The immature or larval stage of the fly is called a maggot. While some of these warrant the designation of pest, particularly when they feed on agricultural crops, a number of these are beneficial too. The maggots of some species play an important role in the decomposition of organic matter. The maggots of some species, raised under sterile conditions, are currently being used by some doctors as microsurgeons to clean necrotic tissue from difficult wounds, with excellent results. The maggots of some flies are predatory. Some fly species have been used for biological control because their larvae are parasitic on the larvae of some plant pests. The maggots of some flower flies can be found feeding on aphids.
Flies belong to the insect order Diptera (from the Greek, di for two and pteron for wing).
Flies have one pair of normal wings. Behind each wing sticking out from the thorax (like a pin stuck in a pin cushion) is a greatly modified wing called a haltere, a slender stalk topped with a knob. Halteres are sense organs which are essential for stable flight. In flight, the halteres move up while the wings are moving down and down while the wings are moving up. In many species they also oscillate during walking. The halteres of the crane fly can be easily seen. (These large flies, often seen resting on walls and ceilings, are sometimes called ``mosquito hawks'' or ``giant mosquitoes'', but they are not predators and do not bite. The adults of some species do not eat. California has more than 400 crane fly species.)
While you are out exploring, watch for flies. Many of them look like bees or wasps (bees and wasps have four normal wings). One species of robber fly even looks like a bumble bee.
Ron Lyons (volunteer 1990-1999)