The term fly is applied to such non-diptera as the butterfly, dragonfly, mayfly and stone fly. Unlike these and other insects, true flies have a single pair of membranous fore-wings, with knob-shaped vestigial wings instead of hind wings. The vestigial wings, called halteres, are used as balancing organs.
The mouthparts are specialized for lapping or sucking in some flies and for piecing and sucking in others. Almost all species have antennae and large compound eyes. A network of thick veins strengthens the wings; the pattern of these venations is used for identification and classification. One theory is that the veins evolve from tracheae, a system of tubes common in insects that supplies oxygen to the tissues.
All files develop through metamorphosis in four stages, egg, larva, pupa, and adult. The larvae, called maggots, lack eyes, legs, wings, antennae, and distinct mouthparts and body regions. The pupa is defenseless but generally in soil or heaps of waste.
The life cycle of a housefly is typical of all true flies and consists of four stages, egg, larva, pupa, and adult. An adult female lays 100 to 160 eggs at a time, usually in decaying organic matter. Each egg, 1 mm (0.04 in) long, hatches into a larva in 12 to 24 hours. After several days the larva changes into a pupa, which is encased in a tough shell. In 3 to 5 days a full-sized adult fly emerges. During warm weather, it will live about 1 month.