Flies are distinguished from all other insects by having one pair of wings instead of two. Instead of a pair of hindwings they have a pair of halteres which are knob-like processes evidently used for balance in flight.
The housefly combines a capacity for producing great numbers of young with one of the shortest life cycles known. A few flies in early summer can produce millions of descendants by fall, and as many as ten generations in that single season. Each female lays about 500 slender whitish eggs in batches of a hundred or more at a time. A half-day later, in warm weather, each egg hatches into a tiny white larva called a maggot. After five days or more, the larva is full-grown and then transforms into a quarter-inch, brown, seed-like pupa or "resting stage". From these pupae, 3 or 4 days later, the adults emerge and are soon ready to start families of their own.
The average life span of an adult housefly is from 20 to 30 days. Occasionally, one may survive freezing weather if hidden away in a warm building but most of the parents of each year's crop spend the winter as maggots or pupae buried in stable litter or under piles of rotting grass. A housefly's two transparent wings buzz at about 160 beats per second. Marked flies have been recaptured after traveling as much as 13 miles but, usually, they travel no farther than the nearest place to feed and lay eggs -- perhaps in the same city block. Each of a fly's six feet ends in a pair of claws and a pad covered with hairs that exude a sticky fluid. That is why they are able to walk up a polished window pane or stroll around on a ceiling.
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