AskDefine | Define harvestman

Dictionary Definition

harvestman n : spider-like arachnid with a small rounded body and very long thin legs [syn: daddy longlegs, Phalangium opilio]

User Contributed Dictionary

English

Noun

  1. A field-worker who works to gather in the harvest.
  2. An order of terrestrial, non-venomous arachnids with often very long legs: Opiliones.

Translations

arachnid

References

  • Pinto-da-Rocha, Ricardo, Machado, Glauco & Giribet, Gonzalez (eds.) (2007): Harvestmen - The Biology of Opiliones. Harvard University Press ISBN 0-674-02343-9 pp. 2,3

Extensive Definition

Harvestmen (also known as daddy longlegs in the U.S. and Canada) 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.. 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).
Although they are arachnids, they are not (contrary to misconceptions based on appearance), spiders.
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, they are good models for biogeographic studies.

Physical description

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 nearly 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 (for example 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, 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.
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
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 up to 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. The eggs can hatch anytime after the first 20 days, up to almost half a year after being layed. Harvestmen need from four to eight nymphal stages to reach maturity, with six the most common.
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, and 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. This is similar to the behavior of the similar looking but unrelated daddy longlegs 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 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.

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. Others species are threatened by the invasion of non-native fire ants.
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 Pérez & Kury, 2002, Iandumoema uai Pinto-da-Rocha, 1996, Pachylospeleus strinatii Šilhavý, 1974 and Spaeleoleptes spaeleus H. Soares, 1966.
Several Opiliones in Argentina appear to be vulnerable, if not endangered. These include Pachyloidellus fulvigranulatus (Mello-Leitão, 1930), which is found only on top of Cerro Uritorco, the highest peak in the Sierras Chicas chain (provincia de Cordoba) and Pachyloides borellii (Roewer, 1925) is in rainforest patches in North West Argentina which are in an area being dramatically destroyed by humans. The cave living Picunchenops spelaeus Maury, 1988 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 Rambla, 1993 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 Goodnight & Goodnight, 1967 and Texella reyesi Ubick & Briggs, 1992 are listed as endangered species in the USA. Both are from caves in central Texas. Texella cokendolpheri Ubick & Briggs, 1992 from a cave in central Texas and Calicina minor (Briggs & Hom, 1966), Microcina edgewoodensis Briggs & Ubick, 1989, Microcina homi Briggs & Ubick, 1989, Microcina jungi Briggs & Ubick, 1989, Microcina leei Briggs & Ubick 1989, Microcina lumi Briggs & Ubick, 1989, and Microcina tiburona (Briggs & Hom, 1966) from around springs and other restricted habitats of central California are being considered for listing as endangered species, but as yet receive no protection.

Venomous myth

An urban legend claims that the harvestman is the most venomous animal in the world, but its fangs are too small to bite a human and therefore is not dangerous. (The same myth applies to the cellar spider, which is also called a daddy longlegs). This is untrue on several counts. None of the known species have venom glands or fangs, instead having chelicerae. The size of its mouth varies by species, but even those with relatively large jaws hardly ever bite humans or other large creatures, even in self-defense. The few known cases of actual bites did not involve envenomation, and had no lasting effect.

Research

Harvestmen are a scientifically much 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 1950, 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 Nathan Banks 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.

Systematics

Currently the harvestmen are divided into four suborders, containing circa 45 families and about 1,500 genera. The limits and relationships of most families and genera are not well understood, and there probably are as yet undescribed families. The tiny Cyphophthalmi are considered the oldest suborder and are distributed worldwide. The Eupnoi are also widely distributed and contain some of the best known harvestmen. The Dyspnoi, including some large species, inhabit mostly the northern hemisphere, while the species-rich Laniatores are found mostly in tropical regions of the southern hemisphere.
  • Eupnoi Hansen & Sørensen 1904 (c. 1,800 species)
  • Dyspnoi Hansen & Sørensen 1904 (c. 320 species)
The family Stygophalangiidae (1 species, Stygophalangium karamani) from underground waters in Macedonia is sometimes misplaced in the Phalangioidea. It is not a harvestman.

Footnotes

References

  • Joel Hallan's Biology Catalog (2005)
  • (eds.) (2007): Harvestmen - The Biology of Opiliones. Harvard University Press ISBN 0-674-02343-9
  • (2003): Third species of Guasiniidae (Opiliones, Laniatores) with comments on familial relationships. Journal of Arachnology 31(3): 394-399. PDF

Further reading

  • (1998): Phylogeny of Opiliones (Arachnida): An Assessment of the "Cyphopalpatores" Concept. Journal of Arachnology 26(3): 257-272. PDF

External links

harvestman in Catalan: Opilió
harvestman in Czech: Sekáči
harvestman in Danish: Mejere
harvestman in German: Weberknechte
harvestman in Estonian: Koibikulised
harvestman in Spanish: Opiliones
harvestman in French: Opiliones
harvestman in Italian: Opiliones
harvestman in Hebrew: קוצרים
harvestman in Georgian: მთიბავები
harvestman in Latin: Opiliones
harvestman in Latvian: Māņzirnekļi
harvestman in Lithuanian: Šienpjoviai
harvestman in Dutch: Hooiwagens
harvestman in Japanese: ザトウムシ
harvestman in Norwegian: Vevkjerringer
harvestman in Norwegian Nynorsk: Vevkjerring
harvestman in Polish: Kosarze
harvestman in Portuguese: Opilião
harvestman in Russian: Сенокосцы
harvestman in Simple English: Opiliones
harvestman in Serbian: Косци
harvestman in Finnish: Lukit
harvestman in Swedish: Lockespindlar
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