Life history patterns in the northern expansion of dragonflies

As basically warm-adapted insects of tropical origin, the Odonata have evolved various life cycle patterns and cold-resistant stages in forms colonizing temperate areas. These patterns can be intricately regulated by photoperiod and temperature. To throw light on the evolution and modification of some of these patterns, my own previous work on the life cycles and photoperiodic responses of species with different seasonal patterns, growth rates and larval overwintering strategies is reviewed; Aeshna viridis and A. cyanea in S Sweden, and Coenagrion hastulatum, Aeshna juncea and Leucorrhinia dubia in S and N Sweden are considered. The recorded average life history duration always exceeded one year. Although voltinism and phenology vary within and between these species, the same basic two-phase pattern of larval photoperiodic responses interacting with temperature provides the framework for seasonal regulation. A long-day diapause, often effective in several instars, prevents emergence during late summer and early autumn. A short-day diapause (hibernation) then follows, with or without a visible termination of the long-day diapause. Thereafter, long days induce rapid development, permitting emergence during spring and summer. The occurrence of these responses at different stages of development and differences in their timing and intensity were found to produce different phenological patterns and to adapt individuals to different climatic conditions. The type of response of overwintering larvae to long days during spring is dependent on size, or developmental stage, and only larvae overwintering above a certain “winter critical size” (WCS) show rapid growth to emergence during the following season. As other, smaller larvae are prevented from emerging by the long-day diapause, a splitting of a year-class cohort is often observed early in the summer. WCS, which is genetically determined at the population level, is a key property in seasonal regulation at high latitudes, and it is instrumental in the determination of the size and location of the seasonal “gate” where emergence can take place. At least in the late-flying Aeshna species, WCS can also be modified in an appropriate fashion by local variation in temperature conditions. In the other, phenologically early species, overwintering close to WCS appears to be reduced by an earlier, less understood cohort splitting, probably partly involving a different timing of the short-day diapause in different instars. Partly speculative discussions of odonate life history changes and the evolution of larval hibernation are centred around the present results. The need for further research in some areas is indicated.