Flies are of great size, generally have a metallic bluish green coloration, have atrophied mouthparts, the ovipositor is not visible; lay eggs in the tissues of vertebrates living or dead or decaying organic substances. The parasite larva is mandatory, but adults are free-living. Obligate parasites of mammalian skin when in the larval stage, the adults have free life; cause myiasis in one host, the parasitic infection due to the infestation of tissues or body cavities by larvae of insects. Some species (Dermatobia hominis) deposited by a single larva whitish lesions known to warble, nodular cutaneous myiasis producing a; other species (Cochliomyia hominivorax) deposit several eggs, resulting in numerous injuries called screwworm larvae. Cause great economic loss, is a major ectoparasites of cattle in the region stretching from Mexico to Argentina; cause infections, weight loss, low milk production, calf death and impaired quality of leather. It is also a public health problem because parasite pets and humans. The flies that produce berne not lay their eggs directly on the host, you need other insects to convey the victim. Those that produce screwworm lay their eggs directly on the wound of the victim.