Migratory dragonflies leave a chemical record of where they grew up. Hydrogen isotopes in the wing chitin are fixed at emergence and reflect the water the larva developed in, so a captured adult carries a stamp of its natal latitude that travel cannot erase.
Applying that method to museum and field specimens of Anax junius spanning more than a century, researchers reconstructed a multi-generation cycle: a spring generation emerges in the south and flies north, breeds there, and its offspring emerge in late summer and fly south. A third, resident southern generation overwinters without a long flight and starts the sequence again. Individual movements of hundreds of kilometres have been logged by radio telemetry, with darners advancing in stages rather than continuously.
What remains unclear is how the insects navigate. There is no plausible mechanism for learning a route that the parent generation flew, so orientation must be inherited or cued by the environment, and neither the cue nor its encoding has been demonstrated.