Wednesday wordphile: ocellus (plural: ocelli)
An ocellus is a simple eye having a single lens. This type of eye does not form an image but acts as a photo receptor, detecting changes in light intensity and direction.
Many arthropods have ocelli. They are especially common in arthropods that fly—such as bees. Bees have three ocelli on the crown of their head–centered between the large compound eyes and looking like shiny black dots.
Experiments have shown that ocelli help honey bees to navigate at flight speeds. Bees with full sight (all five eyes) were found to be much more cautious and tended to decelerate more quickly than bees whose ocelli were covered.
 Kastberger, G. 1990. The ocelli control flight course in honeybees. Physiological Entomology 15(3):337-346.