Opiliones

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iOpiliones

Harvestman Hadrobunus grandis

Scientific classification

Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Order:	Opiliones Sundevall, 1833

The Phalangids (legacy name) or Opiliones (better known as "harvestmen") are eight-legged invertebrate animals belonging to the order Opiliones in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. As of 2005, over 6,300 species of Phalangids have been described worldwide. 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 looks surpringly modern, indicating that the basic structure of the harvestmen hasn't changed much since then. Their closest relatives are probably the mites (Acari).

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).

1 Physical description
2 Behavior
3 Endangered status
4 A venomous myth
5 External links

[edit] Physical description

A macro picture of a harvestman (a male Phalangium opilio, showing the almost fused arrangement of abdomen and cephalothorax that distinguishes these arachnids from spiders.

These harmless arachnids are known for their exceptionally long walking legs, compared to body size. 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. 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 laying next to each other. The second pair of legs are longer than the others and works 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. They have a pair of prosomatic scent glands that secrete a peculiar smelling fluid when disturbed. Harvestmen do not have silk glands and do not possess poison glands, posing absolutely no danger to humans (see below). They do not have book lungs, and breath 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 Opiliones 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 Opiliones escapes.

Typical body length does not exceed 7mm (about 5/16") even in the largest species. However, leg span is much larger and can exceed 160mm (over 6"). Most species live for a year.

[edit] 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. Mating involves direct copulation, rather than the deposition of a spermatophore. 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.

[edit] Endangered status

Some troglobitic 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 U.S.A. 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.

[edit] A venomous myth

There is an urban legend claiming that the harvestman is the most venomous spider in the world, only its fangs are too small to bite a human, and is thus not actually dangerous. This is untrue on several counts [1] (see also cellar spider). Phalangids are not spiders, and none of the known species have venom glands. The size of the jaws varies by species, but even those with relatively large jaws virtually never 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 effects.