Compound eyes are found in most adult insects, and in some myriapods and crustaceans. Most arthropod intercepts with compound eyes will be insects (which may in addition have one or more dorsal ocelli, i.e., single-lens eyes). Some collembolans and myriapods may have eye-like clusters of ocelli.
A few spiders are blind, but most will have 6–8 ocelli (simple single-lensed eyes) along the frontal margin of the prosoma.
A few spiders have their ocelli in discrete clusters, but this is more common in hexapods and myriapods.
Many mites are blind, but when present, eyes are found on the 'shoulders' region of the body, above the legs. The eyes are simple ocelli and typically grouped as two pairs of circular to oval lenses (often red in color). Usually they are sessile, but in some Prostigmata the posterior pair are raised or on short stalks.
Some opilioacarans have 2 sets of 3 ocelli, but most mites have one or two pairs of eyes. In many Prostigmata, the anterior and posterior members of the pair are different in form and the posterior member is sometimes called the ocular body.
Opiliones are either blind or have a pair of ocelli on a median turret-like projection of the prosoma. When such eyes are present, they are a useful character for distinguishing opilionids from mites. Unfortunately, the most mite-like opilionids are blind. Solfugids and some water mites have their ocelli on a median ocular plate (although usually 2 pairs of ocelli) and a few spiders have ocelli on raised processes.
Several groups of oribatid mites appear to have independently evolved a new type of light-sensitive organ called the lenticulus. This structure is median, unpaired, and often a simple oval of lightly colored cuticle, but may have a distinct convex lens. It is usually situated at the anterior edge of the notogaster. Some early derivative acariform mites also have a single median eye, often hidden under a nose-like process above the chelicerae called the naso.
A few prostigmatans (e.g., some velvet mites) have their eyes produced on stalks. Stalked and movable compound eyes are present in many crustaceans (e.g., crabs, lobsters) and some insects have their eyes produced on fixed lateral stalks. However, these are always compound eyes; mite eyes are always simple ocelli.
Eyes are sometimes absent in most groups of small arthropods, and always absent from Palpigradi, Schizomida, Pauropoda, Symphyla, Diplura, and Protura (although the eye-like pseudoculus is present). Copopoda and Ostracoda have a naupliar eye (under the carapace in the latter) that is visible in live specimens as a black dot, but it loses its color on preservation.
Misinterpretations allowed: The ocelli of Onychophora are small and may be difficult to see in the papillate cuticle. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
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