J.A.L. Cooke/Oxford Scientific Films
Flies go through a complete metamorphosis of egg, larva, pupa, and adult. The eggs, which vary greatly among species in size and shape, are usually laid in large numbers in a medium, such as decaying animal flesh, dung, or pond water, that ensures the larvae adequate food. House flies have telescopic ovipositors with which they plant eggs in a soft, decaying medium; gall gnats and fruit flies have rigid ovipositors to penetrate both plant stems and fruit. Mosquitoes and black flies lay eggs on water.
Larval growth, in many species, as in the house fly, is rapid. An egg hatches into a gray-white maggot 2 mm (0.08 in) long. The maggot goes through two molts by the third day, reaches a length of 12 mm (0.5 in) by the sixth or seventh day, and is then ready to pupate inside its last larval skin. For the next four or five days the brown pupa remains inactive while the body cells of the larva dissociate and take on the form of the adult fly. The adult then breaks out of the pupal case, expands its crumpled wings, and is ready to fly and mate.
Among the flies are some of our worst enemies. Some species destroy crops; they live as parasites under the skin of animals, causing myiasis or infestation with fly maggots; and they carry such diseases as typhoid, anthrax, cholera, and dysentery. Mosquitoes carry malaria, dengue fever, encephalitis, and elephantiasis. But most species are harmless to humans and play an important role in the balance of nature. They carry pollen to plants; they are an important link in the food web; and they hasten the decomposition of animal carcasses, manure, and vegetable matter. In addition, they consume a great many other insects. About 5000 species of tachina flies prey on other insects, and are sometimes used as biological control agents in controlling injurious species. Flies are an important food source for many animals such as frogs and toads, lizards, and birds.
Scientific classification: Flies constitute the order Diptera. Primitive flies such as mosquitoes belong to the suborder Nematocera, moderately advanced species such as horse flies constitute the Brachycera, and highly advanced species make up the Cyclorrhapha. The latter group contains the common house fly, Musca domestica.