Initially, the life cycle was going to be synchronised with the half-year, so that the spring hatchlings had reached breeder stage by the end of the same summer, and the creatures never needed to survive a winter at either pole. We abandoned this idea because it would have made sentience tricky to achieve: if you can't pass knowledge and communication between generations, you have a hard time building a civilisation. So instead we decided that the larvae would _also_ have to be able to hibernate, and would last several years at the same pole where they were born, before finally breeding and migrating to the other pole; I suppose this means the only remaining advantage of migration is not having to have winter- resistant eggs, which isn't as convincing a justification as not needing to have winter-resistant anything.
We thought this would be a fun alien to evolve sentience in, because of the fact that the populations at each pole would constantly exchange children but would never communicate any _information_, so there'd be no particular reason why they'd reach (for example) the same tech level at the same time, or speak the same language, or anything. Presumably someone would eventually manage to establish pole-to-pole communication by sellotaping Pioneer-type diagrams to breeders before they took off... In the meantime, though, child- rearing would be entirely communal because nobody knew the ancestry of anyone, and the annual arrival of young would be a stork-style delivery from heaven.