The special thing about fly eye is that everyone may have such an experience. When the fly stops on the table and you catch it with your hand, you will find that your hand has not fallen, and it has flown away from this "land of right and wrong". Why is this? Do flies have eyes behind them? Through the study of fly eyes, scientists found that fly eyes are very special, with five flies. Among them, three smaller eyes are monocular and feel the intensity of brightness, and the other two are compound eyes, each of which is composed of many hexagonal visual unit eyes. These small eyes are self-contained, with independent optical systems and nerves leading to the brain. The optic nerves of these small eyes can cooperate with each other, which can work in coordination and independently. So the fly's eyes not only have speed and high resolution, but also can feel videos from different directions, which is why it is easy to be found when people hit it from behind with a fly swatter. This kind of compound eye has high time resolution. It can divide the moving object into a continuous single lens, and each small eye takes turns to "be on duty". Human eyes are spherical, but flies' eyes are hemispherical. Flies' eyes can't move like human eyes. When a fly looks at something, it turns its eyes to the object by its neck and body. The fly's eyes have no eye sockets, eyelids and eyeballs, and the cornea on the outer layer of the eyes is directly connected with the surface of the head. From the outside, the surface of fly eye (cornea) is smooth and flat. If you put it under a microscope, people will find that the fly eye is composed of many small hexagonal structures. Every small hexagon is a small eye. Scientists call it small eyes. In a fly's eye, there are more than 3000 small eyes, and a pair of fly's eyes have more than 6000 small eyes. This kind of eye, which consists of many small eyes, is called compound eye. Every small eye in a fly's eye has its own system, including an imaging system composed of cornea and lens dimension, a retina composed of photosensitive visual cells, and an optic nerve leading to the brain. So every little eye sees things separately. Scientists have done experiments: peeling off the cornea of a fly as a photographic lens and taking pictures under a microscope can take hundreds of identical images at a time. Scientists are interested in fly eyes because they have many amazing functions. If a person's head does not move, the range that the eyes can see will not exceed 180 degrees, and things behind the body cannot be seen. However, the fly's eyes can see 350 degrees, except for a narrow strip on the back of the head, which is almost a circle. The human eye can only see visible light, but the fly eye can see ultraviolet light that is invisible to the human eye. To see fast-moving objects, the human eye is not even as good as the eyes of flies. Generally speaking, the human eye needs 0.05 seconds to see the outline of an object clearly, while the fly eye only needs 0.0 1 second. The fly's eyes are also a natural speedometer, which can measure its own flight speed at any time, so it can track the target in fast flight. According to this principle, at present, people have developed an electronic instrument for measuring the relative ground speed of aircraft, which is called "aircraft ground speed indicator" and has been tried in aircraft. The structure of this instrument is simply: install two photoelectric receivers at a certain angle to each other on the fuselage (or install one photoelectric receiver on the nose and one photoelectric receiver on the tail) to receive the optical signals at the same point on the ground in turn. According to the time difference between the signals received by the two receivers, the flying height at that time is measured, and then the flying speed of the aircraft relative to the ground can be displayed on the instrument after calculation by the electronic computer.