bat, Any member of more than 1,100 species (order Chiroptera) of the only mammals to have evolved true flight. Their wings are evolutionary modification of the forelimbs, with greatly elongated fingers joined by a membrane that extends down the side of the body. Most bats use echolocation to orient themselves and find prey. Found worldwide, they are particularly abundant in the tropics. Wingspreads vary among species from 6 in. (15 cm) to 5 ft (1.5 m). Nearly all species roost during the day (in caves, crevices, burrows, building, or trees) and feed at night. Most are insectivores, consuming enough insects to affect the balance of insect populations. Others feed on fruit, pollen, nectar, or blood (vampire bats). Some may live more than 20 years. The guano of bats has long been used for agricultural fertilizer. See also free-tailed bat, fruit bat.