Migrating dragonflies coming in from the North Sea in north-western Germany in the autumn of 2021 prompted the authors to consider the cause and biological function of undertaking a risky crossing of the open sea. Individuals of the most prevalent species, Sympetrum striolatum, were all mature, while some sampled specimens were old. In Corbet's system of spatial displacement, published in his seminal book “Dragonflies: behaviour and ecology of Odonata”, S. striolatum is provisionally treated as a species exhibiting seasonal flights to and from aestivation refuges but not as a migratory species. In fact, the species belongs to a group of autumn migrants that begin mass movements when mature. An analysis of our observations and the available literature led us to the conclusion that the chapter “Migration beginning in the reproductive period” of Corbet's book treats the subject inadequately. We argue that the flight type assessment of S. striolatum must be corrected accordingly: it is also a true autumn migrant whose mass flight is particularly noticeable in high mountain ranges and in coastal areas. However, there remain question marks over the biological significance and evolutionary benefits of mature dragonflies performing such risky mass flights over open sea.