Although a lot of flying insects are referred to as "flies" -- butterflies, dragonflies, mayflies, and so on -- the true flies belong to the Diptera. The name means "two wings," and true flies bear only one pair of functional wings. The reduced remnants of the second pair of wings are known as halteres, and seem to function as stabilizers or as airspeed detectors. Dipterans also have large eyes, and may have either long antennae (Nematocera) or short antennae (Brachycera).
The Diptera include files, mosquitos, gnats, midges, and no-see-ums. There are about 120,000 known species of true flies alive today. Dipterans typically have sucking mouthparts, and may feed on plant juices or on decaying organic matter. A number of dipterans feed on blood, and some may transmit vertebrate diseases; certain mosquitos, for instance, transmit human malaria. Other dipterans pollinate certain flowering plants -- although not the kind you'd necessarily want in your corsage! Fly-pollinated plants, like the skunk cabbage of North America or the stinking corpse lily of Madagascar, typically have quite pungent odors.
The oldest fossil flies are Late Triassic in age, from Kyrgyzistan in the former Soviet Union. The picture above depicts a much younger fossil fly from the Oligocene-age Florissant beds of central Colorado, about 35 million years old.
The Smithsonian Institution has recently created a new on-line exhibit, Diptera!
General informtaion about flies is available from the University of Delaware Insect Database. A more detailed phylogeny of the flies is available as part of the Tree of Life project at the University of Arizona. Information on the Chironomidae (midges) is available from the Freshwater Ecology Homepage at the University of Antwerp.
Catalog of the Fossil Flies of the World, by Neal Evenhuis, for more information on fossil flies.
There is quite a lot of information on the Internet about one of the most famous dipterans, Drosophila; the WWW Virtual Library Drosophila page is a good place to start.
Carpenter, F.M. 1992. Treatise on Invertebrate Paleontology. Part R: Arthropoda 4. Volume 4: Superclass Hexapoda. Geological Society of America and University of Kansas Press, Lawrence, Kansas.