Without the feathers the wing would be useless as an instrument of flight. The shorter plumes that shield the bases of the long quill feathers are called the coverts, which are found on both the upper and under surfaces of the wing. They are divided into several sets, according to the position they occupy, and are called the "primary coverts" (because they overlie the bases of the primaries), the "greater coverts," the "middle coverts," and the "lesser coverts." Forming a vast expansion of the bony and fleshy framework are the quills, or flight-feathers, called collectively the "remiges." These plumes mainly determine the contour of the wing, and constitute a thin, elastic surface for striking the air--one that is sufficiently resilient to give the proper rebound and yet firm enough to support the bird's weight. The longest quills are those that grow on the hand or outer extremity of the wing and are known as the primaries. What are called the secondaries are attached to the ulna of the forearm, while the tertiaries occupy the humerus and are next to the body. All these feathers are so placed relatively that the stiff outer vane of each quill overlaps the more flexible inner vane of its successor, like the leaves of certain kinds of fans, thus presenting an unbroken surface to the air. As to the structure of these plumes, they combine firmness, lightness, and mobility, the barbs and barbules knitting the more flexible parts together, so that they do not separate, but only expand, when the wing is unfolded. [Illustration: Barn Swallow] While the primary purpose of wings is flight, there is quite a number of notable exceptions. A concrete example is the ostrich, whose wings are too feeble to lift it from the ground, but evidently aid the great fowl in running, as it holds them outspread while it skims over the plain, perhaps using them mainly as outriggers or balancing poles in its swift passage on its stilt-like legs. The penguins convert their wings into fins while swimming through the water, the feathers closely resembling scales.