Dragonflies leave no ring or tag, so the migration was reconstructed chemically. Hydrogen isotope ratios in wing tissue record the water in which an individual grew, which varies predictably with latitude. Comparing that signature with where a dragonfly was caught reveals whether it had moved.
A study published in 2018, drawing on specimens spanning about 140 years, resolved a three-generation cycle. Adults emerging in the south in spring fly north and breed; their offspring emerge in late summer and fly south; a third generation develops in southern waters over winter and does not migrate, producing the following spring's northbound flight.
Individuals can cover substantial distances daily, and mass movements are visible along coastlines and lake shores in autumn alongside migrating raptors, which follow the same fronts and eat the dragonflies. Comparable multi-generation migration is known in some butterflies, but for dragonflies this is the clearest documented case.