Small flies, 1-6 mm long, slender to moderately robust. Female with biting and sucking mouthparts; bloodsucker of vertebrates or insects, or predator. Male with plumose antennae. Wings held horizontally at rest one above the other, sometimes strongly patterned. Early stages usually found in moist or aquatic habitats; larva apneustic, often elongate in form, and a strong swimmer.