Fly is any of a large group of insects that have one par of wings. The word "fly" is frequently used to only to refer the common housefly. However, there are more than 80,000 species of flies, including horseflies, mosquitoes, midges, gnats, and botflies. On the other hand the term "fly" is incorrectly applied to many flying insects, such as dragonflies, butterflies, and scorpion flies. Despite their names, these insects are not true flies because they have two pairs of wings.
   True flies make up one of the largest groups of insects. Although they live in nearly all parts of the world, probably the greatest variety inhabit the tropic's. In temperate regions, flies are particularly numerous in forests. The common housefly (Musca domestica) is one of the few species found nearly everywhere.
   Although probably best known as pests, flies serve certain valuable functions. They pollinate flowers and serve as food for animals. They destroy decaying organic matter, and some kinds destroy other insect pests. Certain species of fruit flies are useful in the study of genetics.

Housefly (Musca domestica)