A fly cannot see in all directions at once, because one part of its eyes must lie against its head, and in that direction, at least, the fly cannot see. But it is true that the eyes of flies, and of many other insects, can see in more directions at once than ours can. This is especially the case where the eyes are very rounded and bulging. (See the accompanying photo). This does not mean clear vision at the same time in all directions; but it does mean that while looking in one direction, the insect can get a hint of movement much farther round the corner than we can.
The eye of the fly is made somewhat like a precious stone that has been cut into many little faces, or facets. The number of these tiny facets on the eyes of insects is extraordinary. A male ant, for instance, may have twelve hundred facets on each eye, and number on the eye of the dragon fly has been reckoned as high as seventeen thousand. Eyes made in faces or facets, are called compound eyes.