harvard University Press ISBN 0-674-02343-9
- Pinto-da-Rocha, R. & Kury, A.B. (2003): Third species of Guasiniidae (Opiliones, Laniatores) with comments on familial relationships. Journal of Arachnology31(3): 394-399. PDF
Further reading
- Shultz, Jeffrey W. (1998): Phylogeny of Opiliones (Arachnida): An Assessment of the 'Cyphopalpatores' Concept. Journal of Arachnology26(3): 257-272. PDF
External links
- Harvestman: Order Opiliones Diagnostic photographs and information on North American harvestmen
- Harvestman: Order Opiliones Diagnostic photographs and information on European harvestmen
- University of Aberdeen: The Rhynie Chert Harvestmen (fossils)
- National Museum page Classification of Opiliones A synoptic taxonomic arrangement of the order Opiliones, down to family-group level, including some photos of the families
(Redirected from Harvestman anatomy)
Opiliones (commonly known as Harvestmen) are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opliones are divided into two tagmata (arthropod body regions): the abdomen (opisthosoma) and the cephalothorax (prosoma). Unlike spiders, the juncture between the abdomen and cephalothorax is often poorly defined. Harvestmen have chelicerae, pedipalps and four pairs of legs. Most harvestmen have two eyes, although there are eyeless species.[1]
- 1Prosoma
- 6Nervous system
Prosoma[edit]
The frontal portion of the body contains the main part of the centralized nervous system, the feeding organs, defensive glands, and the legs. The upper side is covered by a sclerotized carapace, which is formed by the fused tergites of the six appendage-bearing somites. The feeding apparatus is called the stomotheca. It is formed by extensions from the pedipalps and the first leg pair. The ocularium, an elevated mound placed in the middle of the carapace, usually contains two simple eyes. Some species lack eyes. The prosoma can be smooth or armed with spines. In some members of the suborder Dyspnoi, the eyes are located on projections that form a hood covering the mouthparts.
On the sides of the prosoma there are defensive glands called ozopores, which in Cyphophthalmi open on the tip of an elevated cone called the ozophore. The prosomal muscles attach to the endosternite. The endosternite is a horizontal cartilage like sheet of connective tissue that acts as an internal skeleton. In some harvestmen (Phalangium, Odiellus and Leiobunum) it contains calcium deposits. The endosternite is U-shaped with the arms facing forward and embracing the ganglion above the esophagus.[1]
Chelicerae[edit]
Close-up of the chelicerae of a harvestman
The chelicerae consist of three segments, with the distal two forming a pincer[2] equipped with a row of teeth. The proximal segment is probably homologous to the coxa and trochanter of the pedipalp, the middle segment to the femur through the tarsus of the legs, and the distal segment to the apotele (the claw with its modifications).[1]
Pedipalps[edit]
The pedipalps are used as tactile organs, grasping food, and mating. In predatory species the pedipalps are often enlarged and spiny. Some harvestmen do not have claws, while others (e.g. many Laniatores) may have sickle-like claws. Harvestmen pedipalps are anatomically laid out like the legs, with the difference that the proximal metatarsus and distal tarsus are merged, and simply called tarsus.[1]
Legs[edit]
Phalangium cornutum, profile of male, with legs and palpi truncated. a: ocular tubercle; b: mandible; c: labrum; d: sheath of penis protruded; e: penis; f: glans
The legs consist of coxa, trochanter, femur, patella, tibia, metatarsus, tarsus and claw. In most Eupnoi and many Dyspnoi the coxae are freely movable, while in others they are fused together and immovably attached to the underside of the body. In contrast to spiders, hydraulic pressure does not play a significant role in leg movement. However, a flexed leg of Leiobunum can return 80% in extension. This springlike property is a due to elastic sclerites that span across the joints. The elasticity may be due to resilin in the sclerites.
While the metatarsus is undivided, the tarsus can consist of three to over one hundred tarsomeres. In most Cyphophthalmi the tarsus is entirely undivided. Many long-legged forms in the superfamily Phalangioidea can wrap their tarsi two or three times around twigs. Nevertheless, the tarsi contain no muscles, but only tendons of the claw muscles. These muscles originate in the patella, tibia and metatarsus. Most harvestman legs have only one claw, but in Grassatores, the later two pairs of legs end in two claws, where an additional structure can even give the appearance of three claws.
Nymphal stages of Grassatores and some Insidiatores feature additional structures on the latter two pairs of tarsi, which probably allow adhesion to smooth surfaces during molting, as they are not present in adults.
Legs of Eupnoi and many long-legged Dyspnoi are weak at the base of the femora. When legs are trapped or caught by a predator, these harvestman can detach the restrained leg by a powerful movement of the coxa-trochanter joint. The detached legs of Phalangioidea can twitch for several minutes, with oxygen provided by spiracles in the tibia. The pacemaker neurons responsible for this become active when they lose the connection to the central nervous system. Each of the two twitching leg joints contains an independent pacemaker. Even immature harvestmen cannot regenerate lost legs.
One reason for the hanging stance characteristic for long-legged harvestmen seems to be enhanced stability on exposed surfaces, for example against wind. Leiobunum vittatum (and probably other harvestmen) walks by lifting the 'central' (counting the second pair as antennae) leg of one side and the outer legs of the other side forward while the other three rest on the ground (alternating tripod gait, similar to that of insects).[1]
Opisthosoma[edit]
The hind part of the body, the opisthosoma, consists of ten somites. The last tergite (anal operculum) lacks a corresponding sternite, comparable to the telson of horseshoe crabs, scorpions and whip scorpions. The muscles of the opisthosoma mostly seem to regulate the volume and internal pressure of the body fluid. The openings of the respiratory system, the spiracles or stigmata, are located on the sides of the second somite.[2] While the genital opening, the gonopore, derives from the second opisthosomal somite in all arachnids, it is displaced between the fourth pair of legs in harvestmen, thus appearing to be part of the prosoma.[2] The ovipositor of females or penis of males is withdrawn into the first opisthosomal somite.[1]
Digestion[edit]
Harvestman eating a skink tail
The foregut (stomodeum) develops from the ectoderm. It is called pharynx before passing through the central nervous system, and esophagus inside the CNS. Shortly afterwards it empties into the midgut.
The midgut (mesenteron) is the largest organ in harvestmen and fills most of the opisthosoma. Like in other arthropods it is derived from the endoderm. Unlike the fore- and hindgut, which are derived from ectoderm, it has no cuticular lining. The midgut is surrounded by muscle cells, trachaeae and intermediate tissue, which does not form a fat body like in scorpions and solifuges. The epithelial cells of the midgut are often infected by rickettsia-like parasites, like in some other arachnids. Resorptive cells, which may be unique to harvestmen, contain lipid droplets, glycogen and mineral spherites. These spherites are also common in many arachnids. They seem to function in heavy metal excretion and mineral storage. Overwintering juveniles store calcium and phosphates in it for molting. Silicon is present throughout the life, but decreases with age, as does the number of spherites.
The hindgut is a short invagination of the ectoderm, linking the midgut to the anus. It can be dilated and shortened by muscles.[1]
Circulation[edit]
Unlike arachnids with book lungs (scorpions, most spiders and several others), harvestmen and most other purely tracheate arachnids lack extensive arterial branching and well-defined venous sinuses. The circulatory system consists mainly of a dorsal tubular heart with anterior and posterior aortae. The heart is innervated by a cardiac ganglion. Myofibrils are mostly arranged circularly and constrict the heart during systole. There are two valves present, an anterior valve, extending into the anterior aorta seems to prevent backflow of hemolymph, and a posterior valve, which is possibly actively controlled.
Five types of blood cells have been found, none of which are unique to harvestmen. Prohemocytes are about five µm in diameter and rather scarce. They probably act as stem cells to the amoeboidplasmatocytes, which are about 11 µm and are known to ingestbacteria and dead cells. Granulocytes apparently develop from plasmatocytes and are also amoeboid, although no phagocytosis has been observed. They may act as storage cells. Spherulocytes are up to 15 µm long seem to be non-motile. They contain many spherules, whose composition changes over time, but the function is not known. Coagulocytes, which probably derived from granulocytes, release granules and disintegrate when they come in contact with uncleaned surfaces in vitro, probably due to present bacterial endotoxins. Despite their name, these cells seem not to play a role in hemolymph coagulation.[1]
Respiration[edit]
Harvestmen breathe through tracheae, somewhat like insects. Air enters through a pair of spiracles located behind the last pair of legs, on the opisthosoma. Unlike as in insects, the tracheae end in hemolymph near the organs, instead of leading into them. The oxygen-binding respiratory pigment of harvestmen is hemocyanin, which is typically found in arachnids with book lungs, which harvestmen do not possess. The pattern of tracheal branching seems to be rather consistent in all harvestmen. A main trunk projects upward and forward from each spiracle into the prosoma, where it narrows gradually until it ends in the chelicerae of each side. Lateral branches lead to the other appendages. Except in Cyphophthalmi, the branches from each side meet and form a single transverse trachea. Gonad, genitalia and digestive tract also receive branches.
Phalangioidea have additional spiracles on the tibiae of the pedipalps and legs, thus helping in gas exchange in the long legs. They also provide a severed leg with oxygen, allowing it to continue twitching for some time: A leg of Opilio twitched for about 23 minutes, but stopped after about 40 seconds when the spiracles were sealed.[1]
Nervous system[edit]
The eyes of harvestmen are often placed close together on a small tubercle.[2]
The central nervous system of harvestmen, which evolved from a segmented structure, is centralized around the esophagus. The part above the esophagus (syncerebrum) consists of protocerebrum (associated with the eyes) and deutocerebrum (associated with the chelicerae). The nerves of the pedipalp, walking legs, and some for the opisthosoma emerge from the part below the esophagus. The peripheral nervous system is not well studied.[1]
Harvestman
Eyes[edit]
Arachnids feature two kinds of eyes, lateral and median ocelli. The former, which evolved from compound eyes, are lacking in harvestmen.[2] Many cave- or soil-dwelling species, including most Cyphophthalmi, lack eyes, or they are reduced. Several otherwise blind Cyphophthalmi have small photoreceptors at the base of the ozophores, but it is unclear whether these are derived from lateral or median ocelli. The size and complexity of eyes in harvestmen roughly correlates with the amount of light available in its habitat. Evolutionary reduction of eyes appears to be caused by the early termination of eye development (paedomorphosis) in most species. Adults of those species thus have eyes that resemble embryonic or juvenile forms of their ancestors. However, Ischyropsalis strandi (Ischyropsalididae) nymphal stages have small but complete eyes, but these get partially lost through a degenerative process in adults.[1]
Reproduction[edit]
The basic structure of the reproductive tract is similar in both sexes, with a mesodermalgonoduct (sperm duct or oviduct) emerging from both sides of the U-shaped gonad (testis or ovary). The two gonoducts fuse into a single duct, which leads into a cuticle-lined duct derived from the ectoderm into the open through an organ (penis in males or ovipositor in females) that can be everted through a combination of muscles and hemolymph pressure. These eversible organs play an important role in determining taxonomic relationships.
The penis is often complex, consisting of a long shaft and a shorter glans at the end, which is often equipped with various projections such as spines. In many species muscles move the glans relative to the shaft, but in Grassatores these muscles are lacking, and the movement is achieved purely by hydraulic means. A muscular propulsive organ, which pushes the ejaculate out, is missing in Cyphophthalmi, which have very short penises. The ovipositor is originally segmented and long, but has been reduced in several groups.[1]
References[edit]
- ^ abcdefghijklJeffrey W. Shultz & Ricardo Pinto-da-Rocha (2007). 'Morphology and functional anatomy'. In Ricardo Pinto-da-Rocha, Glauco Machado & Gonzalo Giribet (ed.). Harvestmen: the Biology of Opiliones. Harvard University Press. pp. 14–61. ISBN0-674-02343-9.
- ^ abcdePeter Ax (2000). 'Opiliones'. Multicellular Animals: The Phylogenetic System of the Metazoa. Springer. pp. 128–. ISBN978-3-540-67406-1.
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