When researchers peered at the knees of the Papuan weevil recently, they had a surprise coming to them. Their massive electron micrographs of the joint showed tiny, perfectly formed screws and nuts, with a thread far more delicate than those made by humans. The weevil, it appeared, used them to lock its knees for stability when it landed.
This was the first time scientists had seen such structures in nature. But simple machines--defined by Renaissance natural philosophers as the lever, the wheel and axle, the wedge, the screw, the inclined plane, and the pulley--are hidden beneath the skins and exoskeletons of many a creature. Some of them, like the wheel, are so curiously rare as to have become examples of natural limits on biology. Others, like the lever, are so common that myriad examples exist in the human body alone. Here's a tour of the simple machines that make (the animals of) the world move.