Among mammals, only the rodents exceed bats in number of species. Unfortunately, these terms are somewhat misleading, as some of the Megachiroptera are smaller than some of the Microchiroptera. There are some modifications in the sequence of vespertilionoid families given here, the Hipposideridae are here regarded as a family distinct from the Rhinolophidae, and the Mystacinidae have been transferred to the Phyllostomoidea. Bats vary in head and body length from about 25 to 406 mm. The pelage is usually long and silky, with a dense underfur.
Some species have color phases.
Scent glands, which produce a strong musky odor and are variable in position, are usually present. The membranes that extend from the sides of the body, legs, and tail are actually extensions of the skin of the back and belly. They are elastic and thin, and consist of two layers of skin with no flesh between them and only a small amount of connective tissue, in which the blood vessels and nerves are located. The wing membrane is supported by the elongated fingers of the forelimbs. The third finger is usually equal in length to the head and body plus the legs.