We investigated the emergence and life cycle of the endangered Maghrebian endemic Gomphus lucasii at the Seybouse River in north-eastern Algeria. Starting in mid-April, their emergence, typical of spring species, was highly synchronized and was achieved within two to three weeks. EM50 was reached in three days. Larval sampling indicated that the synchrony was achieved through larvae overwintering in the F-0 stage. Noteworthy was the concomitant presence of a junior cohort throughout the year demonstrating that the species completes a generation in two years. Thus, contrary to what has been reported previously, we argue that G. lucasii is a semivoltine species with a 'slow' developmental rate congruent with its distribution in high-risk permanent habitats. This finding has important conservation implications for this threatened endemic species which is facing severe anthropogenic pressures in the context of global changes.