C. hyperboreus Staeg., C. islandicus K., C. tardus Butler, C. cucini Webb, C. major n.sp. and C. neocorax n.sp. were found to belong to the thummi-complex (chromosome arm combination AB, CD, EF, G). They can be characterized by primitive banding patterns, heterochromatinized centromeres, scarcity of inversion polymorphism, poor chromosome structures and frequently incomplete pairing of the homologous chromosomes. The six species have identical or very similar chromosome arms A, E and F, whereas the banding patterns of arms B, C, D and G as well as the position of nucleolar organizers and Balbiani rings allow clear distinction of the species. Few morphological differences could be detected between larvae and between adults. Freshwater species with salinarius-type larvae are found predominantly in northern areas. Their position in the phylogenetic system of the genus is discussed as well as the possible adaptive significance of the presence or absence of tubuli on the larval abdomen.