Opiliones anatomy
[2] 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.
In some members of the suborder Dyspnoi, the eyes are located on projections that form a hood covering the mouthparts.
[1][4] Functional genetic data have shown that knockdown of the transcription factor dachshund transforms the three-segmented chelicera into the two-segmented variant found in groups like pseudoscorpions and solifuges.
[6] 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.
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.
The detached legs of Phalangioidea can twitch for several minutes, with oxygen provided by spiracles in the tibia.
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).
[3] While the genital opening, the gonopore, derives from the second opisthosomal somite in all arachnids, it is displaced between the fourth pair of legs in harvestmen, thus appearing to be part of the prosoma.
The midgut is surrounded by muscle cells, tracheae and intermediate tissue, which does not form a fat body like in scorpions and solifuges.
The circulatory system consists mainly of a dorsal tubular heart with anterior and posterior aortae.
Coagulocytes, which probably derived from granulocytes, release granules and disintegrate when they come in contact with uncleaned surfaces in vitro, probably due to present bacterial endotoxins.
A main trunk projects upward and forward from each spiracle into the prosoma, where it narrows gradually until it ends in the chelicerae of each side.
[1] When present, most living harvestmen have a single pair of camera-type eyes, which can occur in different positions on the anterior prosoma.
In Cyphophthalmi, eyes typically occur in members of the families Pettalidae and Stylocellidae on the sides of the head, in association with dorsolateral projections called ozophores.
In Phalangida (the remaining harvestmen), eyes typically occur on a median cuticular projection called the ocularium.
The former, which evolved from compound eyes, were thought to be absent in harvestmen, until their recent discovery in embryos of the daddy longlegs Phalangium opilio and the emerald armored harvestman Iporangaia pustulosa.
[2] In addition, Phalangium opilio exhibits a second rudimentary pair of median eyes, homologs of which occur only in sea spiders and horseshoe crabs.
Several otherwise blind Cyphophthalmi have small photoreceptors at the base of the ozophores, but it is unclear whether these are derived from lateral or median ocelli.
However, Ischyropsalis strandi (Ischyropsalididae) nymphal stages have small but complete eyes, but these get partially lost through a degenerative process in adults.
[1] The basic structure of the reproductive tract is similar in both sexes, with a mesodermal gonoduct (sperm duct or oviduct) emerging from both sides of the U-shaped gonad (testis or ovary).
The penis is often complex, consisting of a long shaft and a shorter glans at the end, which is often equipped with various projections such as spines.
