Arthropods have compound eyes made of several visual units called ommatidia. Each ommatidium transmits visual information through the optic nerve to the brain, which interprets the image. Because they are round and numerous, these ommatidia, whose external surfaces point in different 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 simple eye besides their pair of compound eyes.