Every one hundred years, the mantid young hatch in great numbers. Inevitably the swarm migrates east, devouring everything in their path, until they reach the Serpent's Spine. There, thousands are slain by the pandaren, just as they were slain by the mogu who first erected the wall.
Pandaren scholars have tried to understand this cycle, but have reached no consensus. Why do the mantid allow so many of their young to be slaughtered, generation after generation? Only the mantid can say for sure. This cycle has become so predictable that the guardians of the wall plan for it every century. The Shado-Pan, and the mogu before them, know when the swarms will come, and plan their defenses accordingly. As long as the wall continues to stand, there is hope for Pandaria against the seemingly endless swarms of the mantid.