Flies are extremely important animals in the environment. Many species of animals depend on both the adults and larvae as sources of food. Many flower-visiting species are significant as pollinators of plants. Some are considered directly beneficial to humans. For example, the larvae of some flies prey on aphids in gardens and crops, while others eat the tissues of plants that are considered weeds. The presence of certain kinds of flies can be used as indicators of water quality. The presence of midge larvae known as blood worms indicates a polluted environment. The fruit fly, Drosophila melanogaster, is the most intensely studied animal on Earth and is essential to genetic research. Several groups of flies that breed in decaying flesh have proven useful in helping police detectives to investigate human deaths. These flies have very specific food, temperature, and habitat requirements that can be used to establish not only the time of death but also if the body has been moved after death.
Flies are better known as pests because they are the most important carriers of disease that plague humans and other animals. These diseases have affected the movements of humans and changed the course of history. For example, tsetse flies prevented Europeans from colonizing parts of Africa because they spread deadly sleeping sickness in humans and nagana in cattle. Mosquitoes carry four different kinds of Plasmodium—protozoan single-celled animals that cause yellow fever, dengue fever, and malaria—and infect people with their bites. Even today more people die from malaria every year than all other diseases, car accidents, and wars combined. Until the use of insecticides, medicine, and the occasional window screen, humans were unable to live in some lowland areas without getting sick. Horse and deer flies infect wild and domesticated hoofed animals with several deadly diseases. In the tropics, blood-sucking black flies infect humans with parasitic worms that can cause blindness, while sand flies spread protozoans that, if left untreated, destroy all kinds of tissue and lead to death.
The mere presence of the larvae of bat flies, flesh flies, and bottle flies can cause health problems, especially in animals other than humans. Myiasis (my-EYE-ah-sis) is the infestation of an animal by fly larvae. The larvae of some species live in a wound and feed on the host's living or dead tissue and body fluids. Other species live inside the body where they feed on food inside the host's digestive system.
Still other species are attracted to eyes or food. Eye gnats (Hippelates) and face flies (Musca autumnalis) are attracted to the moisture produced around eyes. House flies (Musca domestica), little-house flies (Fannia), and latrine flies
Military surgeons have long noted that untreated wounds infested with maggots healed faster than treated wounds. This is because the maggots only eat tissues infected with bacteria. Using germ-free maggots sometimes prevented having to cut off arms and legs to stop infection. Antibiotics replaced maggots during World War II for treating serious infections. Bacterial resistance to antibiotics has led to a comeback of "maggot therapy," especially for wounds that will not heal.
(Chrysomyia) breed in filth, such as animal waste and garbage. They are considered not only a nuisance but also a potential health hazard when the adults are attracted to food at outdoor parties and picnics.
The larvae of fruit flies chew their way through citrus and other fruit and vegetable crops and are among the most destructive of all agricultural pests. Millions of dollars are lost every year because of the damage they cause, and millions more are spent on efforts to control them. The larvae of other crop pests, such as gall gnats, leaf miner flies, and root miner flies, weaken plants by boring through stems, leaves, and roots.
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