Opiliones is a an order of arachnids commonly known as harvestmen. However, their phylogenetic position within Arachnida is disputed: the closest relatives may be the mites (Araci). They can be easily distinguished from even the 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 the cephalothorax). Unlike spiders, Harvestmen’s chelicera are not hollowed fangs but grasping crab-like claws.

Look closely and one can clearly see the fused arrangement of abdomen and cephalothorax that distinguishes these arachnids from spiders. Opiliones have a single pair of eyes in the middle of the head, oriented sideways. Harvestmen also have a pair of prismatic defensive scent glands (ozopores) that secrete a peculiar smelling fluid when disturbed. They do not have book lungs, and breath 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.

Most hunting harvestmen ambush their prey, although active hunting is also found.  Because their eyes cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making then vulnerable to internal parasites.

Most species are nocturnal and are coloured in hues of brown, although there are a number of diurnal species, some of which have vivid patterns in yellow, green and black with varied reddish and blackish mottling.