How do caterpillars turn into butterflies and moths through metamorphosis?

When I was a kid, I once found an alien in a forest. It was very small and hiding in a shiny green case about the size of a thumb. It dangled from a branch and was completely still. I reached out to touch it, and it cracked and started leaking pink goo all over my fingers.

Horrified, I ran back to my parents. They told me to leave them alone; the green alien cases were actually baby butterflies. I was very confused—how the heck did the green aliens turn into butterflies? And why was there gross pink goo inside of them?


Now I’m a bit older, but I’ve always remembered that experience. What was really going on inside that case? I wanted to learn exactly how all that goo turned into the butterflies we see peacefully floating around during the warmer months of the year.

How Butterflies and Moths Start Life

First, a male butterfly meets a really cute female butterfly, and they mate. After the eggs are laid, they start developing into wee caterpillars.

Instead of developing like most other animals do, caterpillars have something very special inside of them: imaginal disks. These “disks” are just small clusters of cells that match up with the structures they’ll need as adults. There’s one imaginal disk for every adult body part—wing, eye, leg, etc. Remember these imaginal disks; we’ll see them again.

After caterpillars hatch from eggs, they turn into greedy little eating machines. All they want to do is eat, eat, eat (remember The Very Hungry Caterpillar book?). They eat so much that they grow too large for their own bodies, and they need to shed, or molt, their skin, just like a snake. Some caterpillars have tiny bristles or hairs to defend themselves against predators, either as a sort of armor or to inject venom.

The caterpillars repeat this eat-shed-eat process a few times until they reach a size where they’re large enough to undergo metamorphosis.

How do butterflies and moths go through metamorphosis?

Once a caterpillar has eaten its fill, it finds a nice little nook on a branch somewhere (hopefully out of reach of curious children). It hangs itself upside down from the branch and does one of two things, depending on the species. It’ll either wrap itself tight in a silky cocoon, or molt one final time into a hard, sparkling chrysalis (note: these are not actually alien cases).

Here’s a cool video showing how a Monarch butterfly starts the metamorphosis process:

When the larva is tucked neatly away in its cocoon or chrysalis, that’s when the magic starts. Enzymes are released and literally dissolve almost the entire larva into a nutrient soup (the pink goo so vivid from my childhood experience). Only a few other things remain: the nervous system, the breathing tubes, and the imaginal disks (remember them?).

Now that the imaginal disks are free, they start to rebuild the bug. The disks move to the correct positions (no one wants a leg where an eye is supposed to be), and the cells in the disks start to absorb the nutrient soup to grow and multiply.

Very slowly, the new insect starts to take shape. If you ever wanted to see what a caterpillar looks like while undergoing metamorphosis, check out the pictures on this website.


Interestingly enough, even though the entire bug goes through this whole process, some things do stay the same. For example, some scientists have done an experiment to prove that moths can remember things from way back when they were just caterpillars! This shows that even though the body is rearranged, most (if not all) of the nervous system remains intact.

Wrapping Up the Transformation

By the time the transformation is complete, the new butterfly or moth is fully-formed within its cocoon or chrysalis. It then hatches for the second time in its life. The new bug will pause to get its bearings and test its new body; its wings and antenna unfurl and harden. Then, it’s off into the air to start its new life!

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Written by Lindsay VanSomeren

Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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