Temporal range: 245–0Ma Middle Triassic – Recent
|A poster with sixteen different species of flies.|
True flies are insects of the order Diptera (from the Greek di = two, and ptera = wings). The most obvious distinction from other orders of insects is that a typical fly possesses a pair of flight wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. (Some species of flies are exceptional in that they are secondarily flightless). The only other order of insects bearing two true, functional wings plus any form of halteres are the Strepsiptera, and in contrast to the flies, the Strepsiptera bear their halteres on the mesothorax and their flight wings on the metathorax.
The presence of a single pair of patent, mesothoracic flight wings distinguishes most true flies from other insects with "fly" in their names, such as mayflies, dragonflies, damselflies, stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. However, some true flies have become secondarily wingless, for example many members of the superfamily Hippoboscoidea and some species 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, though it is practical this is a comparatively new convention; especially in older books, one commonly might see the likes of "saw fly" and "caddis fly", or hyphenated forms such as house-fly and dragon-fly. In any case, non-entomologists cannot in general be expected to tell dipterans, "true flies", from other insects, so it would be unrealistic to expect rigour in the use of common names.
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. For visual course control, flies optic flow field is analyzed by a set of motion-sensitive neurons. A subset of these neurons is thought to be involved in using the optic flow to estimate the parameters of self-motion, such as yaw, roll, and sideward translation. Other neurons are thought to be involved in analyzing the content of the visual scene itself, such as separating figures from ground using motion parallax. The H1 neuron is responsible for detecting horizontal motion across the entire visual field of the fly, allowing the fly to generate and guide stabilizing motor corrections mid-flight with respect to yaw. 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. Despite the myth that flies only live a day (which may have either arisen from confusion with the mayfly or the fact that a fly inside a house may starve to death in a few days), most adult dipterans can live about one month.
The Nematocera are recognized by their elongated bodies and many-segmented, often feathery antennae as represented by mosquitoes and crane flies. The Brachycera have a more roundly proportioned body and much shorter antennae. In 1964, Boris Borisovitsch 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 reject 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.
Diptera derive from Mecoptera or a strictly related group. The first true dipterans known are from the Middle Triassic (around 240 million years ago), and they became widespread during the Middle and Late Triassic.
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 (66 million years ago).
Maggots found on corpses are useful to forensic scientists, specifically forensic entomologists. Maggot species can be identified by various means such as their anatomy and by matching their DNA. Maggots of various species of flies visit corpses and carcases at fairly well-defined times after the death of the victim, and so do their predators, such as beetles in the family Histeridae. Thus the presence or absence of particular species of fly maggots and other scavenger insects provide evidence as a basis for estimation of the time since death, and sometimes other details such as the place of death.
In food production, certain cheese varieties, such as casu marzu, are exposed to flies known as cheese skippers, members of the family Piophilidae. The digestive activities of the fly larvae soften or liquefy the cheese and modify the aroma as part of the process of maturation.
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 1855 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.
The 1960 Alfred Hitchcock film Psycho shows a close-up view of a fly while the lead character is heard: "Mother: [voice-over] It's sad when a mother has to speak the words that condemn her own son. But I couldn't allow them to believe that I would commit murder. They'll put him away now as I should have years ago. He was always bad and in the end he intended to tell them I killed those girls and that man, as if I could do anything except just sit and stare like one of his stuffed birds. Oh, they know I can't even move a finger and I won't. I'll just sit here and be quiet just in case they do.... suspect me. They're probably watching me. Well, let them. Let them see what kind of a person I am. I'm not even going to swat that fly. I hope they are watching... they'll see. They'll see and they'll know, and they'll say, 'Why, she wouldn't even harm a fly.' " 
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", Alice in Chains album Jar of Flies, 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.
"If only I were a fly on the wall..." is a phrase used when one wishes they could have observed a scene in an omniscient point of view.
"He would bet on two flies crawling up a wall." is an Australian phrase meaning someone who would wager money upon almost anything.
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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 DMOZ
- Manual of Afrotropical Diptera
- The Diptera.info Portal Expert Diptera site and Diptera information portal.
- Fly Species Fact Sheets from the National Pest Management Association