Flies go through stages of transformation such as egg- larva- chrysalis - imago.

There are several larvae such as larvae parasitic on animal or plant, larvae catching and eating small animal or insect, larvae sucking blood, and larvae generating in rotten seaweed, fungi, excretions, and food.

Most flies pupate inside soils. In flies, there are many kinds closely related to interest of mankind. People skin fly is parasitic on human skin and causes maggot symptoms; horse fly, cow fly, sheep fly, blow fly, and blood fly sucking blood are important sanitation noxious insects.

Since numerous kinds of flies such as house fly, bluebottle fly, black fly, blow fly, etc. invade our houses from unsanitary places and cause and infect germs of digestive contagious diseases, parasite egg, and virus such as heterogeneity, typhoid, cholera, amoeba heterogeneity, they are important as environment sanitation noxious insects.

Tsetse fly in Africa propagates Trypanosome, which is pathogen of narcolepsy, when it sucks blood from domestic animals.

Housefly such as black housefly which sticks on domestic animals is an intermediate host of horse's stomach insect, cow's eye insect, and eye vinegar fly is also an intermediate host of dog's eye insect.

Delia platura are known as noxious insect for agricultural products. Also as all flies give an unpleasant feeling to modern people, they can be considered an unpleasant noxious insect. Parasitic flies like Lepidophora lepidocera and parasitic fly and predatory flies like Metasyrphus corollae are considered useful insects as they are natural enemies of noxious insects such as plant louse, green leafhopper, Recilia(Inazuma) dorsalis , and Oriental tussock moth. For the extermination of flies, we need to eliminate generation sources by correctly understanding species and to find out generation sources and by improving animal barns, compost place, bathroom, and waste landfills and if we use various pesticides, fly-catching ribbon, and fly-catching basket as supplementary measures, it will be very effective for comprehensive prevention of breeding and extermination.