An order of the class Insecta known as the true flies and so named because they possess only two wings. This characteristic, together with a pair of balancers or halteres, distinguishes flies from all other orders of the Insecta. The three names for members of the order, known commonly as flies, gnats, or midges, form a part of the common names of most families, genera, and species of the order. The term fly also forms a part of the compound names of the insects in many other orders, such as butterfly, Mayfly, and chalcid fly, but when used alone it is correctly applied only to the members of the Diptera. The forms most commonly known as maggots are actually dipterous larvae, and keds (Hippoboscidae) are parasitic forms of flies that have lost their wings. The Diptera are the most important group of insects considered in medical entomology. Many, especially the mosquitoes, are the vectors for numerous parasites and diseases.