Flies have a remarkable sense of sight. Their eyes are among the largest and most complex in the insect world. While their vision isn't always perfect, they can navigate their environment with ease. In fact, they may have better sight than some humans. They can detect light levels and distinguish between different colors, but they have trouble with depth perception.
One of the biggest reasons for the flies impressive level of vision is their ability to combine multiple images. This allows them to see their surroundings in a variety of ways. They can also judge their distances accurately.
A fly's ommatidium, or eye, contains thousands of tiny pictures. These pictures create a complete image when viewed from a distance. The ommatidium is a crystalline cone-shaped structure with a lens on the outside.
It is these tiny lenses that make up the fly's compound eyes. Although they are classified as individual eyes, they are actually a superposition of two large eyes.
There are three small eyes called ocelli in between the two larger compound eyes. Each ommatidium is also functional, detecting light and motion.
While the fly's eyes aren't the most sophisticated, the complex array they form doesn't need any fancy assembly instructions. Like many of nature's most beautiful structures, it is a combination of a few basic sensory inputs.
While most flies have two or three simple eyes, some insects such as the housefly have five or six. The number of ommatidiums, or individual eye cells, is also a good indicator of the amount of brainpower a particular insect possesses.