Harvestmen are an order of arachnids, with which they share many characteristics: their body is divided into two body regions (tagmata), the abdomen (opisthosoma) and the cephalothorax (prosoma); however, unlike in spiders, the juncture is often poorly defined. They have chelicerae, pedipalps and four pairs of legs. Most harvestmen have two eyes, although there are eyeless species.
Arachnids feature two kinds of eyes, lateral and median ocelli. The former, which evolved from compound eyes, are lacking in harvestmen. Many cave- or soil-dwelling species, including most Cyphophthalmi, lack eyes, or they are reduced. 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. The size and complexity of eyes in harvestmen roughly correlates with the amount of light available in its habitat. Evolutionary reduction of eyes appears to be caused by the early termination of eye development (paedomorphosis) in most species. Adults of those species thus have eyes that resemble embryonic or juvenile forms of their ancestors. However, Ischyropsalis strandi (Ischyropsalididae) nymphal stages have small but complete eyes, but these get partially lost through a degenerative process in adults.