No single monarch butterfly completes the migration

Each autumn, monarch butterflies from the northern United States and Canada fly up to 3,000 miles to a handful of fir-forest groves in central Mexico — a journey of an insect that weighs about half a gram.

The astonishing part is that no individual butterfly makes the round trip. The monarchs that fly south are a special “super generation,” born in late summer, that lives eight or nine months — many times longer than the few weeks a normal summer monarch survives. They overwinter in Mexico, then start north in spring.

Getting all the way back takes a baton-pass of three to four short-lived generations: each lays eggs farther north and dies, and its descendants press on. The monarchs that arrive in Canada are several generations removed from the ones that left it.

That means every fall’s southbound travelers are navigating to overwintering groves they have never seen, guided by inherited cues rather than memory.