Cecidomyiidae are minute to small (0.5–3.0 mm), rarely larger (up to 8 mm, wing length 15 mm) flies with a delicate appearance. Except for a few genera with reduced wings, the eyes are holoptic. The mouthparts are reduced. Cecidomyiid antennae are notably long, with 12–14 segments, (sometimes fewer and up to 40 in some genera). The antennal segments either consist of a basal thickening and petiole or they are binodal, with a proximal node, an intermediate petiole and a distal node. Basal, medial, and apical whorls of hairs occur on the antennal segments. 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. The filaments are thread-like in the Porricondylinae and in all the Cecidomyiinae and take the form of long loops in the supertribe Cecidomyiidi). Ocelli are present only in the Lestremiinae. The wings are usually clear, rarely patterned. The wing bears microtrichia, often as scales, and some species have macrotrichia. The number of longitudinal veins is reduced. Only veins R1, R4+5, M3+4 and Cu1 are well developed in most species. The medial veins M1 and M2 are developed only in primitive groups, and the costa usually has a break just beyond vein R5. The legs are long and slender, without apical bristles.