Butterflies

Nothing about metamorphosis is a gentle costume change. Once a caterpillar seals itself inside its chrysalis, a surge of hormones dissolves almost the entire body into a kind of living soup, muscle, fat and organs breaking down together, with only a handful of tissues, the heart, part of the nervous system, and clusters of dormant cells called imaginal discs, surviving intact.

Those imaginal discs were there from the very start, folded up inside the caterpillar since before it hatched, and it is out of that reserve, not out of anything newly grown, that the adult's eyes, wings, legs and antennae are finally built.

The monarch takes that transformation further than most. A single monarch never makes the full round trip between its northern breeding grounds and its overwintering forests in Mexico; the journey south is finished by one generation, and it takes three or four more generations, born and dying along the way, before a monarch descended from that same butterfly returns north the following spring.

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