In June 1971, intense migratory activity was recorded in L. quadrimaculata L. in Belgium, France and the Netherlands. Enormous swarms gathered in the compound estuary of Zeeland. Part of them apparently moved up the river Scheldt and scattered over a large number of its tributaries in Belgium. The ultimate fate of this swarm was extinction. Compilation of literature data shows that migration in L. quadrimaculata is a cyclical event with a period of about 10 years. The phenomenon is thus related to population dynamics and should be density-dependent. There is a fundamental difference between conditions to migration (crowding, mass emergence, weather condition) and the ethological (optical interaction-synchronization) and ecological stimuli. The latter are believed to act as pacemaker to the former. The ecological mechanism postulated is, indeed, derived from a hypothetical parasite-host relationship, the most likely parasite being a trematode larva. This could produce an internal irritation, reinforcing the optical synchronization.