The wings of bats are their most distinctive -- and perhaps most remarkable -- feature. They give the order Chiroptera its name (literally, "hand-wing"), and functional wings and true flight are characteristics of all bats.
The origin of bat wings is most clearly revealed by their skeleton. Every element of that skeleton is clearly homologous with structures in the forelimbs of other mammals, and there is no question that bat wings evolved as a result of modifications to the forelimbs of their ancestors.
The element of the wing skeleton closest to the body is the humerus. It is long and thin compared to the humerus of other mammals, but its articular surfaces and areas for attachment of muscles are fundamentally like those of most mammals.
Attached to the humerus are the radius and ulna. The radius is also long and thin, but it is a strong bone that supports the wing. The ulna is much reduced in size; the olecranon process (at the articulation with the humerus) is the most substantial part of the ulna; the rest is considerably reduced and fused with the radius.
The wrist region is very similar to that of other mammals, although less flexible. It is specialized to support the particular motions associated with flying.
All bats have a thumb, which sits along the leading edge of the wing. It usually has a substantial claw, which is used for climbing, food handling, and fighting. Bat thumbs vary considerably in size; generally species whose feeding or roosting habits involve much crawling have longer and stronger thumbs.
The rest of the digits (2-5) support the wing. The skeleton of each includes a basal metacarpal, which is the largest element of the digit, and usually 1-3 phalanges. The second digit bears a claw in pteropodids, but not in any microchiropteran.
The wing membrane is an extension of the skin of the body. It is made up of external epidermis and an internal layer of dermis, which contains blood vessels (easily seen in a live bat when the wing is stretched in front of a light) and muscles. These muscles control the curvature of the wing in flight. The membrane is both tough and flexible. If torn, it heals remarkably fast, which is perhaps not surprising considering the importance of flight to bats.
Bat wings usually run from the shoulder region to the ankle, or in some cases, to the digits themselves. The wing membrane joins the body along the sides, except in a few cases in which it arises near the middle of the back. Bat biologists use different names to refer to different parts of the membrane. The propatagium runs from the shoulder to the wrist and is the leading edge of the wing. The plagiopatagium includes the area from the body to the 5th digit. The membrane spanning digits 2-5 is called the dactylopatagium.
The hind legs of many bats are partially or completely joined by a membrane, the uropatagium, which also may enclose the bony tail. It is supported by the legs, the tail skeleton, and by the calcar, a special cartilaginous extension of the ankle. The calcar sometimes bears a distinctive projection called a keel. In some species, both the uropatagium and calcar are absent.
Wing and tail membranes appear naked in most bats, but on close examination they can be seen to be covered with minute hairs, and in some species, with distinctive tufts and fringes of hairs. The function of these hairs may be to modify the aerodynamic properties of the surface of the wings, but this is speculative. A few species have a thick layer of fur on their tail membranes and some parts of their wings; these include tree roosting species such as members of the vespertilionid genus Lasiurus. This fur probably serves to insulate and camouflage the bat.
Bats also differ in the structure of their tails, and tail structure provides important clues for classification. Some bats, such as vespertilionids, have large uropatagiums with tails that run to the end of the membrane or just beyond. In others, such as molossids and rhinopomatids, the tail extends considerably beyond the edge of the membrane. These species seem to use the tail for "feeling" their way as they back into crevices. In yet other groups, the tail is shorter than the membrane, and in some (e.g., emballonurids), it emerges from the membrane well before the end and rises above it. A few bats appear to lack tails altogether.
Phil Myers (author).
Hill, J. E. and J. D. Smith, 1992. Bats: A Natural History. University of Texas Press, Austin.