DIPTERAN

any member of the insect order Diptera, the two-winged or so-called true flies. Although many winged insects are commonly called flies, the name is strictly applicable only to members of Diptera. One of the largest insect orders, it numbers more than 85,000 species that are relatively small, with soft bodies. Although the mouthparts of flies are of the sucking type, individuals show considerable variation in structure. Many flies are of great economic importance. Some bloodsuckers are serious pests of man and other animals. These insects, along with many scavenging flies, are important vectors of disease; others are pests of cultivated plants. Flies are beneficial, tooas scavengers, predators or parasites of certain insect pests, pollinators of plants, and destroyers of weeds noxious to man. Dipterous larvae, often called maggots or grubs, are found in many habitats (e.g., in any kind of water, in plant tissue and soil, beneath bark or stones, in decaying plant and animal matter, even in pools of crude petroleum). Adults feed on plant or animal juices or other insects. Diptera fall into three large groups: Nematocera (e.g., crane flies, midges, gnats, mosquitoes), Brachycera (e.g., horse flies, robber flies, bee flies), and Cyclorrhapha (e.g., flies that breed in vegetable or animal material, both living and dead). a member of the insect order Diptera, the two-winged or true flies. This order contains over 85,000 species divided into three suborders: the Nematocera (midges, gnats, mosquitoes, crane flies), the Brachycera (horse flies, bee flies, robber flies), and the Cyclorrhapha (house flies, fruit flies, leafminers, blow flies). Many other insects are commonly called flies (such as dragonflies, caddisflies, and mayflies), but their wing structures serve to distinguish them from the true flies. Dipterans occur in all habitats throughout the world; one or two species of midges (e.g., Pontomyia natans) are even marine-dwelling. The smallest dipterans are midges only 1 mm (0.04 inch) in length, and the largest are the robber flies (Asilidae), which can attain a length of nearly 8 cm (3 inches). The chief diagnostic feature of the group is wing structure. The anterior wings are functional for flight, while the posterior wings have evolved into halteres, which serve as balancing organs during flight. The earliest dipterans in the fossil record date from the beginning of the Jurassic period (about 208 million years ago). Within the order there is great variation in the relative size of the thorax, abdomen, and legs. While some flies are bright and metallic in appearance, most are covered with a fine coating of dust (tomentum). In many species the ovipositor is a telescoping organ composed of the last few segments of the abdomen; its structure allows the fly to lay her eggs within various organic matter. Dipterans' compound eyes are largein the house fly (Musca domestica) there are 4,000 facets in each eye. Antennae vary greatly in structure but are present in all species of dipterans. In flies the mouthparts have evolved for sucking. In various species of primitive flies, such as mosquitoes, the mandibles of the female are adapted to piercing the skin of vertebrates to obtain blood. Some advanced flies with this habit (tsetse flies and robber flies) have evolutionarily lost the mandibles but have evolved other structures for the purpose. Like many other insects, flies undergo a complete metamorphosis from the egg through larva and pupa to the adult. In the more advanced flies, fewer molt stages, or larval instars, occur. In tsetse flies (Glossina) and three families of parasitic flies (Hippoboscidae, Nycteribiidae, and Strebelidae) only one egg is produced, and it hatches internally. The larva is nourished within the female and when mature is expelled and immediately pupates; these flies lack any free larval stage. By contrast, in many midge species the larvae lead complex, active lives, while the adult lives just long enough to mate and assure a new generation of larvae. Most flies lay eggs. The typical number in a batch of eggs varies among species from one to 250. The site of egg deposition is chosen to place the larvae where food will be readily accessible. Fruit flies (Trypetidae, Drosophilidae) lay their eggs within rotting fruits, while blow flies (Calliphoridae) deposit their eggs in rotting meat or open wounds on living animals. Various other fly larvae, including the frit flies (Chloropidae), leafminers (Agromyzidae), and the gall midges (Cecidomyiidae), develop while feeding on plant tissues. Still other larval species are predatory. Larvae may be found in microhabitats ranging from plant and animal tissue to dung (the beetle flies, Celyphidae), underwater (the mosquitoes, Culicidae, and the black flies, Simuliidae), or in pools of petroleum seepage (the carnivorous petroleum fly [Psilopa petrolei]). The larvae of primitive flies have well-developed heads, chewing mouthparts, and pseudopods or prolegs (false legs, similar to those that support the abdomen of a caterpillar). No fly larvae have true jointed thoracic legs. In the larvae of advanced forms, known as maggots, the head is simple and the mouthparts consist only of a pair of hooks that move vertically. Two spiracles (air holes) are located at the posterior end of the maggot; prolegs are lacking. Fly larvae frequently pupate within either a cocoon or a puparium (a case formed as the larval skin hardens). The adult emerges from the pupa soft, wrinkled, and lacking colour. It swallows air to expand the body and get its blood flowing. Dipterans are ecologically quite important, especially in the function maggots perform in the breakdown and redistribution of organic materials. Flies are also an important link in numerous food chains. Aquatic larvae are an important food for small fish, while all the developmental stages are taken by various other vertebrates including frogs, toads, small mammals, lizards, swallows, swifts, and other species of birds. While some flies are beneficial to man as parasites of insect pests or as scavengers, and many others are important as pollinators of various plants, flies are more familiar for their negative aspects. Many are annoying bloodsucking insectsmosquitoes (Culicidae), deer flies (Chrysops), and stable flies (Stomoxys), for examplewhile swarms of sweat flies and face flies are a nuisance as they gather to suck up sweat and other bodily secretions. In almost all species which take blood, it is done only by the female, who needs nutrients from blood in order to develop a clutch of eggs. Many dipterans are vectors of disease, and because they move from one organism to another they transmit diseases readily. House flies are known agents in the transmission of diseases such as cholera, typhoid, and dysentery. Mosquitoes transmit malaria, yellow fever, and various viral diseases. Deer flies may transmit tularemia. The tsetse flies carry trypanosomes, which cause sleeping sickness in tropical Africa. Fruit flies and frit flies can cause great damage to agricultural crops, as can gall midges and leafminers when they invade ornamental plants. Additional reading J.R. Busvine, Insects and Hygiene, 3rd ed. (1980), comprehensive survey of insects, hygiene, and insecticides; C.N. Colyer and C.O. Hammond, Flies of the British Isles, 2nd ed. (1968), limited to British flies but general application; C.H. Curran, Families and Genera of North American Diptera, 2nd rev. ed. (1965), useful keys for primary identification; M. Demerec (ed.), Biology of Drosophila (1950, reissued 1965), accounts of normal development so that experimental results can be assessed; W. Hennig, Die Larvenformen der Dipteren, 3 vol. in 2 (194852, reissued in 3 vol., 1968), the only comprehensive account of fly larvae, with references to published descriptions; M.T. James, The Flies that Cause Myiasis in Man, U.S. Dept. Agric. Misc. Publ. 631 (1947), useful handbook on larvae that infest the human body; E. Lindner, Die Fliegen der palaearktischen Region (1924, parts still appearing), comprehensive account of flies with much information applicable throughout the world; I.M. Mackerras, The Zoogeography of the Diptera, Aust. J. Sci., 12:157161 (1950); H. Oldroyd, Diptera: Introduction and Key to Families, in the Handbooks for the Identification of British Insects Series (1970); Collecting, Preserving and Studying Insects, rev. ed. (1973); The Natural History of Flies (1964), the only comprehensive account of flies in English; Insects and Their World, 3rd ed. (1973), simple account of the basic principles of structure and function in insects; and Elements of Entomology: An Introduction to the Study of Insects (1970), general reading about basic relationships of insects; L.S. West, The Housefly: Its Natural History, Medical Importance and Control (1951), comprehensive treatise on every aspect of housefly biology. More specialized references on classification may be found in W. Hennig, Die Familien der Diptera Schizophora und ihre phylogenetischen Verwandschaftsbeziehungen, Beitr. Ent., 8:505688 (1958); and in F. Zumpt, Some Remarks on the Classification of the Oestridae, J. Ent. Soc. S. Africa, 20:154161 (1957). Harold Oldroyd