Key to Thalera species of the Baikal region

1. Male genitalia: costal process of valva is more flat than in T. chlorosaria and moderately sclerotized (Figs. 34I–34K); female genitalia: medial sclerite of sterigma with slightly concave lateral margin (see everted sclerite from lateral view, Fig. 38C)................................................................................ T. fimbrialis (Scopoli, 1763)

- Male genitalia: costal process of valva is more strongly sclerotized and bent outwards (Figs. 34L–34N, 35A); female genitalia: medial sclerite of sterigma with straight lateral margin (Fig. 38D)........................ T. chlorosaria Graeser, 1890