Most fly species help break down and consume garbage, feces, decaying vegetation and dead animal bodies. Some species of flies assist with the pollination of plants. The forensic field sometimes uses the appearance of blowfly larvae to help determine the time of death in homicide cases. In the medical field, doctors may use fly larvae, or maggots, when debriding wounds in burn patients.
There are more than 120,000 species of flies in the world, with over 18,000 in North America. A single pair of houseflies can produce five to six egg hatches, and each hatch has over 100 eggs. Flies are scientifically divided into five categories: biting flies, filth flies, small flies, overwintering flies and gnats.
Biting flies include over 300 species, including the deer fly and horse fly, and they feed on the blood of animals to assist with egg development. Filth flies feed on and lay their eggs in trash and animal feces. While they do not bite, they carry serious diseases, including malaria and cholera. Small flies, such as fruit flies, typically have a short lifespans of just eight to 10 days, but they can cause severe damage to crops. Overwintering flies typically inhabit attics or the inside of building walls, and they reproduce in lawns and gardens. Gnats are the smallest type of fly and include midges, biting gnats and sand flies.