Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect

Insect migration is globally ubiquitous and can involve continental-scale movements and complex life-histories. Apart from select species of migratory moths and butterflies, little is known about the structure of the annual cycle for migratory insects. Using stable-hydrogen isotope analysis of 852 wing samples from eight countries spanning 140 years, combined with 21 years of citizen science data, we determined the full annual cycle of a large migratory dragonfly, the common green darner (Anax junius). We demonstrate that darners undertake complex long-distance annual migrations governed largely by temperature that involve at least three generations. In spring, the first generation makes a long-distance northbound movement (>650 km) from southern to northern range limits, lays eggs and dies. A second generation emerges and returns south (>680 km), where they lay eggs and die. Finally, a third resident generation emerges, reproducing locally and giving rise to the cohort that migra...