- Hey, smart people, Joe here.

Here's what your preschool teacher probably taught you about how butterflies are made.

Cute little caterpillar just eats and eats and eats, and gets all fat, and then it eats some more, and then it builds itself a sleeping bag, crawls in, and something happens.

And voila, a butterfly pops out.

Well, I'm here to tell you that it's a lie.

In fact, no book has done more to mislead the world's young people about the biological marvel known as metamorphosis than this one.

That might be a little bit extreme.

What actually happens when a caterpillar morphs into a winged marvelous adult is so flippin' cool, some of nature's most remarkable biological processes and all of that mind blowing stuff happens in this part that the book just skips over.

The transformation is less "Very Hungry Caterpillar" and more "Gremlins."

Unlike humans, who come outta the womb looking like little, squishy versions of our adult selves, caterpillars transform their entire bodies into something completely different with entirely new body parts.

They basically live one life, but as two completely different animals.

Today, I'm gonna unlock the secrets of what actually happens inside this little sack of magic.

All with the goal of answering one question.

Why do butterflies and other insects actually do this metamorphosis thing?

What's it really for?

So forget everything you thought you knew about how a caterpillar to butterfly pipeline works.

It turns out the truth is so much weirder.

(mysterious music) When I was a kid in elementary school, we raised butterflies in science class.

Maybe you did it too.

And maybe you're like some people and you did it again as an adult.

It's me.

Hi, I'm some people.

It's me.

I still play with caterpillars, but I digress.

Back then you probably watched a squirmy, little caterpillar wrap itself up in whatever this is.

Then a whole different flippin' thing comes out complete with wings and legs.

How'd that happen?

If we wanna uncover the real truth about metamorphosis, there's no better place to start than with Tim.

Tim's in charge of the thousands of butterflies at the California Academy of Sciences butterfly rainforest exhibit.

- The process of metamorphosis for a butterfly is pretty fascinating and they grow through four different stages.

Egg first, and then a larval stage where they're feeding on the foliage.

And then they really go through what some people call that black box period.

They are pupating into a chrysalis, and that's where a lot of changes really happen before they enter the adult stage as an adult butterfly.

- This process where an animal completely changes from one form into another is called holometabolism.

Going through this type of complete metamorphosis is surprisingly common.

About 8 in 10 insects, including beetles, flies, bees, and ants live and grow this way.

Butterflies just get all the attention because this is much cuter than this.

But those groups of insects are vast and numerous, which means that about 60% of all earth's animal species use holometabolism and live different phases of life as completely different forms, which means this real-life Pokemon evolution must be a pretty useful way to live.

Like Pokemon, the magic starts with an egg.

So a mommy and daddy butterfly meet, like what happens- - So butterflies will mate end to end.

So you might see their abdomens connected, but that is what it looks like when butterflies are copulating.

And the male will transfer a spermatophore to the female, essentially, a packet of sperm.

She'll hold onto that.

- [Joe] So she's holding onto sperm, and then when she lays that egg does the fertilization not happen until like the moment she's about to lay the egg?

- [Tim] Pretty much, yeah.

The caterpillar doesn't really start forming or developing until that egg is laid, and then the clock really starts ticking.

- Most female butterflies lay eggs on very specific host plants, since nearly all caterpillars eat plant matter.

I said nearly all, because a few have been known to dabble in the occasional side of cannibalism.

Yeah, Eric Carle didn't mention that part, did he?

Anyway, female butterflies choose the ideal leafy nurseries based on leaf color, plant shape, or by tasting leaves with their legs.

Can you imagine a human mom choosing a baby crib at Target that way?

The things we call caterpillars are butterfly larvae, the second stage of this strange life.

All caterpillars come out of the egg equipped with all the genetic programming and cellular building blocks that they'll need to morph into butterflies later.

These building blocks come in the form of imaginal discs.

Basically little seeds made of cells that'll eventually grow into adult butterfly parts like antenna, eyes, and wings.

But that happens later.

Right now, caterpillars are basically stomachs with mouths on a mission to get chonky.

- Their primary goal as a larva is to consume and acquire as many calories as they can to really fuel their process of metamorphosis.

- During this chow down phase of life, caterpillars can grow up to 100 times their original size.

For example, monarch butterfly caterpillars can fit on a pencil eraser when they hatch.

But in a matter of weeks, they'll get more than five times longer and nearly a 1,000 times heavier, sometimes eating more than 100 leaves a day.

But caterpillars are doing more than eating, and pooping, during this life stage.

They're starting the next stage of their metamorphosis now.

As caterpillars grow, they shed their skin, or molt, multiple times.

Each time they look just a little different on the outside and on the inside.

And this, this is where we bust the biggest butterfly myth of all.

I feel like when I was a kid, I was just taught that, you know, they just like liquefy their body and then a butterfly magically emerges from that soup or whatever.

- Yeah, it's kind of like a tale as old as time.

- Look, if you think about it even for like five seconds, bug soup turning into a butterfly isn't even biologically realistic.

I mean, how would that actually work?

But this gooey myth overlooks what I think is the absolute coolest fact about metamorphosis.

A while back when I visited butterfly researcher Arnaud Martin, he made a small incision in the side of the caterpillar and pulled out this.

That is, that's a larval wing.

- [Arnaud] That's a baby wing, here we go.

- The future adult wing of the butterfly already growing inside the caterpillar.

As they grow, caterpillars develop wing buds, like internal proto-wings that will one day grow into the full-size thing.

These proto-wings, along with many other proto-organs like eyes and reproductive business, are already partially there in the larva.

They're just waiting for one special signal that tells them when to grow.

Once caterpillars finish their second to last molt before chrysalisification, their bodies release a big dose of a growth hormone that tells 'em to stop eating and look for a good place to hang out, literally, for a few weeks.

This stage is basically butterfly puberty and it's pretty awkward for them too.

Once the caterpillar finds the perfect spot, it secretes a little dab of silk.

Yeah, spiders aren't the only ones who make that stuff.

And it hangs itself from it.

- [Tim] So at that stage, instead of having a new skin underneath it that looks like a caterpillar, the next skin is really the cuticle made of chitin of that chrysalis stage.

And it's a really cool transition because you can see a lot of the outlines of the future butterfly on that pupil case.

- [Joe] It just hit me like, butterflies are like onions, they just keep layers inside.

- [Tim] Yeah, one layer time.

- So yeah, a chrysalis is not a silk sleeping bag that they spin and then crawl into.

It's their skin.

They take off their old skin like a jacket, and their new body is inside.

This isn't kids book stuff this is more like "Silence of the Lambs."

So now we're on metamorphosis stage three, where the caterpillar is now a pupa.

Thanks to all the energy and protein stored up during the larva phase, the pupa is physiologically self-sufficient and lives off the nutrients it consumed as a caterpillar.

And if you thought the run up to the pupa stage was weird, just wait, things get even stranger and cooler from here.

So if the caterpillar doesn't like completely dissolve, turn into butterfly soup, then abracadabra it's way into a new body, what is happening inside that bag of mystery?

For starters, a whole lot of dying.

See the cells that a caterpillar doesn't need anymore, they self-destruct.

(bomb exploding) This cell death is also triggered by hormones.

Those hormones tell cells to activate digestive enzymes to break apart the cell's proteins and other parts, and the pupa recycles this protein to fuel the rest of metamorphosis.

So, yeah, pupae essentially eat themselves.

A caterpillar's muscles, jaws, most of its gut, and stubby little legs, they all break down because an adult butterfly doesn't need them.

And while things are reorganizing and growing inside the chrysalis, the new parts the future butterfly will need are already there on those imaginal discs, growing into their final form.

For a long time, we couldn't see what was going on inside of a chrysalis.

But with modern technology like CT scans, we can watch the science happen.

This is the inside of a chrysalis.

And you can see that at the beginning, not a lot going on.

You can see the digestive tract in the middle, and the caterpillar's breathing tubes.

You can even see the proto-wings.

But none of this really looks like a butterfly, yet.

But over time, the butterfly's eyes get bigger and more complex, it grows antenna, its body shape changes, and of course, those proto wings-grow into their full, flappy splendor.

Now that the caterpillar is done becoming a completely new animal, the adult butterfly is ready to emerge from the chrysalis.

And this part of the process is known as eclosion.

It starts when the butterflies' brain releases yet another hormone that gives it the strength to bust through the chrysalis wall like the Kool-Aid Man.

- [Kool-Aid Man] Oh yeah!

- And once the butterfly's free, it hangs upside down, pushes a blood-like substance through its wings to help them expand.

This finally allows the wings to dry and harden enough so the butterfly can take its first flight.

One of the coolest things about this total metamorphosis process is that different families of insects put their own spin on it.

Sometimes literally.

For instance, instead of hardening their skin into a protective chrysalis, moths do spin their metamorphic sleeping sack out of silk.

That's probably why they come out with such great hair.

Ants do all this inside enormous colonies pampered by thousands of older sisters.

And baby bees do their change locked inside tiny hexagonal prison cells like some weird dystopian metamorphic "Matrix."

Beetles and cicadas do it too, often buried in the ground for their entire childhood.

And if you're a fly, you might do all this in a dead body or a hunk of poo.

Where's the children's book about that, huh?

So that's the how.

The deeply strange ways insects transform their squishy little larval bodies into their final forms.

But why do they do it?

Why is living your life as multiple life forms such a great recipe for evolutionary success?

Well, I know y'all hate this answer, but we're still not entirely sure why.

Sorry, that's science.

But here's some theories.

Some scientists think that so many insects are fabulously homometabolous because it means that adults and their young don't compete for food.

For instance, while caterpillars eat leaves and sometimes each other, butterflies drink nectar, even sweat, urine, or blood.

Some adult moths don't even eat as adults since they aren't born with mouths, which is the most extreme way to avoid food competition, and another tally in the "Silence of the Lambs" column.

Scientists also theorize that metamorphosis might be a result of adaptive decoupling.

This lets adult and juvenile traits evolve independently from each other, since caterpillars and butterflies face totally different pressures for natural selection.

Whatever the reason, and there may be more than one, the fact that complete metamorphosis is so strikingly common in the animal kingdom means that this is a hugely successful way to live in the eyes of evolution.

And it's my favorite reminder that even seemingly straightforward scientific processes that we can see happen right before our eyes are far more complex and wondrous than we could possibly imagine.

So sorry, "Very Hungry Caterpillar," better luck next time.

(glass shattering) (cat yowling)