Respiration
Harvestmen breathe through tracheae, somewhat like insects. Air enters through a pair of spiracles located behind the last pair of legs, on the opisthosoma. Unlike as in insects, the tracheae end in hemolymph near the organs, instead of leading into them. The oxygen-binding respiratory pigment of harvestmen is hemocyanin, which is typically found in arachnids with book lungs, which harvestmen do not possess. The pattern of tracheal branching seems to be rather consistent in all harvestmen. 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. Lateral branches lead to the other appendages. Except in Cyphophthalmi, the branches from each side meet and form a single transverse trachea. Gonad, genitalia and digestive tract also receive branches.
Phalangioidea have additional spiracles on the tibiae of the pedipalps and legs, thus helping in gas exchange in the long legs. They also provide a severed leg with oxygen, allowing it to continue twitching for some time: A leg of Opilio twitched for about 23 minutes, but stopped after about 40 seconds when the spiracles were sealed.
Read more about this topic: Harvestman Anatomy