Flies are hard to catch, in part because they have compound eyes, which are made up of thousands of individual visual receptors. Their eyes are not able to focus, so they see everything as a blur. This makes it difficult to see flies in a straight line.
Flies do not have eyes that can distinguish between colors, such as red and green. However, they can see polarized light, which has waves that travel in one plane. While this is not very useful to humans, it makes it possible for flies to move around. Their ability to fly is directly related to how fast they process light, as they must react quickly to avoid obstacles.
Most flies do not have teeth, so they cannot chew solid food. Instead, they regurgitate their digestive juices onto the surface of their food, breaking it down into small pieces. Then, they swallow the liquid with their mouthparts, called proboscis. They also use their feet to taste food.
In their natural habitat, flies rely on light for orientation. The amount of light varies from day to day. When light levels increase, their activity increases. As a result, they may hit a window, which might cause disorientation.