Fig. 5 Diagrammatic sketch of the topographic landscape in the Hengduan Mountains (a) and the Himalayas (b) and the potential in-situdisplacement (short arrows) and long-distance dispersal (long arrows) of caddisflies during the LGM. Different colours of the caddisflies indicate high-elevation species (yellow) and low-elevation species (green). During the glacial period, which means a cooler climate, a vertical displacement along the elevational gradient (in-situdisplacement) may occur on both high- and low-elevation caddisfly species. However, compared to the Himalayas, a wider extent of mountain ranges along the North-South orientation in the Hengduan Mountains gives the possibility for species to disperse further south (long-distance dispersal). During the interglacial period, the same dispersal or replacement may happen but in the opposite direction. Therefore, geographic connection and isolation may repeatedly occur during climate fluctuation, thus forming the so-called “species pump” effect and promoting speciation or diversification, according to the MGH (Mosbrugger et al. 2018).