Any of about 90 species of
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 1.6–5 in. (4–13 cm) long, excluding the 0.6–3-in. (1.5–8-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, {{link=guano">guano from such colonies was mined for fertilizer and for sodium nitrate (used to make gunpowder).