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.
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Gall...galling of the fruit; the gall ovoid, more or less hairy, wings present, although not so large as in healthy fruits; window-pit distinct (B. pubescens).
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[Record from B. populifolia was from a cultivated tree in Ontario where "an introduction of infected fruits of Palaearctic birch species is supposed". Only otherwise recorded from North America on introduced B. pendula in Pennsylvania. Host records on other Betula species were all from outside North America (sometimes on North American species in cultivation elsewhere).]

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