Dichoptic - when the eyes are markedly seperated. More primative groups have distinct head with well developed mandibles, mosquitoes, mouthparts chewing, brushes maybe present. Larval mouthparts reduced to a structure called cephalopharyngeal skeleton, an obvious structure is the mouth-hooks, which probably are modified maxillae and the pharyngeal plates. Larvae maggots with no apparent thorax or scleratized parts. Legless and wormlike - bees similar but no mouth hooks.
Some groups have a puparium formed by shed last larval cuticle .
Adults feed on blood. Many are metallic blue or green in color. Many are scaverngers, feeding on dead animals, some are parasites of other insects.