The 986 species of bats belonging to the order Chiroptera are the only truly flying mammals. There are several gliding mammals, however, that are referred to as "flying." Examples of these include the flying squirrel and the flying lemur.
The "wings" of bats are actually double membranes of skin. The skin extends from the bat's back and belly to its hind legs and tail. The membranes are stretched between, and supported by, the elongated fingers of the forelimbs (or arms).
Bats are nocturnal (active at night) and range in length from 1.5 inches (25 millimeters) to 1.3 feet (40.6 centimeters). They inhabit most of the world's temperate and tropical regions, living in caves or crevices.
The majority of bat species eat insects and fruit. Some tropical bat species eat the pollen and nectar of flowers, as well as the insects found inside flowers. Larger bat species prey on small mammals, birds, other bats, lizards, and frogs. Some bats of Central America and South America eat fish. The three genera (plural of genus), or categories, of true vampire bats drink the blood of animals, typically domestic livestock, by making an incision in the victim's skin.
Most bats do not have sight. Rather, they find their way around using "echolocation," a system based on the reflection of sound waves. Bats emit sounds through the nose or mouth while flying. These sounds, usually above the human hearing range, are reflected back as echoes when striking an object. This method enables bats, when flying in darkness, to avoid solid objects and to locate the position of flying insects.
Bats have the most acute sense of hearing of any land animal, picking up sound frequencies as high as 120 to 210 kilohertz. The highest frequency humans can hear is 20 kilohertz.
Sources: Burt, William Henry. A Field Guide to the Mammals, 3rd ed., p. 21; Nowak, Ronald M. Walker's Mammals of the World, 5th ed., vol. 1, pp. 190-92; Travers, Bridget, ed. The Gale of Science, vol.1, pp. 420-25.