There are between 3,000 and 6,000 simple eyes in each of a fly’s two compound eyes. These eyes make small pictures that let the fly see movement from the left, right, front, and above.
Flies can’t see one clear image or focus on one thing like people can. Instead, each simple eye sees a different thing, and all of these views fit together like a mosaic.
Flies have three simple eyes in between their ocelli, which are made up of two eyes. The ocelli help the fly know which way is up. They work like a compass. They help the fly move toward the sun by keeping track of where the sun is.