Flies are one of the most diverse and widespread groups of insects, with over 16,000 species in more than 2,200 genera worldwide. They can be found in nearly every habitat on Earth, from the coldest polar regions to the hottest deserts. Flies are important pollinators of many plants and play a key role in decomposing organic matter. They also serve as food for a variety of animals, including birds, fish, reptiles, amphibians, and mammals.
The majority of flies belong to the order Diptera (“two-winged”), which contains approximately 125 families. The other main group of flies is the Siphonaptera (“fleas”), which contains a single family with about 1,500 species. Fleas are parasites that feed on the blood of mammals and birds; they typically live on their host’s body but can also infest homes and other indoor spaces.
Flies have two wings: the larger wing is called the halteres (or haltere) and helps them balance while in flight; the smaller wing is used for propulsion. Most flies can fly at speeds up to 30 miles per hour (48 kilometers per hour). Their eyes are large and compound, with thousands of individual lenses (ommatidia); this gives them excellent vision and allows them to see in all directions at once. Their mouthparts are adapted for sucking or lapping up liquids; some species can also pierce skin to feed on blood.
Most female flies lay their eggs in or near decaying organic matter such as garbage or carcasses; when conditions are favorable, a single female can lay hundreds or even thousands of eggs during her lifetime. The larvae (maggots) that hatch from these eggs feed on decaying organic material until they mature into adults; this process usually takes 10 days to two weeks but can take longer under unfavorable conditions. Some species of flies undergo complete metamorphosis (egg-larva-pupa-adult), while others undergo partial metamorphosis (egg-nymph-adult).