How Does a Fly Walk Upside Down?
There is a little sucker on the end of each of the fly’s feet which makes his foot stick to the ceiling or any other place he walks, and which he can control at will. It is made very much like the sucker you have seen with which a boy can pick up a flat stone—a circular piece of rubber or leather with a string in the middle and more or less bell shaped underneath. A boy can pick up a flat stone with this kind of a sucker by pressing the rubber or leather part down flat on the stone and then pulling gently on it by the string. When he does this he simply expels the air which is between the leather part of the sucker and the stone, which creates a vacuum and the pressure of the air on the outside part of the leather enables him to pick it up. The fly has little suckers like these on each of his feet, and they act automatically when he puts his foot down. Of course the sticking power of each foot is adjusted to the weight of the fly, just as the sticking or lifting power of the boy’s sucker is regulated by the weight of the stone or other object he tries to pick up. If the weight of the object is sufficient to overcome the sticking power which the vacuum creates, the stone cannot be lifted.