1. Either the axil of the scale is galled, or the galled fruit is distinctly coalescent with the scale; window-pit absent..........skuhravae sp. n.
— Galling of the fruit, the gall is in ripe situation never coalescent with the scale..........2
2. Window-pit absent or indistinct, gall subrotund and glabrous, wings of fruit often completely reduced..........tarda sp. n.
— Window-pit(s) distinct..........3
3. Often a window-pit on both sides of the gall, one of them large; gall glabrous, wings of fruit often completely reduced; East-Nearctic species..........brevipalpis sp. n.
— Only one window-pit developed; gall more or less hairy, wings present..........4
4. Gall usually concave on ad-axial side, window-pit irregularly formed; West-Nearctic species..........steenisi sp. n.
— Gall usually convex on both sides, window-pit subrotund; mainly Palaearctic species..........betulae Wtz.
...
Gall...galling of the fruit; the gall is obovate and usually concave on ad-axial side, more or less hairy, wings present, although not so large as in healthy fruits; window-pit distinct (B. occidentalis).

- J. C. Roskam: (1977) Biosystematics of insects living in female birch catkins. I. Gall midges of the genus Semudobia Kieffer (Diptera, Cecidornyiidae)^©