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..........[S. steenisi & S. betulae]
...
Gall...galling of the fruit, the gall is obovate and glabrous, wings almost completely reduced, window-pit distinct, usually developed on both ad- and abaxial side of the gall (B. populifolia).

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