Adaptation of Aeshna caerulea (Ström) to the severe climate of its environment (Anisoptera: Aeshnidae)

Because the actual range of A. caerulea is restricted to the high latitudes of northern Europe and the higher mountains of central Europe, the species is seen as a typical glacial relict, which needs a cool subarctic climate. However, the thermal demands of the larvae are very complex. Young larvae are thermophilic and need more or less constant temperatures of about 20°C, while older larvae generally tolerate the lower temperatures but prefer higher values for moulting and digestion. Due to the brown colour of the water, the dark (peaty) bottom ground and/or the shallowness, typical breeding sites of A. caerulea, are warm habitats even in cold climates, so that quick larval growth is ensured. In contrast to the aquatic microhabitat, the environment of adults is cool, severe and appears uncomfortable for dragonflies. However, the unusual basking behaviour and physiological specializations, especially the physiological colour change in males, make the species well adapted to such microclimatic conditions. Adult preference for lower temperatures thus restricts the distribution of A. caerulea to the cooler climates of the high North and subalpine and alpine zones.