It's already happened twice to me that people (one of them was a Canadian, the other was a fluent English speaker) didn't know that word - midge - when I used it as a synonym for 'mosquito'. They said that it was not common at all. Is that word out of use indeed, or maybe it's still common in some countries / provinces?
They are a form of minute fly, and are generally found in massive clouds - see here for more information:http://medent.usyd.edu.au/fact/bitmidge.htm
For information on mosquitoes: http://en.wikipedia.org/wiki/Mosquito