Temporal range: Middle Triassic - Recent 245–0 Ma
|A poster with sixteen different species of flies|
True flies are insects of the order Diptera (from the Greek di = two, and ptera = wings). They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. Apart from secondarily flightless insects (including some flies), the only other order of insects with any form of halteres are the Strepsiptera, and theirs are on the mesothorax, with the flight wings on the metathorax.
The presence of a single pair of patent, metathoracic flight wings distinguishes true flies from other insects with "fly" in their names, such as sawflies, mayflies, dragonflies, damselflies, butterflies, but also stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, caddisflies, or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.
Some authors draw a distinction in writing the common names of insects. True flies are written as two words, such as crane fly, robber fly, bee fly, moth fly, and fruit fly. In contrast, common names of nondipteran insects that have "fly" in their names are written as one word, e.g., butterfly, stonefly, dragonfly, scorpionfly, sawfly, caddisfly, whitefly. In practice, however, this is a comparatively new convention, and, especially in older books, one commonly might see the likes of "saw fly" and "caddis fly". In any case, non-entomologists cannot in general be expected to tell dipterans, "true flies", from other insects.
The Diptera comprise a large order, containing an estimated 240,000 species of mosquitoes, gnats, midges and others, although under half of these (about 120,000 species) have been described. It is one of the major insect orders both in terms of ecological and human (medical and economic) importance. The Diptera, in particular the mosquitoes (Culicidae), are of great importance as disease transmitters, acting as vectors for malaria, dengue, West Nile virus, yellow fever, encephalitis and other infectious diseases.
Anatomy and biology
Flies are adapted for aerial movement and typically have short and streamlined bodies. The first tagma of the fly, the head, consists of ocelli, antennae, compound eyes, and the mouthparts (the labrum, labium, mandible and maxilla make up the mouthparts). The second tagma, the thorax, bears the wings and contains the flight muscles on the second segment, which is greatly enlarged; the first and third segments have been reduced to collar-like structures. The third segment of the thorax bears the halteres, which help to balance the insect during flight. A further adaptation for flight is the reduction in number of the neural ganglia, and concentration of nerve tissue in the thorax, a feature that is most extreme in the highly derived Muscomorpha infraorder.
Flies have a mobile head with eyes and in most cases have large compound eyes on the sides of the head, with three small ocelli on the top. The antennae take a variety of forms, but are often short, which reduces drag while flying.
Because no species of fly has teeth or any other organ or limb that allows them to eat solid foods, flies consume only liquid food or finely granular foods, such as pollen, and their mouthparts and digestive tracts show various modifications for such diets. Female Tabanidae use knife-like mandibles and maxillae to make a cross-shaped incision in the hosts' skin and then lap up the blood. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.
For detailed anatomy explore  which uses examples from the 4 major fly groups (Lower Diptera, Lower Brachycera, Acalyptrate, Calyptrate) representing different anatomical expressions.
Reproduction and development
The genitalia of female flies are rotated to a varying degree from the position found in other insects. In some flies, this is a temporary rotation during mating, but in others, it is a permanent torsion of the organs that occurs during the pupal stage. This torsion may lead to the anus being located below the genitals, or, in the case of 360° torsion, to the sperm duct being wrapped around the gut, despite the external organs being in their usual position. When flies mate, the male initially flies on top of the female, facing in the same direction, but then turns round to face in the opposite direction. This forces the male to lie on his back for his genitalia to remain engaged with those of the female, or the torsion of the male genitals allows the male to mate while remaining upright. This leads to flies having more reproduction abilities than most insects, and at a much quicker rate. Flies occur in great populations due to their ability to mate effectively and in a short period of time during the mating season.
The female lays her eggs as close to the food source as possible, and development is rapid, allowing the larvae to consume as much food as possible in a short period of time before transforming into adults. The eggs hatch immediately after being laid, or the flies are ovoviviparous, with the larvae hatching inside the mother.
Larval flies have no true legs. Some Dipteran larvae, such as species of Simuliidae, Tabanidae, and Vermileonidae, have prolegs adapted to such functions as holding onto a substrate in flowing water, holding onto host tissues, or holding prey. Roughly speaking, there is some anatomical distinction between the larvae of the Nematocera and the Brachycera (see Classification section, below); especially in the Brachycera, there is little demarcation between the thorax and abdomen, though the demarcation may be very visible in many Nematocera, such as mosquitoes (see image, both here and in the mosquitoes article); in the Brachycera, the head of the larva is not clearly distinguishable from the rest of the body, and there are few, if any, sclerites. Informally, such Brachyceran larvae are called maggots, but the term is nontechnical and often applied indifferently to fly larvae or insect larvae in general. The eyes and antennae of Brachyceran larvae are reduced or absent, and the abdomen also lacks appendages such as cerci. This lack of features is an adaptation to food such as carrion, decaying detritus, or host tissues surrounding endoparasites. Nematoceran larvae generally have visible eyes and antennae, though usually small and of limited function.
The pupae take various forms, and in some cases develop inside a silk cocoon. After emerging from the pupa, the adult fly rarely lives more than a few days, and serves mainly to reproduce and to disperse in search of new food sources.
The Nematocera are recognized by their elongated bodies and feathery antennae as represented by mosquitoes and crane flies. The Brachycera have a more roundly proportioned body and much shorter antennae. In 1964, B.B. Rohdendorf proposed a classification in which the Nematocera is split into two suborders, the Archidiptera and the Eudiptera.
- Suborder Nematocera (77 families, 35 of them extinct) – long antennae, pronotum distinct from mesonotum, in Nematocera, larvae are either eucephalic or hemicephalic and often aquatic.
- Suborder Brachycera (141 families, 8 of them extinct) – short antennae, the pupa is inside a puparium formed from the last larval skin, they are generally robust flies with larvae having reduced mouthparts.
- Infraorders Tabanomorpha and Asilomorpha – these comprise the majority of what was the Orthorrhapha under older classification schemes. The antennae are short, but differ in structure from those of the Muscomorpha.
- Infraorder Muscomorpha – (largely the Cyclorrhapha of older schemes). Muscomorpha have three-segmented, aristate (with a bristle) antennae and larvae with three acephalic instars (maggots).
Beyond that, considerable revision in the taxonomy of the flies has taken place since the introduction of modern cladistic techniques, and much remains uncertain. The secondary ranks between the suborders and the families are out of practical or historical considerations than out of strict respect for phylogenetic classifications (modern cladists spurn the use of Linnaean rank names). All classifications in use now, including this article, contain some paraphyletic groupings; this is emphasized where the numerous alternative systems are most greatly at odds. See list of families of Diptera.
Dipterans belong to the taxon Mecopterida, that also contains Mecoptera, Siphonaptera, Lepidoptera (butterflies and moths) and Trichoptera. Inside it, they are classified closely together with Mecoptera and Siphonaptera in the superorder Antliophora.
The basal clades in the Diptera are the Deuterophlebiidae and Nymphomyiidae. The Bibionomorpha are a sister clade to Brachycera. The branching order of the remaining clades of the lower Diptera - infraorders Culicomorpha, Psychodomorpha and Tipulomorpha - has yet to be resolved.
Within the Brachycera, several progressively nested groups exist: Eremoneura (three larval instars), Cyclorrhapha (pupation occurs within a puparium), Schizophora (flies that escape from their puparium using the ptilinal sac, an evertable frontal pouch) and Calyptratae (larger flies with wings that have the calypter, an enlarged basal lobe).
The Schizophora include most of the family-level diversity in Diptera (∼85 families) and more than 50,000 species. The Calyptratae form a monophyletic superfamily. Other monophyletic superfamilies include the Ephydroidea, Lauxanioidea, Nerioidea, Sciomyzoidea and Tephritoidea. The relationships between the remaining families have yet to be clarified.
There were three episodes of rapid evolution in the lower Diptera (~220 million years ago), lower Brachycera (180 million years ago) and Schizophora (65 million years ago).
Maggots found on corpses are useful to forensic scientists; specifically forensic entomology. By their stage of development, these maggots indicate the time elapsed since death, as well as the place of death.
Maggot species can be identified using their DNA. The housefly maggot measures 10–20 mm (⅜–¾ in) in size. At the height of the summer season, a generation of flies (egg to adult) may be produced in 12 to 14 days. Other insect families, such as Histeridae, feed on maggots. Thus the lack of maggots would increase the estimated time of death.
Flies in culture
Flies have been depicted in mythology and literature. In the Biblical fourth plague of Egypt, flies represent death and decay. Myiagros was a god in Greek mythology who chased away flies during the sacrifices to Zeus and Athena, and Zeus sent a fly to bite Pegasus, causing Bellerophon to fall back to Earth when he attempted to ride the winged steed to Mount Olympus. In the traditional Navajo religion, Big Fly is an important spirit being.
Emily Dickinson's 1955 poem "I Heard a Fly Buzz When I Died" also makes reference to flies in the context of death. In fact, flies such as the genus Hydrotaea are used in forensic cases to determine time of death for corpses. In William Golding's 1954 novel Lord of the Flies, the fly is a symbol of the children involved.
Musical works that mention flies: Yoko Ono's 1971 album Fly, U2's 1991 song "The Fly", Wire's 1978 song "I Am The Fly", Dave Matthews's 2007 song "The Fly" and Béla Bartók's 1920s piano work "From the Diary of a Fly".
Damien Hirst's provocative 1990 artwork, titled A Thousand Years, featured a severed cow's head contained in a box with thousands of flies and a bug zapper, creating an entire life cycle within a glass box. In 2001 Dr. Garnet Hertz implanted a complete web server into a dead fly.
The ability of flies to cling to surfaces has also inspired the title of "Human Fly" for real and fictional stunt performers whose feats involve climbing buildings.
- Bug guide
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- The Diptera Site
- The Dipterists Forum - The Society for the study of flies
- The Bishop Museum Catalog of Fossil Diptera
- The Tree of Life Project
- Diptera at the Open Directory Project
- Manual of Afrotropical Diptera