The Emperor Dragonfly, Anax Imperator, lives and develops underwater as an immature. It is most abundant in Europe and neighboring areas of Asia and Africa. The adults emerge from early spring until mid-summer when conditions are favorable for adult feeding and reproduction. How do they know when to emerge?
Philip Corbet* studied dragonfly development under a variety of conditions and found that daylength at the time of the molt to final larval instar was critical. If the daylength was increasing (as it does from December to June), the immature would continue development and emerge as an adult. If the daylength was constant or decreasing (as it does from summer to winter), the final instar larva would enter diapause, that is, it would continue to feed, but development was halted.
After several months of diapause, development can proceed but does not if the water temperature is below 10 centigrade. This prevents these dragonflies from becoming adults in the winter cold. In spring, when the water begins to warm, the dragonfly immature will begin the process of metamorphosis to an adult that takes about 30 days. In this way, the Emperor Dragonfly emerges in favorable environmental conditions and not the inhospitable temperatures of late autumn and winter.
*PHILIP S. CORBET. Environmental Factors Influencing the Induction and Termination of Diapause in the Emperor Dragonfly, Anax Imperator. Journal of Experimental Biology 1956 33: 1-14.