What are compound eyes?

Arthropods have compound eyes made of many visual units called ommatidia. Every ommatidium transmits visual information by the optic nerve to the brain, which interprets the image. Because they are round and many, these ommatidia, whose external surfaces point in dissimilar directions creating independent images, cause arthropod eyes have a large visual field, larger than the visual field of vertebrates. Some insects have one or more easy eye besides their pair of compound eyes.