Flies are also characterized by having a pair of balancing organs, called halteres, located just back of the base of the wings. Halteres are used for balancing in flight. There are a few flies, mostly parasites or inhabiting islands or alpine areas, that have no wings at all. The active immature stages of flies are called maggots, and most live in water or in some type of moist, rotting plant or animal tissue. There are exceptions, however, that feed only on living plants or animals.
Maggots living in water usually have some type of breathing tube or gills with which they gain air.
There are more than 110,000 described species of flies in the world. Bee flies are usually fast flying insects with appearances of small bees, while Robber Flies are commonly larger, slower flying, and a few resemble Bumble Bees. A fly known as the Delhi Sands flower-loving fly was designated as an endangered species in 1993 by the U.S. Fish and Wildlife Service. This species has been found in sandy dunes and rolling terrain of two counties in southern California, and has been the subject of much controversy regarding the protection of some species of animals. Flies are found throughout the world, and exist in areas from the Arctic to the rain forests on the equator.