Original subtitles

Our planet is the greatest

living puzzle in the universe.

A collection of worlds

within worlds,

each one a network

of relationships and connections

between all their living parts,

leading to the diverse

and complex world we live in.

And at the heart of many of these worlds

is a very special group of animals -

the insects, and their close relatives -

the arachnids and crustaceans,

classed together as the arthropods.

Together, they account for 80% of

all animal species on our planet.

In these three specials,

we're going to explore the connections and relationships

that they have with us,

our planet...

..and with each other,

ultimately, to understand how this group

hold the key to life itself inside nature's microworlds.

Arthropods are the most abundant

and diverse

group of animals in the world.

They inhabit

every continent and every ocean...

..from the harsh climate

of Antarctica...

..to the driest of deserts around the equator.

From the lushest jungles

to the highest mountain peaks.

There are an estimated ten million species.

More than all the other animal groups combined.

But what is the secret of their success?

What is the single key to their global domination?

To understand this, we need to unpick, one by one,

the factors that influence the lives of arthropods.

First, we need to understand what arthropods are

and where they came from.

And to do that,

we must travel to Delaware Bay,

on the east coast of America.

400 million years ago,

the first land animals

pulled themselves out of the sea.

They were the ancestors of the arthropods

who dominate our planet today.

And each spring, on the highest tides of the full and new moons,

a similar spectacle still occurs.

These are horseshoe crabs -

some of the most primitive arthropods alive today.

They're here to breed.

The males are two-thirds the size of their mates

and cluster along the water's edge as the females arrive.

Clinging to the female's shell,

he's pulled up the beach,

fertilising her eggs as she lays.

For these crabs,

breeding on land is only possible

because of the arthropod blueprint.

Their basic body plan

is the same as all arthropods -

a hard exoskeleton,

segmented body,

and jointed legs.

A body plan that's persisted unchanged for 400 million years.

A body plan as adaptable as it is simple.

The key design feature is no doubt the exoskeleton.

A hard external skeleton,

largely made of chitin, that provides protection,

support for muscles,

but most importantly, prevents water loss from the body.

The ability of the exoskeleton to retain water

is the factor that allows these crabs

to lay their eggs on land

and ultimately allowed arthropods

to be the first animals to colonise the land.

But even this master of design,

the exoskeleton, has a weakness.

How do you grow inside a suit of armour?

To see that,

we must travel 400 miles south

to a freshwater microworld in South Carolina.

This crayfish is getting ready to transform -

something all arthropods must do

to increase their size.

In preparation, he withdraws calcium from his shell

and stores it in little white tablets on the side of his head.

His hard exoskeleton then splits,

and the larger soft-bodied crayfish emerges.

It can take up to a few days for

the new exoskeleton to fully harden,

and before this occurs,

the crayfish is vulnerable.

This queen snake is on a hunt for a meal.

The hardened armour of the crayfish is too much for her,

but she can detect the chemicals

given off by the soft body

of a freshly moulted crayfish

and knows this is her chance.

As soft as a doughnut,

this crayfish provides a hearty meal.

So the exoskeleton,

vital to their success,

also leaves them vulnerable.

And it has another significant defining effect on all arthropods.

It restricts their size.

This is the largest terrestrial arthropod - the coconut crab.

This crab will moult each year,

and can live until he's 60,

yet is still no larger than a newborn baby.

If you must transform

every time you need to grow,

then there's a limit to how big you can get.

But this apparent size disadvantage

can be seen as an asset.

It opens up a whole new world for the arthropods.

And to see how,

we must travel to

the Flow Country, in Scotland.

This is the fairy wasp,

a quarter of a millimetre long.

They spend nearly all their lives underwater.

She is one of the smallest known arthropods alive on the planet

and almost invisible to the naked eye.

She makes a tiny water flea

look like a giant.

This female is looking for

a very specific place to lay her eggs.

She positions

her microscopic ovipositor

to pierce into the stem of a plant,

where the eggs of a water beetle have been laid.

She deposits up to 100 eggs

inside a single beetle egg.

When the young hatch, they have a ready source of food -

the water beetle's undeveloped young.

It's only the size of this wasp

that opens up this niche for her to exploit.

In fact, the arthropods' diminutive size

gives them a real trump card in the game of life.

It allows them to exploit microhabitats.

Be it an egg,

a gall on an oak tree,

or a single leaf.

The wealth of habitats and niches

available to them is virtually infinite.

But being small also makes them vulnerable.

Their size and abundance

makes them an ideal food source

for a whole host of other animals...

..including the biggest animal to have ever lived.

The blue whale.

So if size is not the ultimate key to their success,

we must look elsewhere and examine

why the diversity of arthropods

has evolved so much faster than in all other animal groups.

How do the arthropods speed up their evolution?

To find out, we must travel to a microworld

in our very own back gardens.

This is a female cabbage aphid.

In spring, she produces

100 offspring in just a week.

Her offspring already have babies inside them,

so this mother aphid

is nurturing her granddaughters

even before they're born.

If all her descendants survived and bred,

by the end of the summer,

there'd be 1,560 billion trillion aphids!

She is the ultimate breeding machine.

This ability to reproduce prolifically

is a real trait of the arthropods.

Having lots of offspring and a quick life cycle

increases the chances of genetic mutations occurring.

And it's these mutations,

however bizarre they seem,

that increase the diversity of the gene pool.

They can provide novel solutions

to life's challenges...

..and ultimately lead to

the evolution of new species.

This means that arthropods have the ability to respond to opportunities

and potentially fill niches faster than any other group of animals.

This does gives them a winning hand

and provides another vital piece in our jigsaw.

But it's not the whole story.

We've seen how arthropods have been around for over 400 million years,

how their winning body plan

has stood them in good stead for all this time.

We've seen how their size enables them to exploit microhabitats,

and rapid reproduction has accelerated their evolution.

All factors that have enabled them to fly,

squelch, crawl,

and scuttle

into the myriad of forms we see today.

But there's a twist in the tale.

For the real key to their global success and diversity,

we must look much closer to home.

To really understand

the diversity of the arthropods,

we must examine how the presence

of other arthropods affects them.

Firstly, let's look at what can be achieved

when arthropods work together.

This colony of 40,000 bees

is focused on a single task -

the production of honey.

They have a single queen,

who is the only bee to lay eggs.

She can lay up to 2,000

in a single day.

Each bee is roughly a centimetre and a half in length.

Their exoskeleton prevents them from growing any further

in this adult form.

But there's an animal over a million times its weight on the prowl...

..and he's after their honey.

To lose their honey now would be fatal for their colony.

It'll be their only source of food

during the cold winter months.

But this super society won't give up

its hard-won prize without a fight.

They attack and sting en masse.

The first stinger releases an alarm pheromone

that tells other bees to join in.

The stings are barbed

and hook into the skin of the bear.

Each sting has its own nervous system

and pumps venom into the attacker.

Worker bees can only sting once...

..and then they die.

It's the ultimate sacrifice for their colony.

But the bees are victorious.

The bear is repelled.

Teamwork enables these bees

to defend their precious honey

against an intruder many thousands of times their own size.

So working together can offer great advantages for defence.

But it's one thing if your attacker is an animal like a black bear

and quite another if it's the environment itself.

So how does working as a team allow you to live in

one of the toughest environments in the world?

To find out, we must travel 9,000 miles

to the mangroves of Australia.

They may look like a benign place to live,

but they're actually packed with

potentially deadly natural forces.

Hot tropical sun, salty water,

and the risk of drowning with every high tide.

But in the mangrove mud is where these ants

have chosen to build their nest.

And teamwork has turned the tide to their advantage.

Each high tide brings a fresh bounty of food

that the ants can harvest.

But this tide also brings the threat of drowning

and the precious ant larvae being swept away.

The ants have a cunning strategy.

Every tide, these ants combine their forces

and move the entire nest, larvae and all,

into bell-shaped chambers they've built,

which trap the air, keeping them safely above the waterline.

Here, they'll remain for several hours, until the tide retreats.

So cooperation between arthropods of the same species

offers great advantages.

But we must look elsewhere

for the ultimate key to the success of the arthropods.

We must examine the idea

that diversity stems not only from cooperation,

but also exploitation.

A female bolas spider.

She's hunting for food...

..and she's got a pretty unique way of doing it.

She spins a single thread of silk,

thinner than a human hair,

with a sticky globule on the end.

And from special glands on her abdomen,

she produces a pheromone to attract her prey.

She waits.

An approaching moth,

but she hasn't quite got her eye in.

She doesn't miss twice.

So arthropods affect their own diversity

by providing a food source.

And necessity being the mother of all invention,

they've evolved some diverse and ingenious ways

of catching each other.

But the exploitative relationships between them don't end there.

The Mojave desert in California.

This is a female blister beetle.

She lays her eggs in the scorching sand of the desert,

a few centimetres below the surface.

The perfect temperature for incubation.

When the larvae hatch,

they need to find food - and fast.

En masse, they climb the nearest stem of grass

and form a cluster.

They release a pheromone

identical to that produced by a female digger bee,

and it isn't long before they're noticed.

This is a male digger bee and he's come to mate,

but instead of an amorous reception,

he's boarded by hundreds of larvae.

Temporarily stunned,

he falls to the floor,

but quickly regains his composure...

..and heads off in search of another female.

While he's mating,

his stowaways jump ship.

The female then returns to her burrow,

where she's secreted pollen

for her unhatched young.

The blister beetle's larvae have reached safety

and a ready supply of food,

and when the pollen is finished,

they'll consume the young of the digger bee.

So we know that predation

and exploitation lead to diversity.

But there's one final factor

that affects the success of the arthropods.

One final vital key that unlocks a greater understanding

of their world, and without which,

the array of life we see today

would not be present,

and it's diversity itself.

Diversity breeds diversity.

Arthropods are part of a constant arms race

to outmanoeuvre and exploit each other,

a process called coevolution.

To see coevolution at work,

we must head to a meadow

in the Swiss Alps.

This is a female Alcon Blue butterfly.

She lays her eggs on the gentian plant,

and when they've hatched into caterpillars

and eaten their fill, they drop onto the ground.

Surrounded by foraging ants,

you'd think they'd be at risk,

but the ants dutifully collect them

and transport them back to their nest,

where they're cleaned and fed.

The Alcon Blue caterpillars have bewitched the ants.

They emit chemicals that convince the ants

they're one of their own larvae,

even producing noises

similar to those made by the queen ant,

to ensure the royal treatment.

They've checked in to a five-star ant hotel.

But the ingenuity of this butterfly species

presents an opportunity to any other species canny enough to exploit it.

This is an ichneumon wasp.

Out of hundreds of ants' nests,

she's able to detect the one

that contains an Alcon butterfly caterpillar.

The ants' response to any invader is to attack...

..but she releases a pheromone

that deranges the ants.

They begin to attack each other...

..and this buys her the time she needs.

The wasp makes a beeline for

the butterfly larvae,

where she will lay her eggs.

Job complete, she leaves the nest.

The ants' nest, released from the wasp's spell,

returns to normal.

The caterpillars remain in the nest

until the following summer,

when they transform into a pupa,

and after about a month,

the adult butterflies emerge.

But not every pupa contains

an Alcon Blue adult.

A young ichneumon wasp.

The exploiter has become the exploited.

Diversity has presented opportunity.

The wasp has evolved to exploit the butterfly...

..who exploits the ants.

Their lives are intricately linked.

They have coevolved.

And this is the final vital key

to the abundance

and diversity of the arthropods.

The process of coevolution.

All over the world,

the opportunities presented by other arthropods

are creating the pressure to innovate and evolve.

No doubt this diversity wouldn't exist

if it weren't for that winning body plan,

small size,

ability to reproduce,

work cooperatively,

and finally, predate and exploit each other.

But it's this snowballing effect

of diversity itself

that raises arthropods onto

a whole new level.

The arthropods have lived,

survived and thrived on Earth

for 400 million years...

THUNDER BREAKS

..through extreme climate change,

volcanic events,

ice ages,

and the extinction of the dinosaurs.

In fact, arthropods have survived and thrived

when 99.9% of all species

that ever lived have become extinct.

They are the most successful

and diverse group of animals to have ever lived.

Subtitles by Red Bee Media Ltd