The aim of this study was to analyze the relationship between the surface development of pupal thoracic horn and adaptation to low oxygen concentration in different species of chironomids. A total of 773 samples of larvae were collected in lakes and streams together with measurements of dissolved oxygen and collection of pupal exuviae. The area of the base of the thoracic horn (ring organ, RO) and abdomen length (ABD) of pupae were measured in forty-eight species of Chironomini. RO was significantly correlated with ABD, therefore, the ratio of ring organ/abdomen length (ROA) was calculated to provide a measure less dependent on body size. Chironomus plumosus Linnwus, Chironomus riparius Meigen and Glyptotendipes pallens (Meigen) had the highest ROA, Microtendipes britteni (Edwards), Paralauterbomiella nigrohalteralis (Malloch) and Pseudochironomus prasinatus (Stxger) had the lowest ROA. ROA was still correlated with ABD, so residuals from the regression line (RROA) between RO and ABD were also calculated. An examination of the residuals revealed that some species (C riparius, Cladopelma virescens (Meigen) and Einfeldia pagana (Meigen)) had a RO larger, whereas other species (M. britteni, P. prasinatus and Stenochironomus gibbus (Fabricius)) had a RO smaller than predicted by regression with ABD. RO was significantly correlated with the 25(th) percentile of dissolved oxygen (PCTL) measured in conjunction with benthic macroinvertebrates samples. Hemoglobin concentrations measured in six species were not correlated with ABD and RO, a high hemoglobin content was measured both in small species with a small RO (Polypedilum nubeculosum (Meigen)) and in large species with a large RO (G. pallens). The ring organ is proposed as a morphological trait useful to indicate oxygen concentration in the environment, while other factors such as hemoglobin content are also likely to affect the species ability to respond to oxygen shortage.