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Harvestmen are an order of arachnids, with which they share many characteristics: their body is divided into two body regions (tagmata), the abdomen (opisthosoma) and the cephalothorax (prosoma); however, unlike in spiders, the juncture is often poorly defined. They have chelicerae, pedipalps and four pairs of legs. Most harvestmen have two eyes, although there are eyeless species.
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 plays no significant role in leg movement. However, a flexed leg of Leiobunum returned 80% of its energy by extending. This springlike property is caused by sclerites that span across the joints, which probably consist of resilin.
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).