What's going on here? It turns out that the fly's big eye is not a single eye, but is made up of many, many little eyes. These small eyes one by one, densely arranged together, like a sunflower disk. The small eyes are hexagonal in shape, and when combined form a honeycomb hemisphere.
Each small eye has a tiny conical lens with eight long visual cells at the tip that connect nerves to the brain. Each small eye can see independently. However, the little eyes are so small that they can only see a small part of an object. It takes all the parts that the little eyes see, put together like a puzzle, to make a complete image.
The fly's eyes are called compound eyes. Not only do flies have compound eyes, but all insects, as well as arthropods such as shrimp and crabs, have them.
In addition to the two large compound eyes, insects also have a single eye. The single eye is above the compound eye, it can only feel the intensity of the light, not a real eye, the compound eye is the real eyes of insects.
Different insects, the number of small eyes in the compound eye varies: flies have more than 4,000 small eyes in the compound eye; beetles have 9,000 small eyes in the compound eye; butterflies have 17,000 small eyes in the compound eye, while ants and mosquitoes have only 50 small eyes in the compound eye.
Human eyes are spherical, but fly eyes are hemispherical. The fly's eye cannot rotate like the human eye, and the fly has to turn its neck and body to see in order to turn its eye toward an object. A fly's eyes have no sockets, no eyelids or eyeballs, and the cornea, the outer layer of the eye, is attached directly to the surface of the head