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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.
organs of flight in animals. Wings are characteristic of certain arthropods (most insects), all birds, and some mammals (bats). Among fossil animals, some reptiles—the Pterosauria— had wings. The structure and origin of the wings is different in different groups of animals. Wings can thus serve as an example of analogous organs.
The wings of insects are flat appendages of the thorax. They develop from the protruding lateral folds of the cuticles of the mesothorax and metathorax. When the wing is formed, hemo-lymph (blood) enters the slit between the upper and lower layers of the wing membrane from the body wall. Veins pass through the formed wing; these are tubular thickenings that form the framework of the wing and through which the tracheae and nerves, in addition to the hemolymph, penetrate into the wing. There are usually two pairs of wings. In the Diptera only the front wings are developed (the hind wings are converted into halteres). In Strepsiptera only the hind wings are developed. There are two types of wings: net-veined wings (Odonata, Ephemeroptera, and Neuroptera), which are threaded with many longitudinal and transverse veins, and membranous wings (Hymenoptera and many Homoptera), which have few veins, especially transverse ones. In the Coleoptera the front wings, or protective elytra, are compact and intensely sclerotized and at rest cover the folded hind wings. The structure of the wings is an important feature in insect taxonomy. Wings are set in motion by powerful bundles of striated muscles. Some insects flap their wings up to 1,000 beats per second.
The wings of birds are modified anterior extremities. The wing is formed by the bones of the skeleton—the upper arm, the forearm, and an extremely modified hand, or manus. At rest, the wings are folded and pressed to the body. The flight feathers are attached to the forearm and manus. The bases of the flight feathers are covered by the covert feathers, which lie on the skin of the wing. The feathers of the wing overlap one another, forming a light, firm supporting surface with a system of apertures in the apical part. The wings of birds have aerodynamically efficient vaulted profiles. The pectoral muscles, which are attached to the keel, are the most-developed; they are responsible for the wing’s motion. The shape of the wing depends on the bird’s flight characteristics.
The wings of bats are cutaneous membranes that are stretched between four extremely elongated digits (two to five) of the forelimbs, the trunk, and the hind limbs; in some species the membrane also includes the region of the tail.
M. S. GILIAROV and N. V. KOKSHAISKII