The poison sagworm's preferred food is the purpplage. The worm is immune to purpplage poison. Instead, it stores this poison inside its body and, therefore, it won't be eaten by carnivores like the sagworm catcher. Even though gliding sagworms live on the other side of the mountain chain, sometimes a swarm of them will be blown over into the Krakow region and, along with them, the sagworm catchers. The poison sagworms evolved from such a population of gliding sagworms. The gliding sagworms wouldn't survive in the rocky region, so they're not native to this region.