The Countrey is strangely incommodated with flies, which the English call Musketaes, they are like our gnats, they will sting so fiercely in summer as to make the faces of the English swell?d and scabby, as if the small pox for the first year. Likewise there is a small black fly no bigger than a flea, so numerous up in the Countrey, that a man cannot draw his breath, but he will suck of them in: they continue about thirty days say some, but I say three moneths, and are not only a pesterment but a plague to the countrey. -- John Josselyn, Two Voyages to New-England (1674).
You need the right mind-set to deal with black flies
http://www.mainecampsite.com/blackfly.htm