W. F. Wulker 1991: Chironomus fraternus sp. n. and C. beljaninae sp. n., Sympatric Sister Species of the Aberratus Group in Fennoscandian Reservoirs. - Entomologica Fennica 2: 97-109.
The karyotypes, larvae, pupae and adults of Chironomus fraternus sp. n. and C. beljaninae sp. n. are described. The chromosomes of the two species have almost identical banding patterns; the polymorphisms in at least five of the seven chromosome arms are also the same. However, the chromosome number is 2n = 8 in C. fraternus (AB, CD, EF, G), and 2n = 6 in C. beljaninae (GAB, CD, EF). No hybrid individuals between the two species have been found as yet. Only slight diagnostic differences could be shown between the adults, pupae and larvae of the two species. The sympatric occurrence of C. fraternus and C. beljaninae in lakes of central Finland and Sweden points to speciation due to chromosome fusion. The species are closely related to C. aberratus, C. sororius and C. jonmartini; the five species may therefore be regarded as forming the "aberratus group" of the thummi complex of Chironomus.