Flight characteristics and phylogeography in three large-bodied diving beetle species: evidence that the species with expanded distribution is an active flier

Diving beetles play an important role in fishless freshwater communities. The genus Cybister is included in the Japanese Red Data List owing to its diminished population size. The phylogenetic relationships and genetic structures of Cybister chinensis and Cybister brevis, whose populations are declining, and Cybister tripunctatus lateralis, whose population and distribution is increasing, are poorly understood and must be addressed in future conservation efforts. In this study, we investigated the flight behaviour and phylogeography of the three Cybister species. Cybistyer tripunctatus lateralis and C. brevis flew well in the spring, and the proportion of flight in C. tripunctatus lateralis increased again after the reproductive season. However, C. chinensis did not fly. Relatively, among the three species, C. tripunctatus lateralis has the largest forewings. Phylogenetic analysis based on mitochondrial DNA revealed that C. chinensis showed genetic differentiation between the eastern and western regions of Japan, whereas C. tripunctatus lateralis showed no regional trends. Furthermore, C. brevis showed an intermediate trend between the two species. These findings suggest that C. tripunctatus lateralis has been expanding the area of its distribution rapidly through flight dispersal in recent years. Cybister chinensis flies infrequently, raising concerns about metapopulation fragmentation.