True Flies

The true flies are a large and diverse group of commonly observed insects in the order Diptera, comprising more than 100,000 species. About 107 families of flies occur in North America.

Flies have distinctive, knob-like structures known as halteres on the back of their thorax. Halteres are highly modified from the hind wings of true flies, while the fore wings are membranous and used for flying. The twowinged character of the true flues is reflected in the Latin roots of the scientific name of their order, the Diptera, which means "two wings."

Some species of true flies are of great economic importance as pests of agricultural plants. Other species of flies are great nuisances because they bite humans and domestic animals in order to obtain blood meals, as is the case of mosquitoes, black flies, horse flies, and others. Some of these parasitic, blood-sucking species are also vectors of deadly diseases of humans, as are some of the blow flies and house flies that feed by scavenging dead organic matter. However, many other species of flies provide very useful ecological services, by helping to safely dispose of decaying carcasses and other organic debris, and by serving as predators or parasites of other, injurious insects.

In its popular usage, the word "fly" is often used in reference to insects that are not in the order Diptera, and are therefore not "true flies." The "fly" part of the name of a dipteran should be written separately, as in: house fly, horse fly, or black fly. However, when used to refer to non-dipteran insects, the "fly" portion of a species name should be appended to form a single word, as in: sawfly (species in the family Tenthredinidae, order Hymenoptera), dragonfly and damselfly (order Odonata), mayfly (order Ephemeroptera), stonefly (order Plecoptera), caddisfly (order Trichoptera), and butterfly (order Lepidoptera).