Deep Look

With an extreme makeover, looking fabulous isn't always the goal.

For these four creatures, their growing bodies take radical turns in the course of a lifetime.

Cut this flatworm in half and you get two flatworms.

Even after millions of years of evolution, the axolotl still got its youthful glow.

This caterpillar is super hungry for a good reason.

It's bulking up to fuel a whole new way of life.

But first, the common house fly has to go through a mind-blowing transformation before it's totally ready to annoy you.

Coming of age is awkward, messy, challenging, even for a house fly.

See this pulsing sack on the fly's head?

It's a specialized organ that the fly uses just once to break free from its pupil casing.

Whew, what a headache.

It all started a few weeks back when mom and dad had a brief but spirited courtship, which resulted in these delicate oblong eggs.

And from the eggs, maggots.

Hi.

The little squirmers dig right in wherever mom laid them.

After chowing down for a few days, this chonker is now five times its original size.

What it wants now is privacy.

Over the next four days or so, the maggot's ivory exoskeleton hardens.

Ah, going to my room.

And darkens to a deep mahogany.

The fly is now a pupa.

See these holes?

This is how it gets oxygen and that air just happens to flow through its butt.

While things may look quiet on the outside, there's a lot happening beneath the surface.

This is a blow fly, a house fly's cousin.

Scientists at the Natural History Museum in London X-rayed the pupa to reveal the transformation behind its opaque shell.

See them go from chubby tube to a complex creature with legs and wings and big eyes?

The once blobby maggot has totally transformed, but the cozy casing that protected it now holds it captive.

That's why this inflatable organ called the ptilinum is essential.

The fly pumps it full of clear colored insect blood called hemolymph and pops the lid off its casing.

Once the head's out, the struggle continues.

Next come the front legs, abdomen, and back legs.

The process of freeing itself is called eclosion, but not everyone makes it.

Some get stuck right in the middle forever.

The ones that do survive pull the ptilinum into their head.

All that remains is the ptilinum suture.

It's a scar the adult fly will carry around as a reminder of the heroic effort it made to start its life on the outside.

Now, what to do with all these new body parts?

Its wings extend from crumpled to elegant as they fill with hemolymph.

This insect has now realized its full potential to fly off into your house and drive you mad.

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Don't judge a flatworm by its simple squirminess.

This little worm may help humans learn how to grow new body parts.

This goofy little guy is called a planarian.

Sure, it's just a flatworm at the bottom of a pond, but it has a very special superpower, one that scientists would love to harness.

To show you, well, it's going to get a little bit slasher movie here.

Okay, don't worry.

It's going to be okay.

Seriously, I promise it doesn't even hurt.

That would be curtains for most organisms.

I mean, this piece here is just a chunk with no head and no tail, but watch.

You can see under a microscope that overnight its wound heals closed.

Okay, maybe we could do that more slowly of course, but then it starts to regenerate growing new tissue.

It's that white part called the blastema.

In a week, see those tiny little spots?

Their new eyes grown from scratch.

But the planarian, can I call it that yet?

It still kind of looks like a blob.

By day 12, it's starting to look like a proper planarian.

It's got a head, an entirely new one, and a new tail.

And it's doing its regular planarian things like pooping.

That's because one of the first things it started regrowing was a tube called the pharynx, which is how things go in and out.

It sucks up food like this beef liver that scientists are feeding it.

After three weeks, it looks totally normal.

From that one planarian, you get four.

Other animals, like this nut, can regrow a toe or a tail, but planarians are practically the only animal that can regrow a head.

So why can't we do that?

Well, it all comes down to powerful cells known as stem cells.

They're the green dots here and they make up one fifth of a planarian's body.

They can turn into different kinds of cells and make every new body part.

We only have stem cells that act like these when we're embryos.

Once we grow up, we pretty much lose this ability, though doctors have been able to get us to grow back a fingertip.

But what if we could grow ourselves anew, a whole arm or a liver?

Scientists are trying to figure out exactly how planarians do it, and maybe one day these humble flatworms could inspire new ways to heal our injuries.

This axolotl salamander just doesn't want to grow up.

They can teach us a few things about staying young if we help them out.

The axolotl isn't just a frilly salamander with a half smile and a blank stare.

It's named after an Aztec god who escaped death by morphing into this amphibian shape.

It's a creature that's a bridge between worlds, the spiritual and the natural, the aquatic and the terrestrial, survival and extinction.

Like most of its salamander relatives, the axolotl begins its life as a tiny translucent egg laid underwater.

It grows into a tadpole, a larva with gills, and a flattened fin-shaped tail.

In time, many salamanders shed these features, develop lungs, and crawl onto land, but the axolotl never makes the transition ashore.

Even as it grows up to a foot in length, it hangs onto its larval lifestyle.

The gills are reddish purple from the blood cells pulsing through them, pulling oxygen directly from the water.

To breathe, it just flaps those gills.

But if that doesn't work, like if there's not enough oxygen in the water, the axolotl has a backup, fully functional lungs it can use to gulp air from the surface.

These organs are evolutionary leftovers from when its ancestors lived on land.

The axolotl's lungs are a rare example of cryptic metamorphosis when an animal matures to its next stage of development internally but doesn't show it on the outside.

Despite its flexibility, this magnificent vertebrate is in trouble.

Since the 16th century, the axolotl's native habitat, Lake Xochimilco in Mexico City, has been built over and polluted as this mega city grew.

Now the lake is just a few canals where the axolotls battle invasive predators and warming waters from climate change.

Here in the remnants of their ancestral home, only a few hundred are left.

But scattered across the globe, you'll find thousands of them.

Not in the wild though, there in research labs.

Scientists are impressed by the axolotl's ability to rapidly regenerate damaged limbs, organs, even parts of their brain, and that's amazing for a vertebrate, an animal with a backbone like us.

Harnessing this healing ability for humans would be a breakthrough.

But what does it mean if an animal is thriving only in captivity?

What is a species without its natural habitat?

To preserve what's left of the axolotl's wild home, a team of biologists at the National Autonomous University of Mexico started an innovative collaboration.

They work with farmers of the Chinampas, the traditional floating gardens along the Xochimilco canals where the animals still live.

The farmers filter canal water for crops through native aquatic plants and gravel without the use of pesticides.

This protects axolotls from both pollution and invasive predators.

For many in Mexico City, axolotls have come to symbolize the connection between humans and the natural world.

Humans have put axolotls and our environment in peril, but with traditional knowledge, respect, and some ingenuity, we can give these remarkable animals a chance to come back.

Now, are you on a protein kick?

This hungry caterpillar is storing up enough protein for a radical transformation and a completely new way of life.

Let's face it, butterflies get all the glory, fluttering around with their flashy looks, but caterpillars, they're more than just an awkward adolescent phase.

They do all the hard work so that one day they can look like this.

As adults, butterflies really only have one job, meaning they're basically flying reproduction machines, powered by sugar water.

Their proboscis is made for sipping nectar, but that liquid diet doesn't give them much of the protein they need to make eggs.

So that hungry, hungry caterpillar has a big job to do.

It's not just eating for itself, it's stockpiling the protein it will need later on to make the next generation.

It starts by eating its way out of the egg.

No leftovers, then it's onto the main course.

These pipevine swallowtail caterpillars are chowing down on their favorite and only food, the pipevine.

For a monarch caterpillar, it's all about the milkweed.

In two weeks, they'll grow 20 times longer.

That's like a human baby growing as tall as a three-story building.

A caterpillar can't see very well, so it uses antennae to smell for food.

Not those big ones on top, those are for feeling around.

It's the tiny ones right down there.

It eats and eats and eats, storing up as much protein as possible.

Leaves are about one quarter protein.

After packing on the pounds for a couple of weeks, this monarch caterpillar is finally ready for the big day.

It weaves a ball of silk on a branch, then it hooks onto it with its backside and lets go.

Look through its skin.

There's something wiggling in there.

The caterpillar pumps and pushes until it splits open, shedding all traces of its youth, including its face.

The outer skin hardens into the chrysalis that protects the pupa inside.

Its stomach shrinks.

It's giving up its gluttonous ways.

When the butterfly emerges days later, it uncurls its brand new probosis, two pieces called galeae that have to be zipped together into a single straw before the butterfly can use it, and it lets those glorious wings unfold.

They'll take it to fine mates and a juicy new plant to lay its eggs on using that stored up protein.

This complete metamorphosis is called homometaboly, multiple lives in a single life.

It takes a lot of work and butterflies only live a few weeks, but this strategy is not as rare as you might think.

In the animal kingdom, it's incredibly common.

Mammals like us are the weird ones.

After all, our face doesn't fall off when we grow up.