Slender delicate flies, 3-9 mm long. Legs slender. Proboscis slender, much longer than head; labrum, mandibles, laciniae, and hypopharynx greatly elongate, stylet-like, enclosed in sheath formed by labium; palpus of male usually about as long as proboscis, with terminal segments often bearing dense tufts of long setae. Wing veins and usually most of the body and legs clothed with scales.