Abstract: The Qinling-Daba Mountains, serving as the natural boundary between the Oriental and Palaearctic regions, represent a critical ecological transition zone. Previous studies have demonstrated how this mountain range acts as a geographical barrier for the northward expansion of Oriental species and vice versa. However, the role of the Qinling-Daba Mountains in shaping the distribution of species distributed across the Oriental-Palaearctic region remains poorly understood. Here we performed comparative phylogeographic analysis to investigate three species of the exclusively aquatic holometabolan order Megaloptera, i.e., Protohermes xanthodes, Acanthacorydalis orientalis, and Neochauliodes rotundatus, which are co-distributed in a broad range over the Oriental and Palaearctic regions. Our results show that except for the northeastern population of P. xanthodes, the three species do not exhibit clear geographic boundaries in their population genetic structure. The ancestral distributions of these species are primarily located within western Central China, North China, and Northeast China. Notably, populations from western Central China appear to have served as the source for subsequent dispersals to surrounding areas, with the Qinling-Daba Mountains emerging as a crucial dispersal corridor. Thus, the present overlapping distribution of these three species was formed by multidirectional dispersals in each species. All three species have undergone population contraction from the Last Glacial Maximum to the present, with their suitable habitats likely shrinking due to Pleistocene climate fluctuations, and temperature acting as a key factor influencing their distribution ranges. Our study investigates the phylogeography of aquatic insect species across the Oriental-Palaearctic boundary, providing novel insights into how complex terrain influences the distribution of species.

Key words: aquatic insects, Oriental, Palaearctic, phylogeography, Pleistocene climatic changes