All flies have two compound eyes, which are usually large and conspicuous. Each eye is composed of hundreds or even thousands of facets, which are tiny units that receive separate visual images. In the housefly there are about 4,000 facets in each eye. Compound eyes enable the fly to detect rapid movement and to see in many directions at once.
In addition to the two compound eyes, most flies have small simple eyes, called ocelli. The ocelli are probably used to detect differences in the brightness of light.
