[6] A DNA metabarcoding study published in 2016 estimated the fauna of Canada alone to be in excess of 16,000 species, hinting at a staggering global count of over 1 million cecidomyiid species that have yet to be described, which would make it the most speciose single family in the entire animal kingdom.

[7] A second similar metabarcoding study performed in Costa Rica also found Cecidomyiidae to be the most diverse family of flies, supporting this assertion.

In some species, whorls of loop-shaped sensory filaments are also found, the basal or medial one sometimes being reduced.

Some gall flies have only one (basal) whorl of hairs on the antennal segments, and the sensoria (transparent sensory appendages) differ in size and shape.

Gall midge larvae, and many adults, are orange or yellow in color due to carotenoids.

Supporting structures called apodema are located near the base of the genitalia in males; these are often equipped with two outgrowths.

[12][13] Many species are economically significant, especially the Hessian fly, a wheat pest, as the galls cause severe damage.

[15][16][17] In South Africa, Dasineura rubiformis has been deployed against the invasive Australian Acacia mearnsii; it oviposits eggs into the flowers which develop into galls, thus reducing seed production.

[18] Parasitoids hosted by Cecidomyiidae include Braconidae (Opiinae, Euphorinae), Eurytomidae, Eulophidae, Torymidae, Pteromalidae, Eupelmidae, Trichogrammatidae, and Aphelinidae.

A large number of gall midge species are natural enemies of other crop pests.

As the larvae are incapable of moving considerable distances, a substantial population of prey must be present before the adults lay eggs, and the Cecidiomyiidae are most frequently seen during pest outbreaks.

One species, Aphidoletes aphidimyza, is an important component of biological control programs for greenhouse crops and is widely sold in the United States.