True Flies

The true flies have a complete metamorphosis, characterized by four stages in their life history: egg, larva or maggot, pupa, and adult. Fly maggots are soft-bodied, legless, and worm-like. Most flies are terrestrial animals, but many species spend their larval stages in aquatic habitats, with the adults emerging to the terrestrial environment.

The hind wings of flies are modified into small, distinctive structures known as halteres, which resemble a tiny, stalked knob. Halteres are thought to be used as an aid in achieving a sense of balance and direction. The front wings of flies are membranous and functional in more usual ways, and are used for flying. The smaller flies have very rapid wingbeats, typically 200-400 strokes per second, and as great as 1,000 per second in tiny midges in the genus Forcipmyia.

Other characteristic features of true flies include the division of their tarsus (that is, the leg segment immediately below the tibia) into five segments. Most flies have mouth parts of a haustellate form, that is, adapted for sucking, rather than for chewing. Flies are typically small and soft bodied, and some are minute in size.

There are many feeding strategies among the flies. Most species feed on soft foods and organic debris, while others eat nectar, some scavenge dead bodies of animals, or are predators of smaller arthropods, or are blood-sucking parasites. These various species have mouth parts and behavioral adaptations to their specific modes of feeding and living. Mosquitoes, for example, have piercing and sucking feeding structures, while flies that feed on soft organic materials have sponging or lapping mouth parts.

Some species of flies that feed on plants inject a growth-regulating chemical into the stem, which causes an abnormal tissue, called a gall, to develop. The gall provides habitat for the feeding and development of the larvae of the fly. Gall-inducing species occur in the families Agromyzidae, Cecidomyiidae, and Tephritidae, as well as in other orders of insects.

There is a great diversity of species of flies, with more than 100,000 species being identified so far, and more than 100 families. The families are mostly distinguished using characters related to the morphology of the antennae, legs, wing venation, body bristles, and other anatomical features. Habitat and other ecological information may also be useful. Aspects of body chemistry may also be used in the identification of closely related species, particularly the chemistry of enzymes and nucleic acids. The diversity of flies is too enormous to discuss in much detail in this entry—only a few prominent examples will be described in the following sections.