My understanding of how flies see is that they see many small images instead of one large one. This makes them very good at seeing motion. Imagine that you saw the world by pointing a video camera at things, then viewing them on a TV screen. An insect sees things more like looking at a wall full of small TV screens, all showing the about same scene from slightly different angles. When something moves in the corner, you see it in the corner of your eye/TV screen. The fly sees it all over its eye in the corner of each screen. It can still see that it's in the upper right corner, for example, so it can move off to the left.

Of course it's difficult to know exactly what a fly sees because that has a lot to do with how the brain processes the information. You can test this by looking at optical illusions that fool your brain into seeing something that is not there. There are some fun ones at this site: illusions

Why do you think other animals see things differently from the way you do? Can you think how an animal's habitat, place in the food web, size, or evolutionary history would influence how it sees?

Thanks for asking,

Flies have many small eyes each with a narrow viewing angle.Because of this when light (say reflected off a predator) enters one of these eyes they know which angle it came in from. When the signal from that eye is sent to the brain the brain records which eye sent it the information and therefore the angle of the predator. The fly now knows where the predator is and will fly off in the opposite direction.

Just as with humans or any other complicated animal, the information collected by the eye is then transferred to an image-processing lobe in the animal's brain. This processing lobe puts the data together and produces an image that the creature can react to.

Flies have compound eyes, which are different from human eyes. Compound eyes are made up of lots and lots of light sensors that face at different angles. Each of these sensors, called ommatidia, detect the amount of light coming at it and forms only part of the image. When all of these images are put together they take the form of whatever the fly is seeing. Thus, flies can see which direction that danger is coming from. In a way compound eyes work like a TV screen. A TV screen is made up of many pixels; each pixel has its own color and light intensity. When you put all the pixels together you get a picture.

Although it might seem that a fly does not have vivid vision like humans, the fly is able to extract all the information it needs from those small images you described in your question. You also have to keep in mind that most of visual processing that leads to motion perception and action is done in the brain, so even though the images the fly gets from its eyes are small, most of its brain is dedicated to visual processing.

The main process by which the fly knows about danger is called looming. When an object moves closer to the fly, it is perceived as getting bigger. The change in size tells the fly that danger is approaching.