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 fives 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 small quantities of liquid after a meal.
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 flys 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. This leads to flies having more reproduction abilities than most insects and at a much quicker rate. This is why the flies come 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 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.
Some types of maggots found on corpses have been found to be of great use to forensic scientists; specifically Forensic Entomology. By their stage of development, these maggots (and other insects) 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 designed to rot as part of their 'aging' process.