A host tree is a species of tree native to Rathillien whose leaves are like migratory insects, which migrate from a northern host to a southern host tree and back every year. They hatch from cocoons on the northern host in the fall and fly to their southern host, where they over-winter before returning to the northern host in the spring. When the cocoons are about to hatch, a rap on the tree's trunk can initiate the hatching and flying away. The migratory leaves are golden-veined.