Key to the species of Nearctic Ditomyia

1 Wing patterned (Figs. 2 & 11); male terminalia as Figs. 13–15 and female terminalia as Fig. 16; eastern Nearctic…............................................................................................... D. euzona Loew

- Wing hyaline; terminalia not as above…................................................................... 2

2 Antennal flagellomeres pale yellow (sometimes becoming light greyish-brown distally), nearly concolorous with pale yellow pedicel and scape, black spot marking base of each antennal seta strongly contrasting against yellow background color of flagellomere (Fig 5); mesonotum black to very dark brown (Fig. 19); male terminalia as Figs. 20–22, female terminalia as Fig. 23; eastern Nearctic…...................................................................... D. potomaca Fisher

- Antennal flagellomeres grey-brown, contrasting with pale yellow scape and pedicel, black spot marking base of each antennal seta not strongly contrasting against background color of flagellomere (Fig. 6); mesonotum pale yellow with light brown stripes (Figs. 7 & 24); western Nearctic…........................................................................ 3

3 Macrotrichia of wing evenly distributed on basal third (cell br+bm with evenly spaced macrotrichia) (Fig. 4); male terminalia as Figs. 25–27, female terminalia as Fig. 28, with second segment of cerci broadly rounded and not tapered apically; female fore tibia ~1.1 times as long as fore basitarsus; British Columbia to California ............... … D. sohnsi Fitzgerald n. sp.

- Basal third of wing with macrotrichia unevenly distributed and with large bare areas (cell br+bm mostly bare) (Fig. 1); male unknown; female terminalia as Figs. 8–10, with second cercus more narrow and slightly tapered to a round point apically (Fig. 10); female fore tibia ~1.5 times as long as fore basitarsus (Fig. 7); Washington …............ D. elongata Fitzgerald n. sp.