The karyotype of Chironomus tenuistylus, unlike that of C. longistylus, lacks large heterochromatic blocks at the centromeres and a nucteotus in arm C. Also, the banding pattern in 4 of the 7 chromosome arms differs between the species. The banding patterns belong to the most primitive ones in the thummi-complex. An inversion polymorphism has been found in arms A and B of C. tenuistylus, in arm B (rarely D and F) of C. longistylus. The larvae of C. tenuistylus have a distinct dark stripe on the clypeus and blackish coloured antennal pedicel and gula, and also differ in certain head measurements. Male adults of C. tenuistylus are larger than those of C. longistylus and the anal point of the male hypopygium is not as broad. Both species show an holarctic distribution (Fennoscandia and northern USA). The breeding habitats overlap to some extent, but C. tenuistylus apparently prefers polyhumic waters, whereas C. longistylus prefers flat, eutrophied lakes and pools. Adults of C. tenuistylus emerge in autumn, those of C. longistylus in summer.