Opiliones (commonly known as harvestmen) are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opiliones are divided into two tagmata (arthropod body regions): the abdomen (opisthosoma) and the cephalothorax (prosoma). Unlike spiders, the juncture between the abdomen and cephalothorax is often poorly defined. Harvestmen have chelicerae, pedipalps and four pairs of legs. Most harvestmen have two eyes, although there are eyeless species.
Prosoma
The frontal portion of the body contains the main part of the centralized nervous system, the feeding organs, defensive glands, and the legs. The upper side is covered by a sclerotized carapace, which is formed by the fused tergites of the six appendage-bearing somites. The feeding apparatus is called the stomotheca. It is formed by extensions from the pedipalps and the first leg pair. The ocularium, an elevated mound placed in the middle of the carapace, usually contains two simple eyes. Some species lack eyes. The prosoma can be smooth or armed with spines. In some members of the suborder Dyspnoi, the eyes are located on projections that form a hood covering the mouthparts.
On the sides of the prosoma there are defensive glands called ozopores, which in Cyphophthalmi open on the tip of an elevated cone called the ozophore. The prosomal muscles attach to the endosternite. The endosternite is a horizontal cartilage like sheet of connective tissue that acts as an internal skeleton. In some harvestmen (Phalangium, Odiellus and Leiobunum) it contains calcium deposits. The endosternite is U-shaped with the arms facing forward and embracing the ganglion above the esophagus.
Chelicerae
The chelicerae consist of three segments, with the distal two forming a pincer equipped with a row of teeth. The proximal segment is probably homologous to the coxa and trochanter of the pedipalp, the middle segment to the femur through the tarsus of the legs, and the distal segment to the apotele (the claw with its modifications).
Pedipalps
The pedipalps are used as tactile organs, for grasping food, and mating. In predatory species the pedipalps are often enlarged and spiny. Some harvestmen do not have claws, while others (e.g. many Laniatores) may have sickle-like claws. Harvestmen pedipalps are anatomically laid out like the legs, the difference being that the proximal metatarsus and distal tarsus are merged, and simply called tarsus.
Legs
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 does not play a significant role in leg movement. However, a flexed leg of Leiobunum can return 80% in extension. This springlike property is a due to elastic sclerites that span across the joints. The elasticity may be due to resilin in the sclerites.
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.