Signal flies are very variable in external appearance, ranging from small (2.5 mm), slender species to large (20 mm), robust individuals, often with body colours having a distinctive metallic lustre and with face and wings usually patterned with dark spots or bands. The head is large. Frontal bristles on head are absent. Two orbital bristles are on the head. The frontal stripe is pubescent and the arista is more or less long and pubescent. The antennal grooves are deep and divided by a median keel. Radial vein 4+5 bears bristles. The costa is without interruptions and the anal cell is elongated, bordered on outer side by an arcuate or straight vein. The abdomen of male has five visible segments and the female has six.