Stacked macrophotograph of the head of a black slip wasp (Pimpla rufipes) with compound eyes and ocelli. The ocelli (singular: ocellus) are secondary visual structures on the head of bees and wasps, a trio of small shiny bumps in the space between the compound eyes. The ocelli are generally considered to be far more sensitive to light than the compound eyes. They are thought to assist in maintaining flight stability by measuring changes in the perceived brightness of the external world as an insect rolls or pitches around its body axis during flight.