  The common house fly, seen at the right, lays its eggs in rotting, moist material. The maggots emerge from the eggs in warm weather within 8 to 20 hours, and they immediately feed on, and grow in, the material where the eggs were laid. These larvae are 3 to 9 mm long, and creamy white in color. 
When the maggots are full-grown, (picture at left) they crawl away to a dry, cool place near breeding material, and transform to the pupal stage.  Flies spread disease to animals. Myiasis is the infection of an animal with fly larvae (maggots). Common in cattle, (picture at right), sheep, and deer, myiasis also occurs in humans, when certain flies are attracted to wounds, where they lay eggs. The hatched larvae (maggots) then burrow into the skin. Depending on the type of fly, the larvae remain in the skin and cause lesions, or they move through the body and cause damage to various organs. Intestinal myiasis occurs when humans ingest food containing the eggs of larvae. Six or more generations of fly eggs may hatch in a single summer, resulting in a large number of flies. Maggots are useful in several ways. As decomposers, like bacteria and fungi, they help to break down rotting biomass and return nutrients to the soil. The rotting flesh of dead animals, after other scavengers are done, is quickly reduced to bone by the action of maggots.  Maggots are also used occasionally in the field of medicine, as are leeches. Maggots are used to eat dead tissue, helping to clean open wounds. During the Civil War in the United States, and World War I, battlefield physicians saw that soldiers' wounds that were infested with maggots tended to heal better than non-infested wounds. Soon 'maggot therapy' was being used to clean festering and foul-smelling wounds. Maggots not only eat the rotten flesh, they also get rid of harmful bacteria in the wound.
 More than 200 hospitals in the U.S. and Europe have prescribed maggots of the Blowfly to treat patients with infections from injuries like pressure ulcers ("bed sores"), leg and foot ulcers, stab wounds, and post-surgical wounds that won't heal properly. About 5,000 laboratory grown and microorganism-free maggots (see picture at right) are delivered to hospitals across the U.S. every week. Using maggots is sometimes faster than other non-surgical methods for wound-healing, and is not as likely to injure healthy tissue. The larvae are placed in special bandages which hold the maggots in after they are applied to a human wound. Open, untreated wounds can often become infected and gangrenous if left untreated; gangrene is the death of human tissues, caused by a blockage of blood supply to a wound. If gangrenous tissue isn't removed, the affected limb eventually begins to rot, and becomes infected by poisonous bacteria, which can be fatal. Maggots are implanted directly onto a wound, where they eat dead flesh, clean out dead skin, and kill harmful bacteria Once maggots reach their fill of dead and dying flesh, they're removed from the wound, and new maggots are applied. Blood can then flow throughout the tissue, promoting the growth of new flesh.  Naturally occurring maggot infestations can of course be very harmful. In some fly species, the larval forms feed upon live tissue as well as decaying material, and the larvae can be a big problem. For example, the screw worm fly (see picture at right) is sometimes called the 'flesh-eater'. About twice the size of typical house fly, it lays its eggs on the edges of wounds, or in the mucous membranes of body openings. The newly hatched larvae burrow downwards into the tissue, causing massive tissue damage and sometimes death.
 Livestock are very vulnerable to screw-fly larvae, in places where the fly can be found (mostly in Central America, the southern hemisphere, and occasionally in the southern U.S.) There are two species, known as Old World and New World screw-worm fly. All warm-blooded animals are susceptible, including livestock, pets, wildlife and humans.
Screw-worm flies only breed on the wounds of living animals. The female fly lays up to 250 eggs on the edge of a fresh wound. After hatching, the larvae (or maggots) enter the wound and burrow into the underlying tissue to feed. As they grow, they form a huge mass of maggots, and cause extensive tissue damage. Infested animals can't dislodge the larvae by licking or biting, and may eventually be 'eaten alive'. See also our page about the BotFly. The United States successfully wiped out most screw-worm fly populations in the western hemisphere by mating it with sterile males, producing eggs that did not hatch. Populations that still exist are limited to deep rain forests. |