Midges and mosquitoes are members of the order Diptera, the group of two-winged insects which also includes flies. Midges, like mosquitoes, are humpbacked and many are similar in coloration (brown, black or gray). They are from different "families" within this order, and there are variations in their (1) mouthparts and food habits - mosquitoes have proboscis, a piercing/ sucking mouth; midges have well-adapted cutting mouth parts which slice through the host animal's skin to extract blood; (2) biology - midges lack scales on their wings which mosquitoes have; (3) midges swarm but mosquitoes don't. For a run-down on the Highland midge -
http://www.albawest.com/midges.html