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Flies belong to the order Diptera, a group that also includes mosquitoes, gnats, and midges. Flies make up the fourth largest order of insects, with about 100,000 species recognized. Dipterans are amongst the most advanced insects in terms of morphology and biological adaptations. Their versatility and extreme range of anatomical and behavioral adaptations have enabled them to thrive in almost every corner of the globe—in soils, plants, and around water bodies. A large number of species have developed special relations with other The life cycle of a common housefly. Illustration by Hans & Cassidy. Courtesy of Gale Group.
animals as well as plants: many are free-living, feeding off a wide range of plants, while others are parasites and scavengers. A number are of economic importance in controlling pest species, while others serve as vectors for a range of human, animal, and plant diseases.

Dipterans are characterized by a single pair of functional wings positioned high on the thorax, behind which rest a pair of knoblike vestigial wings known as halteres. The head is free-moving and attached by a slender neck to the thorax. Two large compound eyes are prominent features on the head, as are a pair of segmented antennae. Also attached to the thorax are three pairs of legs, each ending in a pair of claws. In most species, these are short and powerful, even capable of grasping and carrying prey their own size. Crane flies (Tipulidae) are exceptional in having extremely long, delicate legs, an adaptation which together with their slender bodies has evolved in species that frequent damp habitats, frequently around streams and lakes.

Many species are a dull dung color which assists equally well as camouflage for avoiding the ever-watchful eyes of predators, as well as for ambushing prey. Hover flies (Syrphidae) though are among the most colorful species, many of which are boldly colored in similar patterns to bees and wasps that carry a venomous sting. While hoverflies carry no such defenses, the act of mimicking the garish black and yellow colors of these other insects guarantees them a greater security from predators than many other species might enjoy. Most hover flies feed on nectar and pollen.

One part of the anatomy which exhibits considerable variation among flies is the structure of the mouth, a feature which dictates the way of life of many species. The proboscis in blood-feeding and other predatory flies, for example, is in some species a hollow piercing needle, while in others it resembles a broad, dagger-shaped weapon. Only the females of the bloodsucking species practice this habit and the mouthparts of the males are therefore quite different. In other species the proboscis is quite short and equipped with an absorbent type of soft pad through which liquids and small solid objects can pass. The mouthparts of some other species are nonfunctional and no food is taken during the adult stage, which is usually of short duration and intended only for dispersion and reproduction.

One of the most widely known groups of flies is the family Culcidae, which consists of mosquitoes and gnats. Most are slender built with long thin legs and narrow wings. All members of this family have well-developed toughened mouthparts for piercing plant or animal cells and long slender probosces for sucking up fluids. The probosces are used in some species for sucking up nectar and others for sucking up blood from animal prey. Mosquitoes and gnats are lightweight flies so that their animal victims rarely feel them when they alight to feed. Once settled, they make a tiny incision in the skin and inject a small amount of anticoagulant to the wound; this prevents the blood from clotting in and around the wound and guarantees the free flow of blood through the proboscis.

Other predatory and often blood-feeding flies include the much larger and more robust horse flies (Tabanidae) and the highly specialized robber flies (Asilidae) that lie in ambush for a passing insect; once the robberfly has captured its prey, the prey is killed immediately with an injection of poison. The body fluids from the hapless prey are then sucked up through the versatile proboscis.

Reproduction in diptera proceeds in a series of well-defined stages, from egg to adult. Eggs are laid on or near a food source (plant or animal material). Some species such as mosquitoes lay their eggs in standing water bodies. When they hatch, the larvae live suspended in the water in a horizontal position, just below the surface, feeding on tiny food particles. The pupae may also develop and remain in water up to the stage when they finally hatch into adults. Hover fly larvae, in contrast, may lay their eggs on plants infested with aphids. When the larvae hatch they devour the harmful aphids and are therefore important for many gardeners and horticulturalists. Some species of parasitic flies bypass the egg-laying phase and lay larvae directly into their hosts. Most larvae, or maggots, have short or reduced legs and the head is also quite indistinct. The only other remarkable feature about most larvae is the range of siphonlike appendages near the hind end, which assist with respiration. As they grow, the larvae shed their skin, a feature which may be repeated four or five times before it finally pupates. During the pupal phase, the larvae undergoes a complete transformation in a process known as metamorphosis. After several weeks in a cocoon that the larvae spins about itself, an adult fly emerges to begin the life cycle all over again.

Many species of flies are an economic concern to humans. Bot flies (Muscidae) and horse flies can be serious pests among livestock and other wild animals, such as deer. Although males feed exclusively on nectar and plant juices, females are blood-suckers and lay their eggs on the hair of cattle, horses, and other species. As the animals groom themselves, the eggs are taken into the mouth from where they move through the digestive tract to the stomach. Here they hook onto the stomach lining and feed until they are ready to pupate, at which time they release their grip and pass further along the intestine where they are deposited with the animal's feces. Other species when they hatch chew their way directly through the skin where they cause a small swelling. Here they will remain, feeding off flesh, until such time as they pupate and leave their host. Such species are not only a painful irritation for the host animal, but also seriously reduce the beast's condition and overall value of its hide.

Other fly species are important from a medical point of view as many are vectors of diseases such as malaria, yellow fever, typhoid, sand-fly fever, and others. Considerable amounts of money continue to be spent in the tropics in an attempt to eradicate malaria, which is caused by a tiny protozoan, but which is spread by female mosquitoes of the genus Anopheles, which require blood proteins for the development of their eggs.

Not all species of flies are harmful. Many species fulfil an important role in pollinating fruit crops, as well as a great many wild flowers, while even the distasteful actions of many scavenging flies serve an important role as they remove and recycle carrion and other animal and plant wastes that might otherwise pose a serious health hazard. Increasing attention is now being given to the possibility of using some flies, such as predatory species, to control a number of destructive flies and their larvae, such as those which attack livestock and plants.

See also True flies.

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