Ocelli (sing. ocellus) are the 'simple' eyes of adult and nymphal insects, typically three in a triangle formation on the vertex, with one median and two lateral ocelli.
Many adult insects, as well as some nymphs, have ocelli in addition to their compound eyes. The three small ocelli lie in a triangle on top of the insect's head. There is transparent cuticle covering the ocelli, which can be modified and curved into a lens.
Ocelli are, in essence, light receptors. They are highly sensitive to changes in light intensity, but do not appear to function as high-resolution receptors. Ocelli are thought to function as 'horizon detectors' for control of roll and pitch movements while flying, and to register the cyclical, seasonal changes in light intensity that accompany diurnal behaviour.
sourced, in part, by The Insects: An outline of entomology, second ed. Gullan, P.J. and P.S. Cranston. Blackwell Science, Great Britain, 2000.