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The thickness of the costa is variable in tenuicosta, although stagnalis is generally thicker. The shape of the apical white spot (with constricted middle) is right for tenuicosta. S.stagnalis has a rounded spot.
Although S.stagnalis is often recorded from greenhouses, I have looked at several thousand greenhouse Scatella, and never found stagnalis. In the Holarctic region, only tenuicosta and obscura are found in greenhouses. (paper in preparation).