wings, flight organs of the bird, the bat, and the insect. Birds' wings are pectoral appendages that are basically the same in skeletal structure as the forelimbs of all higher vertebrates, including the human arm. Bird bones are specialized for strength and lightness, and the wing bones are further modified to act as a sturdy anchor for the wing feathers and for the powerful muscles and tendons necessary for flight. The main inner part of the bird's wing is like an airplane wing, concave below and convex above, and supplies lift. The secondary flight feathers also function in lifting; they are attached to a "forearm" bone, the ulna. The ulna locks with a parallel bone, the radius, in flight. The wingtip, or primary, feathers attach to the fused "hand" bones; their circular movement in flight provides the thrust to pull the bird forward. The primaries can be spread and maneuvered to control speed and direction. A mobile "thumb," bearing one or more feathers called alulae that lie along the front edge of the wing, can also be lifted to direct airstreams over the wing when its angle is too great (as in climbing) for the air to flow smoothly around it. There is much variation in the size, shape, and strength of wings and in the number and arrangement of their feathers. Soaring birds, such as the eagle and the pelican, have long, broad wings; in gliding and diving birds, like the gull and the albatross, wings are long and narrow; and in hoverers and darters, like the hummingbird and the swallow, wings are narrow and the primaries especially long to facilitate a rapid, erratic flight. The ostrich's vestigial wings are used for balance in running, and the wings of aquatic birds such as the penguin and the puffin are flipperlike for underwater swimming. The wings of bats are really membranes extending from the "arm,""hand," and "finger" bones to the ankles; the elongated finger bones form a frame to support the folds of skin. Insects' wings are not modified limbs but special lateral outgrowths of the cuticle of the thorax comprising a light membrane strengthened by thick-walled veins. The number, kind, and venation of the wings are bases for classification.