|MadSci Network: General Biology|
Thank you for your question about Houseflies. Houseflies belong to a group of flies called Filth Flies. Filth Flies get their names because they are important for breaking down decaying material. Any species that is important for breaking down dead organic material is called a saprophyte. If their were no saprophytes, then there would be no top soil for plants to grow in. Saprophytes, such as the Filth Flies, digest dead trees, dead animals, and even animal wastes, such as feces. (It is mostly because they break down feces that Houseflies and their cousins are called Filth Flies!)
It is the young fly (called a maggot) that lives as a saprophyte. When you see a fly buzzing around, it is in its adult stage. As an adult, the fly is important for providing food for insect-eating animals, such as frogs. (To see the different stages of a housefly's life, see this picture on the website of Clemson University. The white ones are the maggots. West Virginia University has a web site that describes the life cycle, and many other interesting facts about Houseflies.)
While most of the Filth Flies play a role in their natural ecosystems, Houseflies are a little different. They are a single species (Musca domestica) that has adapted to living off of human waste. This includes not only the organic material in their houses and dumps, but also the fecal waste of their animals, such as cattle. Because Houseflies, unlike most Filth Flies, do not play a role in maintaining a natural ecosystem, they are considered pests.
Other species that have adapted to live off of humans are certain species of mosquitos (such as the Asian Tiger Mosquito named Aedes albopictus) and certain roaches (such as the American Cockroach named Periplaneta americana). These species, like the housefly, have spread all over the world, wherever humans have gone. Not all flies (or roaches or mosquitos) are human pests, and so people must be very careful in using pesticides to kill all of one kind of insect in an area. It would be very bad for natural ecosystems if all of the natural Filth Flies were killed off!
Ecosystems are very interesting, and biologists spend their lives studying why different species are important. It is great that you're already thinking about how species fit in with their environments! Please write again if you have any comments or questions!
Try the links in the MadSci Library for more information on General Biology.