Opiliones
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Harvestmen (also known as daddy long-legs) 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 species. 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.

In some places, harvestmen are known by the name "daddy longlegs" or "granddaddy longlegs", but this name can also refer to two other unrelated arthropods: the crane fly (Tipulidae) and the cellar spider (Pholcidae).

Because they are an 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.

They have a single pair of eyes in the middle of their heads, oriented sideways, which cannot produce pictures. However, there are eyeless species (for example the Brazilian Caecobunus termitarium (Grassatores) from termite nests, Giupponia chagasi (Gonyleptidae) from caves, and all species of Guasiniidae).

Harvestmen have a pair of prosomatic scent glands 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 trachea 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), although the largest species Trogulus torosus (Trogulidae) can reach a length of 22 mm. However, leg span is much larger and can exceed 160 mm (over 6"). 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 of the decays of any dead animal, 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. Also unlike most other arachnids, harvestmen do not have a sucking stomach and a filtering mechanism, but 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 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 hatch after anything from 20 days to almost half a year. 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 wildly vibrates 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.

A 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 active 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 Eug??ne Simon, Tord Tamerlan Teodor Thorell, William Sørensen and Nathan Banks around the turn of the 20th century, and later C??ndido Firmino de Mello-Leit??o and Reginald Frederick Lawrence. Since 1980, study of the biology and ecology of harvestmen has intensified, especially in South America.

Phylogeny

Harvestmen are very old arachnids. Fossils from the Devonian, 400 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.

http://en.wikipedia.org/wiki/Opiliones

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