any of about six species of vesper bats (vesper bat) (family Vespertilionidae) found in Europe and Asia. Noctules are golden to yellowish or dark brown, with a paler underside. They are 5–10 cm (2–4 inches) long without the 3.5–6.5-cm (1.4–2.6-inch) tail. They are swift, erratic fliers and commonly leave their roosts (generally in caves or buildings) at or before sunset. They eat insects and apparently are fond of beetles. The best-known and most widely distributed species is the Eurasian N. noctula, a reddish brown migratory inhabitant of wooded regions.