Green’s Dictionary of Slang

(US) a tiny biting fly, a midge.

1848		H.D. Thoreau Maine Woods 2: In the summer myriads of black flies, mosquitoes, and midges, or, as the Indians call them, ‘no-see-ems,’ make travelling in the woods almost impossible [DA].
1909		Carr & Chase in ‘Word-List From Aroostook’ in DN III:v 414: noseeums, n. Midges.
1914		Outing (N.Y.) June 187/2: A No-seeum is a species of guerrilla gnat having two stingers in each foot and nine in the head [DA].
1949		Pacific Discovery May–June 18/2: We regretted leaving everything except those diminutive torturers, the ‘no-see-ums’ [DA].
1986		C. Hiaasen Tourist Season (1987) 359: Wiley swatted no-see-’ems in the darkness.