Original subtitles

Our world is not always the same.

Hidden from our view lies a different world.

Creatures utterly unlike us...

..almost alien.

Yet they are more numerous than any other group on the planet.

Welcome to the fascinating world of the arthropods -

spiders, scorpions and insects.

Today we have new camera techniques that will allow us

to reveal in greater detail than ever before their lives.

The way they fight and feed and reproduce.

This series uses specially developed 3D camera technology to study

the micro world in extraordinary detail, both on location

and in specially constructed environments.

We'll witness their births, the challenges they face,

and the moments when their lives hang in the balance.

And that may help us understand how it is that today

over 80% of all animal species on this planet, are arthropods.

In this series we'll see the way they have evolved,

from the comparative simplicity of the millipede, to vast

colonies that contain hundreds, even millions of individuals.

We'll witness the most extraordinary transformations

in the animal kingdom...

We'll meet ants that farm...

Spiders that can cast their webs.

And the bug that wears the bodies of its victims as a disguise.

Welcome to a strange and dangerous world.

Every species of animal must reproduce.

If it didn't it would go extinct.

Arthropods have developed many ways of doing so.

From courtship and mating...

..to egg laying.

The hatching of larvae...

..to caring for the newly-born young.

And some insects meet the reproductive challenge

by splitting their life cycle in two.

And all in order to produce offspring

and ensure they get the best possible start in life.

The arthropods' success in doing so has lead them

to becoming one of the most abundant forms of animals on this planet.

In the woodlands of Madagascar and parts of Southern Africa lives

a spider that has to rely on stealth in order to mate and father young.

This is the male golden orb web spider.

He hatched two months ago and is now looking for a mate.

He's found a female's web and he's lurking at its edge.

This is the female.

As spiders go, she's huge.

Her body alone is as long as your thumb and her legs span some 15cm.

She's about 20 times the size of the male.

Not only that, she's a deadly predator...

..with a voracious appetite.

And all this makes mating a risky business for the male.

The start of their courtship is triggered by the insect

the female has already captured.

She is distracted by it, so he seizes his opportunity.

He cautiously begins an approach...

He climbs very tentatively onto her abdomen.

Now he's in position, he deposits his sperm.

Success!

His alertness has saved him from becoming the female's next meal.

He will only mate once in his short life

and this is his reward.

He is father to the 400 or so eggs in this egg sac.

A few weeks later the young emerge from it.

The spiderlings are each no bigger than a pinhead.

To begin with,

they stay close to the egg sac from which they emerged.

They moult and then, after 30 days, they start to disperse.

Golden orb web spiders live for only a year.

Mating is the culmination of their lives.

For some creatures though, time is simply too short for mating,

so the females reproduce without a male.

Spring is the season when most arthropod eggs hatch.

But in colder climates, spring arrives late

and the summer is short,

leaving little time to mate

and for the young to grow strong enough to survive the coming winter.

Megabunus, a species of harvestmen, has a way of dealing with that.

The female lives in Alpine forests

and spends the freezing winter sheltered in the leaf litter.

She emerges in spring and starts to hunt.

Her long legs help her to clamber over the moss.

In fact, her legs are so long, that she

has breathing holes in them to supply them directly with oxygen.

But she must reproduce if the species is to survive.

So she does so without mating.

She lays unfertilised eggs which hatch into exact genetic

copies of herself -

clones.

She adapted her reproduction to the harsh climate and

so sacrificed the genetic variation that could've come with sex.

But other plant-eaters in gentler climates use the same

technique to take advantage of the glut of food that comes with spring.

Aphids also clone their offspring, and what is more,

a female produces her young alive.

And she can do so ten times a day or more.

Not only that, each of her offspring will start producing

young of their own within days.

If the descendants of a single female all survived,

they would, by the end of summer, number 600 billion.

All of them identical clones.

But as the winter approaches the aphids

change their way of reproducing.

They lay eggs.

Aphids cannot survive the cold of the winter,

but the eggs are hardy and will hatch next spring.

And then, once again, the aphid population will explode.

The million or so species of arthropod on our planet have matched

the way they reproduce to suit the particular

environment in which they live.

Most of them lay eggs.

And some do so in scarcely believable numbers.

Once such lives on a hedgehog.

Ixodes is a tick - a parasite.

The female is so well adapted to life on a hedgehog that she

rarely lives anywhere else.

She has a limitless supply of food immediately beside her -

blood.

She stays on the hedgehog until she's ready to lay her eggs.

Then she lets go, falls to the ground...

..and starts to deposit her eggs in the undergrowth.

The eggs make up 50% of her entire body weight.

She can lay around 1,500 of them

and it takes her up to 20 days to lay them all.

Producing so many is her way of ensuring that at least one or

two of her young will find their own hedgehog host.

For those that do the cycle can begin again.

By the time Ixodes has produced them all,

her once plump body is deflated and she dies.

Some insects, among them butterflies, have developed

a way of growing that involves a truly astonishing transformation.

This is a Heliconius butterfly.

And THIS is its offspring -

a caterpillar.

The two look as though they're completely different creatures,

but of course, they're not.

The butterfly has divided its life into two halves.

The first half, the caterpillar,

is devoted almost exclusively to gathering food and growing.

And the second, the adult,

is devoted almost entirely to reproduction.

Adult butterflies feed on nectar which they locate

with their antennae, taste through their feet

and collect with long, tube-like mouthparts.

This sugar-rich food fuels their search for a mate.

Once a male and female have found one another, the male uses special

claspers at the end of his abdomen to transfer his sperm to her.

Once fertilised, the female Heliconius lays her

eggs on the leaves of the passion flower plant.

Her young, the caterpillars, are fussy eaters

and these leaves are almost the only ones they will eat.

She lays around 50 eggs and then her work is done.

About a week later the caterpillars emerge.

They are little more than eating machines

and they get down to work immediately.

Some, over a month or two, can grow to 40 times their original size.

They have protective spines to ward off their predators,

but no reproductive organs.

Then, when they've grown enough, their behaviour changes.

They stop eating and settle in a suitable resting place.

Then their skins hardens to form a shell.

This is a chrysalis.

If we could see inside we would witness one of the most

extraordinary changes in the animal kingdom...

..metamorphosis.

Some parts of the caterpillar are transformed

and others disappear completely.

The caterpillar had a massive gut for processing food,

that shrinks for nectar will be easier to digest than

the leaves the caterpillar consume.

The mouth parts must change - the adult needs not munching jaws,

but a tube-like tongue.

And the caterpillar's simple eyes are also transformed.

Searching for a mate needs better eyesight than finding leaves.

Antennae sprout form its head.

It will use them to sniff out the scent of a female or a flower.

And finally, its wings, their shape and colour will

warn off predators and enable it to find and select a suitable mate.

An adult Heliconius butterfly emerges after

eight days of transformation.

Its delicate wings are crumpled and wet.

It stretches them by pumping blood along their veins

and then waits for them to dry before attempting to fly.

From this point on its body will not grow or change...

It will live for just a month or two

and feed just enough to keep itself going.

This body is purely for mating.

A male's antennae can detect females' scent from more than

a kilometre away.

And he's off to find a female.

Success for this butterfly is reproduction,

as it is for all species.

And that need has shaped the bodies, the behaviour,

the entire life cycle of all arthropods, and produced

the dazzling range of forms that we see around the world today.

Every generation must reproduce itself,

if it does not a species will disappear.

From the cunning, tiny make golden orb web spider...

..to the amazingly fertile aphids that clone themselves to make

the most of summer.

And the tick that leaves its hedgehog host to lay its eggs.

The arthropods have evolved reproductive strategies

that are surely among the most fascinating,

almost unbelievable stories, in the natural world.

In our next programme,

I'll be looking at what happens after reproduction.

Over 400 million years ago some early arthropods began to

care for their young and to live in groups.

And for these too, life was about more than just staying alive,

it was about giving the next generation the best

chance of survival.