Harvestmen are members of the class Arachnida, together with spiders, scorpions, pseudoscorpions, ticks and mites. Like all arachnids, harvestmen have four pairs of walking legs, a pair of chelicerae, and a pair of small, leg-like appendages called pedipalps.
The body of a harvestman is globular and rounded, and is formed by the fusion of two parts, the cephalothorax and the abdomen. The fused body and abdominal segmentation distinguish harvestmen from closely related groups - spiders (Araneae) and mites (Acari). The body of most species is only 3-7 mm long, but the animals often appear larger due to their long legs (in one group of harvestmen the legs are exceptionally long - up to 20 times the length of the body). The second pair of legs is often the longest, having a tactile sensory function.
The colour of harvestmen is usually cryptic - dull shades of brown and black, although some have brighter coloration with patches of orange, white, and green.