Key to Thetidia species of the Baikal region

1. Forewing: white discal spot present; antemedial transverse line wavy (rarely not pronounced). In male genitalia (Figs. 33H, 33I) socii significantly widened in base, slightly curved; uncus comparatively wide, its apices does not reach apices of socii; vinculum with moderately deep medial notch; aedeagus comparatively curved. In female genitalia (Fig. 37C) sterigma comparatively large, about 0.8 mm in length, without antero-medial hollow......................... T. smaragdaria (Fabricius, 1787)

- Forewing: discal spot absent; antemedial transverse line not wavy, angled in middle of discal cell. In male genitalia (Figs. 33F, 33G) socii rather narrow, more curved than in T. smaragdaria, hook-shaped; uncus comparatively narrow, its tips more distal than tips of socii; vinculum with strongly deep medial notch; aedeagus slightly curved. In female genitalia (Fig. 37B) sterigma smaller than in T. smaragdaria, about 0.5 mm in length, with small antero-medial hollow.................................................................................................. T. chlorophyllaria (Hedemann, 1879)