An insect of the order Diptera (meaning ‘two wings’), in which adults have a single pair of membranous wings, the hindwings having been modified to form halteres, drumstick-shaped structures used for balance. Mouthparts are usually adapted for sucking. Metamorphosis is complete, larvae being legless and segmented. Flies occur worldwide. About 150,000 species have been described, but there may be more than 1,000,000.