How does a caterpillar become a butterfly?

CHILD: You're listening to Brains On!, where we're serious about being curious. Brains On! is supported in part by a grant from the National Science Foundation.

DAD: Look, kiddo, we need to talk. You're not going to be a caterpillar forever.

DAUGHTER: Dad, we normally listen to bugs on while we drive. Can we talk after my bugs-ketball game?

DAD: Honey, I think this is the best time. We don't have to make direct eye contact in the car, plus there's a clear ending to our talk when we get to the gym.

DAUGHTER: Fine.

DAD: All right. You already know you don't look like me, your moth-dad.

DAUGHTER: Well, yeah, you have wings, plus you're like a million years old, right?

DAD: Very funny. Now, you might be noticing some changes to your caterpillar body. So let's start there.

DAUGHTER: Well, I am changing color. I think my body is shrinking too.

DAD: Those things, they're totally normal. And soon your outer skin will change even more. It'll sort of grow into a case for your body.

DAUGHTER: Oh, dad, they covered this at school. I'm sort of a worm-shape now, but soon I'll transform. And then eventually, I'll be a moth with wings like you.

DAD: Oh, that's great. I am glad you covered it. I just wanted you to have as many details as possible. You might start feeling interested in certain activities like spinning silk with your mouth.

DAUGHTER: I have been thinking about that lately.

DAD: Metamorphosis is going to be really different from your life so far.

DAUGHTER: I know, dad.

DAD: At the end, you're going to look completely different.

DAUGHTER: Dad, I know. Oh, look, we're at the gym. Is the talk over yet?

DAD: Oh, OK. OK. Well, feel free to ask me any of the questions you might have. I'm all ears and rooting for you all the way. Oh, and honey, have a good game. Break a couple of legs. I'll be watching.

DAUGHTER: Bye, dad.

[MUSIC PLAYING]

MOLLY BLOOM: Welcome to Brains On! from American Public Media. I'm Molly Bloom, and I'm here with Tilly from Los Osos, California. Hi, Tilly.

TILLY: Hi.

MOLLY BLOOM: So this episode is all about metamorphosis.

TILLY: How caterpillars change from walking wormy larva into fantastic flying adults.

MOLLY BLOOM: Larva is just another word for the baby form of an insect.

TILLY: When an insect is transforming from a larva to an adult, it's called a pupa.

MOLLY BLOOM: And Tilly you asked us a great question about one kind of pupa, the chrysalis. Now, that's a caterpillar's hard outer shell where it transforms. So do you remember that question you sent us?

TILLY: What happens inside of chrysalis?

MOLLY BLOOM: That is a really good question. So I'm just wondering what made you curious about that.

TILLY: I saw chrysalis first because we had them in our second grade classroom.

MOLLY BLOOM: So you were curious about what happens inside of a chrysalis. So what do you think happens inside of a chrysalis?

TILLY: Well, I heard that they like it liquefied, sort of.

MOLLY BLOOM: Well, a lot of other listeners wondered about metamorphosis too.

EMMETT: My name is Emmett.

BEN: My name is Ben. We're From Irvington, New York. And our question is, how do caterpillars transform into butterflies?

AVA: Hello, my name is Ava, and I'm from Smyrna, Georgia. My question is, how do caterpillars' chrysalises stick together?

JOHANNA: My name is Johanna, and I'm from Romeo, Michigan. My question is, how do caterpillars make their cocoons?

MOLLY BLOOM: Hmm, so Sanden was super into your question about the chrysalis, and he was supposed to come talk about it. But he seems to be missing. I'm sure he'll turn up soon.

Let's morph this segment into the--

GIRL: Shh. Mystery sound.

MOLLY BLOOM: Here it is.

[MYSTERY SOUND]

Do you want to hear that one more time?

TILLY: Sure.

[MYSTERY SOUND]

MOLLY BLOOM: So Tilly, what is your guess of what we're hearing there?

TILLY: Sounds kind of maybe like a bird.

MOLLY BLOOM: OK. So some kind of bird is what we're thinking at the moment. So we will listen to it again and have some more guesses a little bit later in the show.

SANDEN TOTTEN: Molly, Tilly, ugh, I'm so sorry. I'm late. I totally lost track of time.

You see, I found this amazing new show where insects look at different pupation stations for metamorphosis. It's called My Dream Pupa.

MOLLY BLOOM: Huh, maybe we'll check it out later, but do you want to tell us about what happens inside a chrysalis?

SANDEN TOTTEN: Oh, totally. And since you and Tilly have rhyming names, I've decided to compose a little poetry.

MOLLY BLOOM: That sounds fine.

TILLY: I do like a rhyme.

SANDEN TOTTEN: Awesome. [CLEARS THROAT] OK. If you look inside a chrysalis, you'll find all the caterpillar bits.

But here's the real surprise. Lots of bits get liquefied. Don't worry. That bug is totally alive.

It's always breathing the whole time. It keeps some bits intact from the caterpillar starter pack. Those bits inside that stay the same. They have a pretty special name.

They're called-- get ready-- imaginal disks. Think of them as bags in a caterpillar bisque. Bisque is soup. [SLURPING SOUND]

So those bags jump-start construction. A bug's DNA has all the instructions. A larva's old bits grow into other things. New legs, new eyes, and don't forget the wings.

[CLAPS]

MOLLY BLOOM: Very impressive. So inside the chrysalis, bugs break down some of their old body parts into a mushy goo. Then those mushy bits morph into new body parts?

SANDEN TOTTEN: Yeah, you got it. It's delightful, isn't it?

TILLY: It's almost like a squishy recycling center.

SANDEN TOTTEN: Yeah. I like that. Squishy recycling center. So that's how metamorphosis works overall.

OK, I just really want to show you my new TV obsession, My Dream Pupa now. Is it OK if we watch an episode?

TILLY: Sure.

SANDEN TOTTEN: Aw, that's what I was hoping you'd say. All right, let me just hit play and--

WOMAN: Monty and Cat are starting to feel different, so they're looking for a place to transform.

MONTY: Hi, I'm Monty. I'm a Luna moth larva.

CAT: And I am Cat. I'm a peach tree borer caterpillar.

WOMAN: And these two friends are hoping to be roommates during pupation.

MONTY: It's just so nice to have a pal.

CAT: I love coming home and telling Monty about all the leaves I've eaten.

WOMAN: Hmm, well, that might not be biologically possible. But these two have another issue, different taste in pupation stations.

MONTY: I'm thinking something wispy, thin, sort of an open pupa plan.

WOMAN: While Monty wants to keep things simple, Cat is looking for a dark, cozy home.

CAT: I have this strange new urge to just burrow deep down into something.

WOMAN: Lucky for these two larva we're introducing them to Alison Brackley. She's a scientist who studies bugs.

ALLISON BRACKLEY: Well, hi. So I am actually not a caterpillar. I'm a human being. But I am really interested in caterpillars.

CAT: Hmm. Hmm. Mmm.

MONTY: She means great to meet you.

CAT: Yep. Sorry. I cannot put down these bark chips. I love this peach tree flavor.

WOMAN: Before these three head out to look at options, Allison sits them down to lay everything out.

ALLISON BRACKLEY: When you're a pupa, you're in a really vulnerable place because you can't move around. If something comes to try to attack you, you can't fight back. If you're hungry, you can't eat.

CAT: Ah! So many threats. Do I love these bark chips, or am I stress eating?

MONTY: Cat, no need to worry. This next phase of our life is about our transformation. So hopefully, we'll be shielded from the weather and from predators like birds or yellow jackets that want to eat us. We might even have some protection against disease.

WOMAN: First, Alison shows them a simple option, the butterfly's chrysalis.

ALLISON BRACKLEY: Which is just kind of its outer layer, outer skin.

CAT: Right. Yeah. I'm seeing that this chrysalis is just basically a hard outer shell that butterflies make. Kind of an oval shape, very bare bones.

ALLISON BRACKLEY: If that suits your needs, if you are still safe in that scenario, then it's kind of the best option because you don't have to spend all your energy stores on protecting yourself, essentially.

MONTY: I love the simplicity. Some chrysalises can still protect butterflies from birds or disease. Think of all the energy we'd save.

CAT: I hear you, Monty. But I'm not sure I'd feel safe building new organs in this space. I need more structure.

WOMAN: Next up, Allison shows them the every bug's pupa, a cocoon.

ALLISON BRACKLEY: So a cocoon is basically kind of-- it's like a sleeping bag essentially that you make using the silk glands that are in your mouth.

CAT: Some cocoons are pretty heavy-duty. I like that protective aspect.

MONTY: Oh, look how wispy this one is. I'd love to snuggle into that. But also, I love the chrysalis option. It seems so great to have a hard outer shell for a pupa.

ALLISON BRACKLEY: You're going to have that no matter what. But if you have a cocoon also, then the cocoon goes around that. And it's kind of like you'll make your cocoon, and then you'll go into the cocoon.

MONTY: Wow. I'm loving that we'd make both that outer shell and the cocoon out of materials in our bodies. Nothing else.

CAT: Ugh, both of these options are just so exposed. Neither one satisfies my urge to be deeply hidden from the world.

WOMAN: Looks like Monty and Cat want pretty different things. More on these two larva after a break.

MOLLY BLOOM: We're working on an episode where we're giving awards for the things that are the most you know, most long or most deep, most fast. Or longest, deepest, fastest is how we usually say these in English. And we want to hear from you.

If you are going to give a Mostie award, what would you give it to? Maybe it's the funniest joke or the tastiest food or the most stylish color. Truly whatever category you can dream up. But we'd like to hear your winner.

The award I like to give is the most pleasant weather. I would give it to a spring day that is 68 degrees, a light breeze and sunny skies. Sounds lovely.

You can send your Mostie award ideas to us at brainson.org/contact. And while you're there, you can also send us mystery sounds, drawings, and questions like this one.

CHILD: My question is, why do we have tongues?

MOLLY BLOOM: You can find an answer to that question on the Moment of Um podcast. It's our new podcast feed that's full of bite-sized answers to your brilliant questions. Search for Moment of Um wherever you listen to Brains On! And at the end of the show, we'll read the most recent group of listeners to be added to the Brains Honor Roll.

TILLY: So keep listening.

Welcome back to Brains On! I'm Tilly.

MOLLY BLOOM: I'm Molly.

WOMAN: I'm the narrator of My Dream Pupa. When we left off, Monty and Cat were hunting for a pupation station. Let's get back to it.

So far, Monty's excited about the low effort options. But Cat wants something with more protection.

CAT: I know I've only used my jaws for chewing leave so far, but I have the nagging feeling that I'm also supposed to use them for something else.

MONTY: Not me. The only thing I want to do is spin up a simple silk sleeping bag.

WOMAN: Next, these two are checking out something a bit more complicated, a leaf shelter.

ALLISON BRACKLEY: One example of a leaf shelter is one that is made out of a dead leaf that you find on the ground.

CAT: Ooh, more complexity. It looks like some caterpillars have a hard pupa shell then a little cocoon. And then they crawl into this dead leaf pocket like a little burrito.

MONTY: Ugh, the upkeep. You'd have to put in so much effort to make the shelter work. What if you never found a leaf? What if your leaf decomposed?

CAT: I trust Alison. What's her take on leaf shelters?

ALLISON BRACKLEY: I did some experiments where I put out different pupae of the silver-spotted skipper, and I had some of them in their natural leaf shelters and others that didn't have leaf shelters. And I found that in the summer months basically are when they have a lot of predators coming at them. During those times, the leaf shelters really help them to survive longer.

MONTY: Cat, Allison, I worked so hard to munch all these leaves and store up energy for my transformation. I just don't know if it makes sense to burn all that energy dragging around some dead leaf.

CAT: Oh, you're just hungry. I bet you'll feel different after a snack. Bark chip?

[MUSIC PLAYING]

WOMAN: The last shelter is the most protective one yet, a bark burrow.

CAT: Oh. It's like a little personal cave in the bark of a tree. This is the opportunity to use my teeth I've been waiting for.

I could go find a tree trunk, sink my teeth into the bark, and then chew up lots of bark, and burrow in--

MONTY: That's already so much work. And then what?

ALLISON BRACKLEY: Once you're in there, you're going to eject any waste that you have, any poop that you have, anything like that. And you're actually going to make a little hatch for yourself so that anything that is walking by that tree on the outside would never be able to see you, your predators wouldn't be able to smell you.

CAT: Ah, my own private bark burrow bunker with a door.

MONTY: Ugh. I've heard of other caterpillars burrowing into the ground. And burrowing into a bark is even more work.

WOMAN: With lots of options on the table, it's time for Cat and Monty to talk it out.

MONTY: To be honest, Cat, I don't even really have a choice about this. I absolutely can't go with anything besides building the cocoon that spoke to me.

CAT: And I can't imagine any strategy besides chomping down on a tree trunk. It's like encoded into my very being.

MONTY: It feels like this isn't even a decision we make. Allison?

ALLISON BRACKLEY: Each caterpillar species really has its own way that it undergoes metamorphosis. And so you can kind of look around and see the different ways that other caterpillars are going to go through it. But you won't actually know what you're going to do until you get to that point in your development.

CAT: What happened to the point of this show choosing a pupation station?

MONTY: It's not even up to us?

WOMAN: Well, no. But even though these two won't be roommates, everything is going to be fine.

ALLISON BRACKLEY: The really great news about that is that for each species, it seems like they have a strategy that really works for them.

WOMAN: Turns out no caterpillar chooses what structure to build. They all follow their instincts. But choice or no choice, looking at options is just plain fun.

Tune in next week when two beetles check out some beetle pupation options, including a capsule made of poo.

[MUSIC PLAYING]

MOLLY BLOOM: Well, there are a lot of different ways for insects to transform. We wanted to learn a little bit more about the hows and whys of this metamorphosis. So we called up Ana Carvalho. She researches butterflies at the University of Florida.

TILLY: Welcome, Ana.

ANA CARVALHO: Hi. Thank you for having me, Tilly.

TILLY: How do caterpillars know it's time to transform?

ANA CARVALHO: So the caterpillar knows when it's time to transform when it has eaten enough. The caterpillar spends most of their time eating and growing. And when the caterpillar is big enough, the body will release certain hormones that indicate that it's time to stop eating and pupate.

TILLY: What do caterpillars get rid of when they become butterflies?

ANA CARVALHO: So they get rid of pretty much everything. When the caterpillars pupate, they release enzymes that breaks down their whole bodies. So if you open a pupa before the transformation is complete, you would mostly find liquids.

The only things that do not get dissolved during this transformation are something that we call imaginal discs. But before metamorphosis, these imaginal discs do not have a very definitive form. They are just a group of cells. And they really only assume the shape and structure of the body parts they will become during metamorphosis.

So when the butterfly and the moth hatches from the pupa, it has a brand-new body with new wings, new legs, antennae, and other parts.

MOLLY BLOOM: And so I know we always think about the butterflies and moth when it comes to metamorphosis, but it sounds like there's a lot of insects that do it.

ANA CARVALHO: Well, a lot of them are doing this. Some of the most diverse groups of insects are butterflies and moths, flies, wasps, and beetles. And these all go through metamorphosis. So I'd say it's a very common thing in insects to go through a metamorphosis.

So to me, a very cool thing about metamorphosis is that it can take a really long time for some species of insects. I once reared a swallowtail caterpillar that took a whole year to hatch from the pupa. And in some species of beetles which also do metamorphosis, they can pupate for several years.

Sometimes people would buy furniture, and a jewel beetle would hatch out of it because it pupate inside the wood that was used to make that piece of furniture. So there are cases where a beetle hatched 25 years after the wood was harvested for that furniture.

MOLLY BLOOM: Well, thanks, Ana, for sharing how cool these insects are.

ANA CARVALHO: They are really cool. Yeah. [CHUCKLES]

TILLY: Thanks, Ana. Bye.

ANA CARVALHO: Bye, Tilly. Thank you so much for having me.

MEN: (SINGING) Ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, Brains On!

MOLLY BLOOM: Are there other animal transformations that you've heard about that also go through metamorphosis?

TILLY: I haven't heard of animals that go through metamorphosis, but I've heard of like birds change their color and stuff like that.

MOLLY BLOOM: Oh, that's super cool. The one that always springs to my mind is a frog, the egg to the tadpole to the frog, which is a really cool transformation.

And jellyfish go through a crazy transformation. They start in these tube-like shapes, and then they turn into these individual rings, kind of look like a stack of tires. And then each tire basically pops off and becomes an individual jellyfish.

It is super, super cool. Now it's time for the reveal we've all been waiting for. But first, let's hear that mystery sound one more time.

[MYSTERY SOUND]

Tilly, what is your guest this time? Last time, you're thinking a bird. Do you have new thoughts?

TILLY: Well, it kind of sounds like maybe something rub against each other and making like a squeaky sound.

MOLLY BLOOM: Hmm. Excellent guess. What do you think that squeaky sound might be?

TILLY: I have no clue.

MOLLY BLOOM: [LAUGHS] It's a tough one. Here is the answer.

TIM WONG: My name's Tim Wong, and I'm a biologist here at the California Academy of Sciences. The sound you just heard was our butterfly release hamper unzipping.

It looks like a laundry hamper that you might have at home, and it's a soft mesh enclosure that we use to transport newly hatched butterflies from our emergence chamber. And then we unzip the hamper, and we let them fly out into the exhibit.

MOLLY BLOOM: Tim helps take care of the butterflies in the rainforest exhibit, so that's what it sounds like for them to release butterflies into that exhibit. Does that make sense now that you heard it?

TILLY: Yeah.

MOLLY BLOOM: So yeah, they unzip that hamper and give it a few taps. Brains On! contributor Mary Harvin was there to see it happen.

TIM WONG: I'm going to unzip it right now and see if you can hear this.

MARY HARVIN: Oh, my gosh. It's fluttering out.

TIM WONG: Today we're standing in the Osher Rainforest Exhibit. And in this space, I'm taking care of tropical plants and live tropical butterflies. So every morning, we're in here. We're releasing butterflies from different tropical regions around the world.

And we're usually receiving them in what we call the pupa stage, which for a butterfly, we call a chrysalis. This stage is actually really ideal for transport because the butterfly chrysalis has a hardened shell or a cuticle. And that means they're protected during transport when they're being shipped in the mail.

But also, it means that they don't need any food or water until we receive them here at the academy.

MARY HARVIN: They come to you in the mail. Like, what does that look like?

TIM WONG: Yeah, it's actually-- it's kind of like receiving a box of variety of chocolates or candies, except the different varieties. There are different kinds of butterflies. Most of our butterflies hatch first thing in the morning, so it's a pretty special time to start my day up here. [INAUDIBLE].

MARY HARVIN: I never seen a butterfly hatch before. This is amazing. Oh, my gosh. It's like the leaf is breaking open.

TIM WONG: Mm-hmm. The hatching process is a pretty delicate process. The wings are soft. They have these new legs.

They have to put their proboscis, their tube-like mouth part together. And all of that has to go right in order for them to become a successful adult butterfly.

Now you can see his neighbor's just wiggling. So that chrysalis right there is just simply wiggling because the butterfly hatching next to it is touching it. And some chrysalis will wiggle around to try to deter any potential threat or predator.

MARY HARVIN: Oh, my gosh. It looks like someone in a sleeping bag made out of a leaf zipped up all the way wiggling.

TIM WONG: Wiggling, yeah. So once the butterfly's wings are fully dried from our emergence chamber, we'll then put them into this soft hamper that kind of looks like a laundry hamper. And that's what we use to just transport them from the hatching habitat out into our rainforest exhibit, where they can fly around and feed on flowers.

I guess I could kind of like tap the side a little bit. It usually helps the butterflies kind of start flying around so that when I unzip it-- so that they can kind of just like--

MARY HARVIN: Because there's still some left in here from when you release them earlier.

TIM WONG: There is some, yeah. Sometimes it takes them a little while to take off. They are new butterflies, so sometimes it takes a little encouragement to get them to explore the world.

But just a gentle tap.

MARY HARVIN: Come on, guys. You're free. Welcome to the world. Here they go.

[MUSIC PLAYING]

TILLY: Insect's larva like caterpillars have everything they need to become adults inside their bodies already.

MOLLY BLOOM: When a caterpillar goes to transform, it breaks down some body parts to build its new adult body.

TILLY: Bugs use lots of different pupation stations to protect themselves as they transform.

MOLLY BLOOM: That's it for this episode of Brains On!

TILLY: It was produced by Menaka Wilhelm, Molly Bloom, Sanden Totten, and Marc Sanchez.

MOLLY BLOOM: We had production help from Mary Harvin and Ruby Guthrie. Our executive producer is Beth Pearlman. And the executives in charge of APM Studios are Chandra Kavati, Alex Schaffert, and Joanne Griffith.

We had engineering help from [? Greta ?] [? Mart ?] and Cameron Wiley. Special thanks to [? Greg ?] [? Domber, ?] Jennifer Lai, Rosie duPont, and Aron Woldaslassie.

Now before we go, it's time for the Brains Honor Roll. These are the incredible kids who keep the show going with their questions, ideas, mystery sounds, drawings, and high fives.

[MUSIC PLAYING]

[LISTING HONOR ROLL]

ROBOT: Brains Honor Roll. Bye. Bye.

MOLLY BLOOM: Brains On! will be back soon with more answers to your questions.

TILLY: Thanks for listening.