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Opiliones (; formerly ''Phalangida'') is an order of arachnids commonly known as harvestmen. , over 6,500 species of harvestmen have been discovered worldwide, although the total number of extant species may exceed 10,000. The order Opiliones includes five suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and the recently named Tetrophthalmi.〔 With the exception of Dyspnoi (which is restricted to North America and Eurasia), representatives of each can be found on every continent except Antarctica. Well-preserved fossils have been found in the 400-million-year-old Rhynie cherts of Scotland, and 305-million-year-old rocks in France, which look surprisingly modern, indicating that their basic body plan appeared very early on, and, at least in some taxa, has changed little since that time. Their phylogenetic position within Arachnida is disputed: their closest relatives may be the mites (Acari) or the Novogenuata (the Scorpiones, Pseudoscorpiones and Solifugae). Although superficially similar to and often confused with spiders (order Araneae), Opiliones is a distinct order that is not closely related to spiders within Arachnida. They can be easily distinguished from even long-legged spiders by their fused body regions and single pair of eyes in the middle of their cephalothorax (spiders have an 'abdomen' that is separated from the cephalothorax by a constriction, as well as three to four pairs of eyes, usually around the margins of their cephalothorax). English speakers colloquially refer to species of Opiliones as "daddy longlegs" or "granddaddy longlegs", but this name is also used for two other unrelated groups of arthropods: the crane flies of the family Tipulidae, and the cellar spiders of Pholcidae, most likely because of their similar appearance. They are also referred to as "shepherd spiders" in reference to how their unusually long legs reminded observers of the ways that some European shepherds used stilts to better observe their wandering flocks from a distance. ==Physical description== (詳細は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 and therefore pose no danger to humans. They also have no silk glands and therefore 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 the head, 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. 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 male Opiliones have a penis, unlike other arachnids. 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 , and some species are smaller than one mm, although the largest known species ''Trogulus torosus'' (''Trogulidae'') grow as long as .〔 The leg span of many species is much greater than the body length and sometimes exceeds all the way up to in south eastern Asia.〔http://www.senckenberg.de/root/index.php?page_id=5210&year=2012&action=press&id=2480〕 Most species live for a year. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Opiliones」の詳細全文を読む スポンサード リンク
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