Continental scale migration by the common green darner dragonfly - mosquito sashimi and mixed genes

Dragonflies are known both as voracious predators and unusually strong fliers. Among insects with fully aquatic immatures, they are nearly unique in that adults are capable of long distance, directed dispersal under their own power, i.e., without relying primarily on wind for transport. A number of Anisoptera species are known to undertake regular seasonal migrations, among which the North American Common Green Darner, Anax junius, is perhaps best known. Several studies combining radio telemetry, visual observation, and stable isotope analysis show that individuals can move many hundreds of kilometers from their point of origin, occasionally covering up to 150 km in a day. They apparently alternate bouts of active migration with periods of several days when they linger in a confined area, sometimes in large numbers, to feed and replenish fat stores. Unlike many migratory insects, Green Darners evidently mature sexually en route, and mating pairs and ovipositing females have been observed in migratory swarms. This apparent “bet-hedging” strategy of reproduction results in extensive gene exchange over long distances. Individuals throughout the entire eastern US, at least, effectively form a single population. Interestingly, studies of larval development and emergence phenology show that, through most of the species’ range, only a fraction of each local subpopulation migrates. Early emergers reproduce in or near their natal pond. Initial results suggested that early and late-emerging adults might be temporally isolated, but further study of phenology and genetics shows this not to be so. One of many unanswered questions, then, is what cues determine the tendency of adults to migrate if they emerge late in the year.