BugInfo True Flies (Diptera)

Definition. Although many insects are termed "flies," only those having one pair of wings belong to the insect Order Diptera. Flies are also characterized by having a pair of balancing organs, called halteres, located just back of the base of the wings. Halteres are used for balancing in flight. There are a few flies, mostly parasites or inhabiting islands or alpine areas, that have no wings at all.

Immatures. The active immature stages of flies are called maggots, and most live in water or in some type of moist, rotting plant or animal tissue. There are exceptions, however, that feed only on living plants or animals. Maggots living in water usually have some type of breathing tube or gills with which they gain air.

General information. There are more than 110,000 described species of flies in the world. Flies are superb mimics, especially in the Families Bombyliidae (Bee flies), Syrphidae (Flower flies) , and Asilidae (Robber flies). Bee flies are usually fast flying insects with appearances of small bees, while Robber Flies are commonly larger, slower flying, and a few resemble Bumble Bees. A fly known as the Delhi Sands flower-loving fly was designated as an endangered species in 1993 by the U.S. Fish and Wildlife Service. This species has been found in sandy dunes and rolling terrain of two counties in southern California, and has been the subject of much controversy regarding the protection of some species of animals. Flies are found throughout the world, and exist in areas from the Arctic to the rain forests on the equator.

Benefits of flies to mankind. As in other kinds of insects, there are in the Order Diptera many flies that are beneficial to mankind. Flies that visit flowers are helpful in the pollination of flowering plants. The maggots of hover flies are welcomed by gardeners because they feed on aphids, reducing numbers of those pests. Many flies are parasitic, feeding on moth caterpillars, beetle grubs, and other pest species. Some species of gall midges, fruit flies and other families have been introduced to North America as weed biocontrol agents. Species of Drosophilidae have been used extensively in studies of genetics because of their short lifespans, giant salivary gland chromosomes, and ease of culturing.

Detriments of flies to mankind. Unfortunately, some flies, especially mosquitoes, transmit some of the most destructive diseases, notably malaria and yellow fever. The House Fly is known to be a carrier (vector) of many diseases, including anthrax, some forms of conjunctivitis, dysentery, typhoid fever and yaws. Some flies, such as the Mediterranean Fruit Fly, attack and spoil citrus and other fruits.

Kinds of flies. Some of the larger, common fly groups are placed in the following Families:

Crane Flies, Family Tipulidae. With over 10,000 species in the world, are large and long-legged, resembling and sometimes mistaken for mosquitoes.

Tachinid Flies, Family Tachinidae. A large Family of flies, with larvae parasitic on other insects. Are beneficial for that reason.

Hover Flies, Family Syrphidae. Usually with bright colors of yellow and black, and are often mistaken for small bees or wasps. Many are common, and most are capable of hovering in one spot.

Bee Flies, Family Bombyliidae. This Family includes many parasitic insects. one species parasitizes the Carpenter Bee. They mimic bees and wasps, and are excellent fliers.

Robber Flies (Assassin Flies), Family Asilidae. These flies are predators that hunt and feed on other insects. They are the third most diverse group of flies, with more than seven thousand species already identified and countless others yet to be discovered. 

Selected References:

Cole, F. R. 1969. The Flies of Western North America. University of California Press, Berkeley.

McAlpine, J. F., et al, coordinators. 1981. Manual of Nearctic Diptera, 3 volumes. Canadian Govt. Publishing Centre, Quebec.

Merritt, Richard W., and Cummins, Kenneth W, editors. 1996. Introduction to the Aquatic Insects of North America, 3rd edition, Kendall/Hunt Publishing Company, Dubuque, Iowa.

Oldroyd, H. 1964. The Natural History of Flies. London, Weidenfeld and Nicolson.

Prepared by the Department of Systematic Biology, Entomology Section,
National Museum of Natural History, in cooperation with Public Inquiry Services, 
Smithsonian Institution

Information Sheet Number 188, May 1999, 2015