The true flies are a large and diverse group of commonly observed insects in the order Diptera, comprising more than 100,000 species. About 107 families of flies occur in North America.
Flies have distinctive, knob like structures known as halteres on the back of their thorax. Halteres are highly modified from the hind wings of true flies, while the fore wings are membranous and used for flying. The two-winged character of the true flues is reflected in the Latin roots of the scientific name of their order, the Diptera, which means ‘two wings.’
Some species of true flies are of great economic importance as pests of agricultural plants. Other species of flies are great nuisances because they bite humans and domestic animals in order to obtain blood meals, as is the case of mosquitoes, black flies, horse flies, and others. Some of these parasitic, blood sucking species are also vectors of deadly diseases of humans, as are some of the blow flies and house flies that feed by scavenging dead organic matter. However, many other species of flies provide very useful ecological services, by helping to safely dispose of decaying carcasses and other organic debris, and by serving as predators or parasites of other, injurious insects.
In itspopularusage, the word ‘fly’isoften used in reference to insects that are not in the order Diptera, and are therefore not ‘true flies.’ The ‘fly’ part of the name of a dipteran should be written separately, as in: house fly, horse fly, or black fly. However, when used to refer to non-dipteran insects, the ‘fly’ portion of a species name should be appended to form a single word, as in: sawfly (species in the family Tenthredinidae, order Hymenoptera), dragonfly and damselfly (order Odonata), mayfly (order Ephemeroptera), stonefly (order Plecoptera), caddisfly (order Trichoptera), and butterfly (order Lepidoptera).
The true flies have a complete metamorphosis, characterized by four stages in their life history: egg, larva or maggot, pupa, and adult. Fly maggots are soft-bodied, legless, and worm like. Most flies are terrestrial animals, but many species spend their larval stages in aquatic habitats, with the adults emerging to the terrestrial environment.
The hind wings of flies are modified into small, distinctive structures known as halteres, which resemble a tiny, stalked knob. Halteres are thought to be used as an aid in achieving a sense of balance and direction. The front wings of flies are membranous and functional in more usual ways, and are used for flying. The smaller flies have very rapid wingbeats, typically 200-400 strokes per second, and as great as 1,000 per second in tiny midges in the genus Forcipmyia.
Other characteristic features of true flies include the division of their tarsus (that is, the leg segment immediately below the tibia) into five segments. Most flies have mouth parts of a haustellate form, that is, adapted for sucking, rather than for chewing. Flies are typically small and soft bodied, and some are minute in size.
There are many feeding strategies among the flies. Most species feed on soft foods and organic debris, while others eat nectar, some scavenge dead bodies of animals, or are predators of smaller arthropods, or are blood-sucking parasites. These various species have mouth parts and behavioral adaptations to their specific modes of feeding and living. Mosquitoes, for example, have piercing and sucking feeding structures, while flies that feed on soft organic materials have sponging or lapping mouth parts.
Some species of flies that feed on plants inject a growth-regulating chemical into the stem, which causes an abnormal tissue, called a gall, to develop. The gall provides habitat for the feeding and development of the larvae of the fly. Gall-inducing species occur in the families Agromyzidae, Cecidomyiidae, and Tephritidae, as well as in other orders of insects.
There is a great diversity of species of flies, with more than 100,000 species being identified so far, and more than 100 families. The families are mostly distinguished using characters related to the morphology of the antennae, legs, wing venation, body bristles, and other anatomical features. Habitat and other ecological information may also be useful. Aspects of body chemistry may also be used in the identification of closely related species, particularly the chemistry of enzymes and nucleic acids. The diversity of flies is too enormous to discuss in much detail in this entry—only a few prominent examples will be described in the following sections.
The muscid flies (family Muscidae) include more than 700 species in North America, including some important pests. The house fly (Musca domestica) is one of the most familiar flies to most people, because it breeds readily in garbage and other organic debris, and can be very abundant in dirty places around homes, villages, and cities. The house fly does not bite, but it can be a vector of some diseases of humans, spreading the pathogens by contact, for example, by walking on food that is later eaten by people. The face fly (Musca autumnalis) tends to cluster around the face of cows, where it feeds on mucous secretions around the nostrils and eyes, causing great irritation to the livestock. The stable fly (Stomoxys calcitrans) and horn fly (Haematobia irritans) are biting flies that greatly irritate livestock. Species of tsetse flies (Glossina spp.) occur in Africa, and are the vectors of sleeping sickness and related diseases of people, livestock, and wild large mammals.
Blow flies (family Calliphoridae) are scavengers, whose larvae feed on dead animals, excrement, and similar, rotting debris. Although a carcass teeming with writhing maggots is a rather disgusting spectacle, it must be remembered that blow flies provide a very useful ecological service by helping to safely dispose of unsanitary animal carcasses. A few species of blowflies lay their eggs in wounds on living animals, and the larvae may then attack living tissues, causing considerable damage. The screw-worm (Cochliomyia hominivorax) is an important pest in this regard, causing severe damages to cattle populations in some areas.
Flower flies or syrphids (family Syrphidae) include about 1,000 species in North America. Adult syrphids can be quite common in some habitats, where they are typically seen hovering in the vicinity of flowers. Many species of syrphids are brightly colored, sometimes with a black and yellow banding that is an obvious mimicry of bees and wasps.
The fruit fly family (Tephritidae) is made up of several hundred North American species, some of which are important pests in agriculture. The apple maggot (Rhagoletis pomonella ) burrows in the fruits of apples and other fruits, while the Mediterranean fruit fly or medfly (Ceratitis capitata ) is a serious pest of citrus fruits.
The small fruit flies (family Drosophilidae) are common in the vicinity of decaying vegetation and fruit. The species Drosophila melanogaster has been commonly used in biological laboratories for studies of genetics, because it is easily and quickly bred, and has much larger chromosomes that can be readily studied using a microscope.
The warble and bot flies (family Oestridae) are large, stout, fast-flying flies that have a superficial resemblance to bees. The larvae of these flies are parasitic on large mammals, living in the flesh just beneath the skin. Some species are serious pests of agricultural animals, for example, the sheep bot fly (Oestrus ovis ) and the ox warble fly (Hypoderma bovis ). Warble flies are extremely irritating to cattle and to wild ungulates—there are reports of caribou being driven to distraction by warble flies, and jumping off cliffs in desperate attempts to escape these nasty pests.
Tachinid flies (family Tachinidae) include more than 1300 species in North America. The larvae of tachinids are parasitic on other species of insects, including some economically important pests, which are essentially eaten alive by the tachinid. As such, some species of tachinids provide a useful service to humans.
The robber flies (family Asilidae) are a diverse group, with more than 800 species in North America. Robber flies are predators of other insects, which are captured in flight.
Seaweed or wrack flies (family Coelopidae) are dark-colored flies that can be very abundant along marine shores, where they breed in natural piles of seaweed compost, with the adults swarming abundantly above, often attracting large numbers of shore-birds and swallows.
There are many families of flies that have aquatic larval and pupal stages, but are terrestrial as adults.
Some of the more prominent of these flies are described briefly in the following paragraphs.
The mosquitoes (family Culicidae) are a diverse, well known, and important group of biting flies. Larval mosquitoes, also known as ‘wrigglers,’ are aquatic and feed on algae and organic debris, and the adult males feed on flower nectar. Female mosquitoes, however, require a blood meal from a bird or mammal before they can develop eggs. Many species of mosquitoes bite humans, and they can be an enormous cause of annoyance, as well as the means of spreading some important, even deadly, diseases. About 150 species of mosquitoes occur in North America, and in some habitats, for example, in northern forests during the summer, these blood-suckers can be enormously abundant and bothersome.
The black flies or buffalo gnats (family Simulidae) are small, dark-colored, hunch-backed flies. Female black flies require a blood meal to develop their eggs, and they obtain this food by biting the skin of a victim, and then sucking the blood that emerges from the wound. Black flies breed in cool streams, and they can be very abundant in some northern habitats. Unprotected animals have actually been killed as a result of the enormous numbers of bites that can be delivered during periods when black flies are abundant.
The horse flies and deer flies (family Tabanidae) are another group of fierce, biting flies with aquatic larvae and pupae, and terrestrial adults. Only the females require a blood meal—the males feed on nectar and plant juices. The eyes of deer flies are often extremely bright-colored, even iridescent.
The biting midges or no-see-ums (family Ceratopogonidae) are very small, blood-sucking flies with aquatic larval and pupal stages, but terrestrial adults. These diminutive pests can be quite abundant along the shores of lakes and oceans. These tiny flies can easily penetrate through fly screens and many types of clothing, and deliver bites that are much more painful than might be expected on the basis of the diminutive, less-than one millimeter size of these insects.
The crane flies (family Tipulidae) have a superficial resemblance to gigantic mosquitoes with extremely long and delicate legs. The larvae of crane flies occur in aquatic or moist terrestrial habitats and mostly feed on decaying organic matter, while the adults are terrestrial and feed on nectar.
The phantom midges (family Chaoboridae) are mosquito-like insects, but they do not bite. Larval phantom midges are predators of other bottom-dwelling arthropods, and have almost transparent bodies—hence their common name.
Some of the biting and blood-sucking flies are the vectors of important diseases of humans, domestic animals, and wild animals. The microorganisms that cause malaria, yellow fever, encephalitis, filariasis, dengue fever, sleeping sickness, typhoid fever, dysentery, and some other important diseases are all spread to humans by species of Diptera.
Malaria is one of the best-known cases of a disease that is spread by biting flies. Malaria is caused by the protozoan Plasmodium falciparum, and is spread to humans by a mosquito vector, especially species of Anopheles, which infect people when they bite them to obtain a blood meal. Malaria is an important disease in the tropics and subtropics. During the 1950s, about 5% of the world’s population was infected with malaria, and during the early 1960s two to five million children died of malaria each year in Africa alone. The incidence of malaria has been greatly reduced by the use of insecticides to decrease the abundance of the Anopheles vectors, and by the use of prophylactic drugs that help to prevent infections in people exposed to the Plasmodium parasite. However, some of the pesticide-based control programs are becoming less effective, because many populations of Anopheles mosquitoes have developed a tolerance of the toxic effects of some insecticides.
The house fly (Musca domestica ) is a non-biting fly, but it can carry some pathogens on its feet and body, and when it walks over food intended for consumption by humans, contamination can result. The house fly is known to be a contact vector of some deadly diseases, including typhoid fever, dysentery, yaws, anthrax, and conjunctivitis. House flies can be controlled using insecticides, although some populations of this insect have developed resistance to insecticides, rendering these chemicals increasing less effective as agents of control.
Carde, Ring, and Vincent H. Resh, eds. Encyclopedia of
Insects. San Diego, CA: Academic Press, 2003.