The signpost is genetic memory

The Chestnut Tiger is a very graceful looking butterfly, but despite its elegant appearance, it is known for its long-distance migration of over 2,000 km. During the winter, they spend their time in warm areas such as the Nansei Islands and Taiwan, and in the summer they move north to spend their time in the cool forested regions of Honshu. Then, in autumn, they return to the warmer south. In Japan, there are very few butterflies that travel such long distances.

There are dragonflies that come to Japan on similar long-distance migrations, but they have bodies that are harder to fly than dragonflies and make good use of updrafts and wind to travel long distances.
It takes so much energy to migrate and it is a very difficult path. Despite this, the strength of the creatures that cross the ocean and come to Japan every year is astonishing.