Flies have two compound eyes with thousands of individual visual receptors called ommatidia. Each ommatidia works as a separate functioning eye with its own nerve fiber that connects to the optic nerve.
The vast number of ommatidia in a fly's eye allows it to receive visual information simultaneously from several different points. Although its eye is immobile, the spherical shape and level of protrusion from the head give flies an almost 360-degree field of vision. Flies do not have pupils, so they cannot control how much light passes through the lenses of their eyes. This lack of control prevents the eye from focusing on images.