In cockroach the eyes are large sessile, paired beanshaped and present on either side of head. The are compound in nature. Each compound eye consists of a large number of visual elements called ommatidia.
Each ommatidium is composed of a diopteric region and reticular (receptor) region. It is capable of producing a separate image of a small part of object seen.
Thus, the image of the object viewed consists of several pieces and is known as mosaic image. Fine nerve fibres arise from the inner end of each ommatidium all of which combine to form one optic nerve connected to the brain.