According to all known laws of neuroscience, there is no way that a butterfly  should remember being a caterpillar.

It melts into goop when it transforms and loses all its brain connections. The butterfly, of course, remembers anyway, because butterflies don’t care  what humans think is impossible. …No, this entire episode will not  be the script of the Bee Movie… sorry about that.

Also We actually do know how bees fly, contrary to what a certain movie might tell you. But there’s a real mystery in the insect world: Some insects that undergo metamorphosis can remember things from before they transformed. And that really does seem  like it should be impossible. [♪ INTRO] Hi!

I’m Tom Lum, and for the next week and a half I’ll be hosting SciShow. Holometabolous insects – butterflies, moths, beetles,  flies, bees, and others – go through complete metamorphosis, where they undergo a drastic bodily transformation by way of a pupal stage. Much more drastic than my  teenage change in music taste… It was never a phase mom!

And Metamorphosis is extremely common – more than 80% of animals go through some degree of change throughout their lives. For insects, this number is more like 98%. Most scientists think it's an  evolutionary strategy used to keep the amount of resource competition  experienced between parents and offspring to a minimum, since  different life stages generally happen in different ecological niches  and require different resources.

But despite how common it is,  metamorphosis is bizarre and difficult to study, and complete metamorphosis  is especially enigmatic. Because, to be clear, I wasn’t  really exaggerating earlier. An insect basically kinda turns  into gloop inside its pupa.

Virtually everything in their  body is melted down and reordered. Digestive enzymes consume the  vast majority of the cells, which just rupture into a slurry of  the basic building blocks of life -- proteins, amino acids and other  nutrients all sliding around together. Complete metamorphosis is  cell death on a massive scale, and you might find yourself  questioning whether the snot inside a butterfly chrysalis  is even a living thing.

Essentially, the animal's  instructions for being a larva have to be turned off completely in  order to make way for the parts of the DNA that tell it to  be an adult to be turned on. It's a bit like unplugging a microwave,  putting it in a nuclear reactor, taking the resulting sludge and  rebuilding it into a refrigerator, and then plugging it back in. It's a pretty scorched earth version of puberty.

A groundbreaking 2023 study teased out the reason complete metamorphosis  should leave no memories of an insect's larval stage for  the adult to reminisce over. These researchers successfully  traced individual brain cells and the connections between them through  the process of metamorphosis. The authors scoured the mushroom  bodies of Drosophila fruit flies –  that’s the part of their brains  in charge of learning and memory.

They found there wasn't a single  anatomical hook in the adult's brain upon which to hang a  memory formed in childhood. They showed that, although  individual larval neurons in the study survived the process,  the neural connections in the mushroom body were completely  dissolved and reconstructed. And those connections, as we understand it, are what memories are made of.

Therefore, adult metamorphosing insects should have no memories from their larval lives. This was a 5-star, top shelf,  roundly celebrated study – everybody agrees! Nice job, scientists!

Only one little catch: it  certainly seems from past research that adult butterflies and moths  can totally remember things from before their metamorphosis. But before that let’s take a quick  pause to hear from our sponsor Thanks to Brilliant for  supporting this SciShow video! Brilliant is a learning platform made  by people who have put a lot of thought   and design into being as effective as possible.

Everything you interact with on  Brilliant was crafted by teachers,   researchers, and professionals from MIT, Caltech,   Duke, Microsoft, Google, and more. So  they know what they’re talking about. But it’s way better than listening to a lecture  from one of them because each lesson is filled   with hands-on problem solving that lets you  play with concepts.

You know, active learning. You can start building understanding from  the ground up at Brilliant.org/SciShow,   the QR code, or the link in the description.  That link gives you 20% off an annual premium   Brilliant subscription. And you’ll  get the first 30 days for free.

A 2008 paper found that the  larvae of tobacco hornworm moths could be trained to avoid the smell  of ethyl acetate by giving them a mild electric shock whenever  they came close to it. When those larvae matured into moths, the adult moths also avoided  the smell of ethyl acetate. Seems like they remembered their  little adventures just fine.

Which begs the question: if they  don't even share the same brain, how do the lessons of the juvenile carry over into the understanding of the adult? The first question we've gotta  ask is, what aren't we seeing? One potential confounder here is called the chemical legacy hypothesis, which says that during a certain period of metamorphosis, if the emerging adult is exposed to odors that its larval self had associations with, those associations can carry  over into its adult life.

However, researchers are generally aware of this and do their best to control for it. The authors of the 2023 paper concede  that just because a Drosophila fruit fly's memories could never  survive complete metamorphosis, that doesn't mean it can't be different  for insects with more complex brains and a greater number of neurons –  like moths, beetles, and butterflies. Because those insects have more  complex brains, it could be they don't get the complete renovation  the researchers saw in Drosophila.

However, a 1994 paper trained  Drosophila to avoid a specific smell with electroshocks, and the  result seemed to be the same -- they remembered to steer  clear of that smell as adults. So, why do adults on the other side of complete metamorphosis have childhood memories? According to the extremely rigorous 2023 study, it shouldn't be possible.

And  yet, nature appears to find a way. The authors of the 2023 paper  suggest the possibility that memory for metamorphosing insects  looks different than we think, and is about more than which neuron  is connected to which other one. The rearrangement of their brains might even look different depending on the  environment they live in.

More research is obviously needed, and it could be a while before someone gets there. There appears to be a limited number of neuro-entomologists in the world. Anyway, the answer to the  burning question of whether adult metamorphosing insects retain  memories from before they pupate is– Sure seems like they can!

But do we know why or how? Absolutely not! But hey, that just gives us  something to work toward.

I mean, who wants science to be /done/, right? I can only hope that one day  we can play jazz for a pupa and ask the butterfly, do you still like jazz? [♪ OUTRO]