Even the high-level, detail-free version of metamorphosis we get taught in grade school is pretty amazing. An animal begins its life as one thing (the usual example is a caterpillar), enters a period of pseudo-hibernation and emerges as something totally different. As incredible as this seems, you may not have ever stopped and put together the fact that the majority of animals follow this sort of life plan. Nearly all insects look very different at the beginning of their lives than they do at the end, and since insects make up the vast majority of animal species, metamorphosis is more the rule than the exception. Generally speaking, it is weird that you didn’t spend your teenage years in a chrysalis.
The reason that evolution has selected this approach to life for so many animals is amazingly straightforward. If the adult version of an animal inhabits a completely different ecological niche than its offspring, the mature and immature versions of that animal are not competing against each other for food. Caterpillars spend their days munching leaves whereas most butterflies get all their meals through their straw-like proboscis. In the case of the Atlas Moth, the adult version doesn’t even have a mouth. It relies only on fat storage to survive its adult life. It is hard to complete with your children for food when you don’t even eat.
So metamorphosis is a cool, albeit common approach to life on earth, but how does it happen? Most of us are under the impression that after it has munched its last leaf, a caterpillar finds a safe spot and encases itself in a hard shell, inside which it grows wings, antennae, and the other anatomical necessities of adulthood. In fact, what happens it far weirder. The freshly fattened up caterpillar will either attach itself to the underside of a stick, or use silk to create a hammock of sorts to suspend itself. From there, the caterpillar sheds its skin to reveal the chrysalis, which eventually hardens into a shell.
What happens inside that shell is decidedly stranger than you probably think. Rather than simply adding wings onto its usual body plan and calling it a day, the caterpillar effectively digests its own body. Most of the structures get broken down to their constituent proteins and put back together in a totally new form. It is a bit like taking apart your Lego skyscraper to build a car, only the Legos are alive and a lot more gooey. If you cut open a chrysalis, you would basically just find a soup of animal fluids that don’t at all resemble a caterpillar.
Interestingly, at least some of the memories that caterpillars form survive the metamorphosis process. Research at Georgetown University has shown that if you train caterpillars to avoid certain smells, the butterflies they become show the same behaviour. This is because not all the parts of a caterpillar’s body get broken down. The guts and breathing tubes (in addition to the brain, presumably) remain intact and undergo only slight changes.