flies

True flies are insects of the order Diptera (di = two, and pteron = wing), possessing a single pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax.

The presence of a single pair of wings distinguishes true flies from other insects with "fly" in their name, such as mayflies, dragonflies, damselflies, stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.

Diptera is a large order, containing an estimated 240,000 species of mosquitos, 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 well adapted for aerial movement, and typically have short and streamlined bodies. The second segment of the thorax, which bears the wings and contains the flight muscles, is greatly enlarged, with the other two segments being reduced to mere collar-like structures. The third segment 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 dervied 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, to reduce drag while flying.

Flies consume only liquid food, and their mouthparts and digestive tract show various modifications for this diet. The most apparently primitive flies have piercing blade-like mandibles and fleshy palps, but these have become adapted into numerous different forms in different groups. These include both the fine stilleto-like sucking mouthparts of mosquitos, and the fleshy proboscis of houseflies. The gut typically includes large diverticulae, allowing the insect to store large quantities of liquid after a meal.

a) reproduction and development

The genitalia of male 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 lies on top of the female, facing in the same direction, but then turns round to face in the opposite direction. In some species, this forces the male to lie on its back in order for its genitalia to remain engaged with those of the female, but in most cases, the torsion of the male genitals allows the male to mate while remaining upright.

The female lays her eggs as close to the food source as possible, and development is generally rapid, allowing the larva to consume as much food as possible in a short period of time before transforming into the adult. In extreme cases, the eggs hatch immediately after being laid, while a few flies are ovoviviparous, with the larva hatching inside the mother.

Larval flies, or maggots, have no true legs, and often little demarcation between the thorax and abdomen; in the more derived species, even the head is not clearly distinguishable from the rest of the body. In some species, there are small prolegs on some segments, but maggots are more commonly entirely limbless. The eyes and antennae are reduced, or even absent, and the abdomen also lacks appendages such as cerci. This general lack of features is an adaptation to the extremely food rich environment, such as within rotting organic matter, or as an endoparasite.

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.

classification

There are two generally accepted suborders of Diptera. The Nematocera are usually recognized by their elongated bodies and feathery antennae as represented by mosquitoes and crane flies. The Brachycera tend to have a more roundly proportioned body and very short antennae. A more recent classification has been proposed in which the Nematocera is split into two suborders, the Archidiptera and the Eudiptera, but this has not yet gained widespread acceptance among dipterists.

1. 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.
2. Suborder Brachycera (141 families, 8 of them extinct) – short antennae, the pupa is inside a puparium formed from the last larval skin. Brachycera are generally robust flies with larvae having reduced mouthparts.
1. 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.
2. Infraorder Muscomorpha – (largely the Cyclorrhapha of older schemes). Muscomorpha have 3-segmented, aristate (with a bristle) antennae and larvae with three instars that are acephalic (maggots).

Most of the Muscomorpha are further subdivided into the Acalyptratae and Calyptratae based on whether or not they have a calypter (a wing flap that extends over the halteres).

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 more out of practical or historical considerations than out of any strict respect for phylogenetic classifications (some modern cladists tend to spurn the use of Linnaean rank names). Nearly 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 sometimes classified closely together with Mecoptera and Siphonaptera in the superorder Antliophora.

evolution

Diptera are usually thought to derive from Mecoptera or a strictly related group. First true dipterans are known from the Middle Triassic, becoming widespread during the Middle and Late Triassic.

flies in culture

Flies have often been used in mythology and literature to represent agents of death and decay, such as the Biblical fourth plague of Egypt, or portrayed as nuisances (e.g., in Greek mythology, Myiagros was a god who chased away flies during the sacrifices to Zeus and Athena, and Zeus sent a fly to bite the horse Pegasus causing Bellerophon to fall back to Earth when he attempted to ride to Mount Olympus), though in a few cultures the connotation is not so negative (e.g., in the traditional Navajo religion, Big Fly is an important spirit being). Emily Dickinson's poem "I Heard a Fly Buzz When I Died" also makes reference to flies in the context of death. In fact, many flies, such as the genus Hydrotaea are used in forensic cases to determine time of death for many corpses. In William Goldings lord of the flies, the fly is a symbol of the children involved.

Not surprisingly, in art and entertainment, flies are also used primarily to introduce elements of horror or the simply mundane; an example of the former is the 1958 science fiction film The Fly (remade in 1986), in which a scientist accidentally exchanges parts of his body with those of a fly. Examples of the latter include trompe l'oeil paintings of the fifteenth century such as Portrait of a Carthusian by Petrus Christus, showing a fly sitting on a fake frame , a 2001 art project by Garnet Hertz in which a complete web server was implanted into a dead fly , and various musical works (such as Yoko Ono's album Fly, U2's song "The Fly," Dave Matthews' song "The Fly" and Béla Bartók's "From the Diary of a Fly"). Damien Hirst's famous work 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.The ability of flies to cling to almost any surface has also inspired the title of Human Fly for stunt performers whose stunts involve climbing buildings, including both real life and fictional individuals.

Aside from the fictional and conceptual role flies play in culture, there are practical roles that flies can play (e.g., flies are reared in large numbers in Japan to serve as pollinators of sunflowers in greenhouses), especially the maggots of various species.

a) maggots

Some types of maggots found on corpses can be of great use to forensic scientists. By their stage of development, these maggots can be used to give an indication of the time elapsed since death, as well as the place the organism died. The lack of maggot presence is also telling in an investigation.

Maggot species can be identified using their DNA. The size of the house fly maggot is 10–20 mm (⅜–¾ in). At the height of the summer season, a generation of flies (egg to adult) may be produced in 12–14 days. Some other families of Insecta, such as Histeridae, feed on maggots. Thus, the lack of maggots would increase the estimated time of death.

Other types of maggots are bred commercially, as a popular bait in angling, and a food for carnivorous pets such as reptiles or birds.

Maggots have been used in medicine to clean out necrotic wounds, and in food production, particularly of cheeses.