Thoracic horn structure in Orthocladiini pupae (Diptera: Chironomidae: Orthocladiinae)

The pupal thoracic horn in Chironomidae is a respiratory organ specialized for oxygen adsorption and the horn surface was suggested as a measure of a species’ oxygen requirements. This structure was analyzed in 31 species belonging to the tribe Orthocladiini within the genera Cricotopus, Eukiefferiella, Orthocladius and Tvetenia. Optima for dissolved oxygen and water temperature were calculated for each taxon using information from a large database. The species with extensive horn are the most resistant to low oxygen levels and high temperature, and no significant correlation was detected between horn surface and species optima. The species of the genus Cricotopus, characterized by reduced thoracic horn, tolerate lower oxygen levels then Orthocladius: it is suggested that higher tolerance is bound to more frequent respiratory undulations. At taxonomic levels below tribes behavioural and physiological adaptations more than morphological ones may explain the results