The eyes occupy a fairly large portion of the surface of insect heads, and they facilitate a rather wide field of vision. Compounds eyes are not present in all insects. They are reduced or absent in parasitic forms, many soil insects, and in some species that live in very dark places, such as caves. Ommatidia vary in size and number among insect groups. The sizes of ommatidia vary from about 5 to 40 microns in diameter.
Sizes vary among species and even within a single compound eye.
In some dragonflies, for example, the dorsal units are considerably larger than the ventral ones. A figure shows the structure of a single ommatidium. The ommatidium rests on a basement membrane. The height of the eye comes from the long retinula cells and secondary pigment cells. The crystalline cone lies within the corneagen cells.