Any of about 90 species of bats (family Molossidae), found worldwide in warm regions, that are named for the way part of the tail extends beyond the membrane attached between the hind legs.
Also known as mastiff or bulldog bats because of their facial resemblance to those dogs, free-tailed bats are swift fliers with a stout body and long, slender wings. They are about 1.65 in. (413 cm) long, excluding the 0.63-in. (1.58-cm) tail, and typically have small eyes, a heavy snout, large ears, and dark fur. They eat insects and roost in tree hollows, caves, and buildings. Most species live in groups; some, including the Mexican free-tailed bat, form colonies of several million. In the past, guano from such colonies was mined for fertilizer and for sodium nitrate (used to make gunpowder).