Ctenosciara hyalipennis (Meigen, 1804)
Ctenosciara thiedei Thiede, 1977 (nomen nudum, see below)

Ctenosciara hyalipennis occurs in two seasonal forms, a spring form and a summer form, that differ in genital morphology, size and coloration. The summer form has been named "thiedei", but without any description and therefore the name is a nomen nudum. The differences between the two forms are quite significant and it has been suggested that they are in fact two different species; however, there are no differences in COI gene sequence between the two forms, indicating that at least on the level of this nuclear marker the two forms are identical (Heller and Rulik 2016). Therefore, the two forms are currently regarded as two seasonal forms of the same species. However, a new species has been described from Finland, Ctenosciara exigua Salmela et Vilkamaa, 2005, and this species is very similar to the summer form of Ctenosciara hyalipennis (see table below). It seems likely, therefore, that the summer form of Ctenosciara hyalipennis is actually a separate species, and identical with Ctenosciara exigua. This matter urgently requires revision; however, until more data are available I provisionally retain the summer form as a infrasubspecific form of Ctenosciara hyalipennis.

HYALIPENNIS (SPRING FORM)	HYALIPENNIS (SUMMER FORM)	EXIGUA
large	smaller	smaller
dark brown	lighter brown	lighter brown
gonostylus clavate	gonostylus straight	gonostylus straight
- - - -  COI gene sequence identical in all three taxa  - - - -

Table 1: Comparison between the two putative seasonal forms of Ctenosciara hyalipennis and the taxon Ctenosciara exigua Salmela et Vilkamaa, 2005.