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Our planet is the greatest
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living puzzle in the universe.
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A collection of worlds
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within worlds,
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each one a network
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of relationships and connections
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between all their living parts,
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leading to the diverse
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and complex world we live in.
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And at the heart of
many of these worlds
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is a very special group of animals -
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the insects,
and their close relatives -
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the arachnids and crustaceans,
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classed together as the arthropods.
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Together, they account for 80% of
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all animal species on our planet.
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In these three specials,
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we're going to explore
the connections and relationships
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that they have with us,
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our planet...
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..and with each other,
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ultimately, to understand
how this group
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hold the key to life itself
inside nature's microworlds.
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Arthropods are the most abundant
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and diverse
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group of animals in the world.
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They inhabit
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every continent and every ocean...
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..from the harsh climate
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of Antarctica...
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..to the driest of deserts
around the equator.
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From the lushest jungles
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to the highest mountain peaks.
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There are an estimated
ten million species.
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More than all the other
animal groups combined.
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But what is
the secret of their success?
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What is the single key
to their global domination?
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To understand this,
we need to unpick, one by one,
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the factors that influence
the lives of arthropods.
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First, we need to understand
what arthropods are
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and where they came from.
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And to do that,
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we must travel to Delaware Bay,
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on the east coast of America.
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400 million years ago,
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the first land animals
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pulled themselves out of the sea.
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They were the ancestors of
the arthropods
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who dominate our planet today.
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And each spring, on the highest
tides of the full and new moons,
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a similar spectacle still occurs.
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These are horseshoe crabs -
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some of the most primitive
arthropods alive today.
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They're here to breed.
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The males are two-thirds
the size of their mates
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and cluster along the water's edge
as the females arrive.
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Clinging to the female's shell,
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he's pulled up the beach,
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fertilising her eggs as she lays.
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For these crabs,
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breeding on land is only possible
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because of the arthropod blueprint.
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Their basic body plan
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is the same as all arthropods -
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a hard exoskeleton,
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segmented body,
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and jointed legs.
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A body plan that's persisted
unchanged for 400 million years.
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A body plan
as adaptable as it is simple.
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The key design feature
is no doubt the exoskeleton.
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A hard external skeleton,
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largely made of chitin,
that provides protection,
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support for muscles,
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but most importantly,
prevents water loss from the body.
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The ability of the exoskeleton
to retain water
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is the factor
that allows these crabs
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to lay their eggs on land
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and ultimately allowed arthropods
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to be the first animals
to colonise the land.
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But even this master of design,
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the exoskeleton, has a weakness.
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How do you grow
inside a suit of armour?
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To see that,
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we must travel
400 miles south
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to a freshwater microworld
in South Carolina.
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This crayfish
is getting ready to transform -
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something all arthropods must do
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to increase their size.
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In preparation,
he withdraws calcium from his shell
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and stores it in little white
tablets on the side of his head.
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His hard exoskeleton then splits,
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and the larger
soft-bodied crayfish emerges.
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It can take up to a few days for
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the new exoskeleton to fully harden,
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and before this occurs,
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the crayfish is vulnerable.
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This queen snake
is on a hunt for a meal.
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The hardened armour of the crayfish
is too much for her,
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but she can detect the chemicals
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given off by the soft body
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of a freshly moulted crayfish
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and knows this is her chance.
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As soft as a doughnut,
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this crayfish provides
a hearty meal.
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So the exoskeleton,
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vital to their success,
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also leaves them vulnerable.
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And it has another significant
defining effect on all arthropods.
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It restricts their size.
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This is the largest terrestrial
arthropod - the coconut crab.
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This crab will moult each year,
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and can live until he's 60,
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yet is still no larger than
a newborn baby.
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If you must transform
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every time you need to grow,
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then there's a limit to
how big you can get.
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But this apparent size disadvantage
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can be seen as an asset.
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It opens up a whole new world
for the arthropods.
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And to see how,
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we must travel to
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the Flow Country,
in Scotland.
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This is the fairy wasp,
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a quarter of a millimetre long.
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They spend nearly
all their lives underwater.
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She is one of the smallest known
arthropods alive on the planet
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and almost invisible
to the naked eye.
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She makes a tiny water flea
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look like a giant.
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This female is looking for
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a very specific place
to lay her eggs.
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She positions
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her microscopic ovipositor
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to pierce into the stem of a plant,
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where the eggs of a water beetle
have been laid.
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She deposits up to 100 eggs
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inside a single beetle egg.
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When the young hatch,
they have a ready source of food -
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the water beetle's
undeveloped young.
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It's only the size of this wasp
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that opens up this niche
for her to exploit.
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In fact, the arthropods'
diminutive size
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gives them a real trump card
in the game of life.
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It allows them
to exploit microhabitats.
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Be it an egg,
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a gall on an oak tree,
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or a single leaf.
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The wealth of habitats and niches
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available to them
is virtually infinite.
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But being small
also makes them vulnerable.
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Their size and abundance
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makes them an ideal food source
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for a whole host of other animals...
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..including the biggest animal
to have ever lived.
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The blue whale.
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So if size is not the ultimate
key to their success,
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we must look elsewhere and examine
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why the diversity of arthropods
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has evolved so much faster
than in all other animal groups.
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How do the arthropods
speed up their evolution?
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To find out,
we must travel to a microworld
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in our very own back gardens.
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This is a female cabbage aphid.
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In spring, she produces
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100 offspring in just a week.
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Her offspring
already have babies inside them,
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so this mother aphid
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is nurturing her granddaughters
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even before they're born.
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If all her descendants
survived and bred,
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by the end of the summer,
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there'd be
1,560 billion trillion aphids!
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She is the ultimate
breeding machine.
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This ability to reproduce
prolifically
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is a real trait of the arthropods.
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Having lots of offspring
and a quick life cycle
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increases the chances of
genetic mutations occurring.
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And it's these mutations,
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however bizarre they seem,
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that increase the diversity
of the gene pool.
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They can provide novel solutions
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to life's challenges...
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..and ultimately lead to
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the evolution of new species.
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This means that arthropods have the
ability to respond to opportunities
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and potentially fill niches faster
than any other group of animals.
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This does gives them a winning hand
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and provides another
vital piece in our jigsaw.
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But it's not the whole story.
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We've seen how arthropods have been
around for over 400 million years,
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how their winning body plan
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has stood them in good stead
for all this time.
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We've seen how their size enables
them to exploit microhabitats,
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and rapid reproduction
has accelerated their evolution.
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All factors that have enabled them
to fly,
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squelch, crawl,
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and scuttle
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into the myriad of forms
we see today.
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But there's a twist in the tale.
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For the real key to their
global success and diversity,
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we must look much closer to home.
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To really understand
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the diversity of
the arthropods,
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we must examine
how the presence
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of other arthropods
affects them.
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Firstly, let's look at
what can be achieved
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when arthropods work together.
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This colony of 40,000 bees
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is focused on a single task -
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the production of honey.
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They have a single queen,
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who is the only bee to lay eggs.
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She can lay up to 2,000
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in a single day.
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Each bee is roughly
a centimetre and a half in length.
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Their exoskeleton prevents them
from growing any further
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in this adult form.
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But there's an animal over a million
times its weight on the prowl...
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..and he's after their honey.
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00:15:23,980 --> 00:15:27,460
To lose their honey now
would be fatal for their colony.
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It'll be their only source of food
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during the cold winter months.
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But this super society won't give up
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its hard-won prize without a fight.
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They attack and sting en masse.
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The first stinger
releases an alarm pheromone
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that tells other bees to join in.
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The stings are barbed
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and hook into the skin of the bear.
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Each sting has its
own nervous system
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and pumps venom
into the attacker.
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Worker bees can only sting once...
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..and then they die.
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It's the ultimate sacrifice
for their colony.
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But the bees are victorious.
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The bear is repelled.
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Teamwork enables these bees
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to defend their precious honey
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against an intruder many
thousands of times their own size.
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So working together can offer
great advantages for defence.
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00:16:49,220 --> 00:16:53,380
But it's one thing if your attacker
is an animal like a black bear
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and quite another
if it's the environment itself.
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So how does working as a team
allow you to live in
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one of the toughest environments
in the world?
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To find out, we must
travel 9,000 miles
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to the mangroves
of Australia.
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They may look like
a benign place to live,
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00:17:20,380 --> 00:17:22,620
but they're actually packed with
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00:17:22,620 --> 00:17:24,860
potentially deadly natural forces.
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00:17:28,260 --> 00:17:31,620
Hot tropical sun, salty water,
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00:17:31,620 --> 00:17:34,300
and the risk of drowning
with every high tide.
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00:17:39,020 --> 00:17:41,220
But in the mangrove mud
is where these ants
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have chosen to build their nest.
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And teamwork has turned the tide
to their advantage.
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Each high tide
brings a fresh bounty of food
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00:17:54,700 --> 00:17:56,740
that the ants can harvest.
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00:17:59,940 --> 00:18:02,180
But this tide also brings
the threat of drowning
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00:18:02,180 --> 00:18:06,100
and the precious ant larvae
being swept away.
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00:18:07,460 --> 00:18:09,260
The ants have a cunning strategy.
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00:18:21,580 --> 00:18:24,620
Every tide,
these ants combine their forces
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00:18:24,620 --> 00:18:26,860
and move the entire nest,
larvae and all,
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into bell-shaped chambers
they've built,
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which trap the air, keeping them
safely above the waterline.
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00:18:35,220 --> 00:18:39,300
Here, they'll remain for several
hours, until the tide retreats.
265
00:18:45,460 --> 00:18:49,460
So cooperation between arthropods
of the same species
266
00:18:49,460 --> 00:18:51,980
offers great advantages.
267
00:18:56,460 --> 00:18:58,300
But we must look elsewhere
268
00:18:58,300 --> 00:19:01,300
for the ultimate key
to the success of the arthropods.
269
00:19:03,580 --> 00:19:05,340
We must examine the idea
270
00:19:05,340 --> 00:19:08,100
that diversity stems
not only from cooperation,
271
00:19:08,100 --> 00:19:11,300
but also exploitation.
272
00:19:17,380 --> 00:19:20,380
A female bolas spider.
273
00:19:20,380 --> 00:19:22,500
She's hunting for food...
274
00:19:24,380 --> 00:19:27,260
..and she's got
a pretty unique way of doing it.
275
00:19:30,660 --> 00:19:33,900
She spins a single thread of silk,
276
00:19:33,900 --> 00:19:35,380
thinner than a human hair,
277
00:19:35,380 --> 00:19:37,100
with a sticky globule on the end.
278
00:19:39,500 --> 00:19:43,300
And from special glands
on her abdomen,
279
00:19:43,300 --> 00:19:45,300
she produces a pheromone
to attract her prey.
280
00:19:49,180 --> 00:19:50,580
She waits.
281
00:19:53,740 --> 00:19:55,380
An approaching moth,
282
00:19:55,380 --> 00:19:58,620
but she hasn't quite got her eye in.
283
00:20:05,540 --> 00:20:07,300
She doesn't miss twice.
284
00:20:14,140 --> 00:20:17,460
So arthropods affect
their own diversity
285
00:20:17,460 --> 00:20:19,140
by providing a food source.
286
00:20:21,100 --> 00:20:23,860
And necessity being
the mother of all invention,
287
00:20:23,860 --> 00:20:26,860
they've evolved
some diverse and ingenious ways
288
00:20:26,860 --> 00:20:28,620
of catching each other.
289
00:20:34,380 --> 00:20:38,860
But the exploitative relationships
between them don't end there.
290
00:20:42,500 --> 00:20:44,540
The Mojave desert in California.
291
00:20:45,980 --> 00:20:48,820
This is a female blister beetle.
292
00:20:52,860 --> 00:20:55,700
She lays her eggs
in the scorching sand of the desert,
293
00:20:55,700 --> 00:20:57,980
a few centimetres below the surface.
294
00:21:00,580 --> 00:21:02,620
The perfect temperature
for incubation.
295
00:21:03,820 --> 00:21:06,140
When the larvae hatch,
296
00:21:06,140 --> 00:21:08,940
they need to find food - and fast.
297
00:21:10,940 --> 00:21:12,980
En masse,
they climb the nearest stem of grass
298
00:21:12,980 --> 00:21:14,860
and form a cluster.
299
00:21:18,500 --> 00:21:20,500
They release a pheromone
300
00:21:20,500 --> 00:21:23,580
identical to that produced by
a female digger bee,
301
00:21:23,580 --> 00:21:25,700
and it isn't long
before they're noticed.
302
00:21:28,100 --> 00:21:32,820
This is a male digger bee
and he's come to mate,
303
00:21:32,820 --> 00:21:34,780
but instead of an amorous reception,
304
00:21:34,780 --> 00:21:37,380
he's boarded by hundreds of larvae.
305
00:21:39,980 --> 00:21:41,980
Temporarily stunned,
306
00:21:41,980 --> 00:21:43,420
he falls to the floor,
307
00:21:43,420 --> 00:21:45,060
but quickly regains his composure...
308
00:21:46,260 --> 00:21:48,820
..and heads off
in search of another female.
309
00:21:51,580 --> 00:21:53,380
While he's mating,
310
00:21:53,380 --> 00:21:55,740
his stowaways jump ship.
311
00:22:02,540 --> 00:22:05,220
The female then returns to
her burrow,
312
00:22:05,220 --> 00:22:07,220
where she's secreted pollen
313
00:22:07,220 --> 00:22:08,820
for her unhatched young.
314
00:22:14,140 --> 00:22:17,540
The blister beetle's larvae
have reached safety
315
00:22:17,540 --> 00:22:20,340
and a ready supply of food,
316
00:22:20,340 --> 00:22:22,940
and when the pollen is finished,
317
00:22:22,940 --> 00:22:25,420
they'll consume
the young of the digger bee.
318
00:22:28,780 --> 00:22:30,740
So we know that predation
319
00:22:30,740 --> 00:22:33,420
and exploitation lead to diversity.
320
00:22:37,340 --> 00:22:39,460
But there's one final factor
321
00:22:39,460 --> 00:22:42,700
that affects
the success of the arthropods.
322
00:22:42,700 --> 00:22:45,460
One final vital key that unlocks
a greater understanding
323
00:22:45,460 --> 00:22:48,660
of their world, and without which,
324
00:22:48,660 --> 00:22:50,580
the array of life we see today
325
00:22:50,580 --> 00:22:53,140
would not be present,
326
00:22:53,140 --> 00:22:55,740
and it's diversity itself.
327
00:23:09,740 --> 00:23:13,500
Diversity breeds diversity.
328
00:23:13,500 --> 00:23:17,620
Arthropods are part of
a constant arms race
329
00:23:17,620 --> 00:23:21,420
to outmanoeuvre
and exploit each other,
330
00:23:21,420 --> 00:23:22,940
a process called coevolution.
331
00:23:22,940 --> 00:23:26,260
To see coevolution
at work,
332
00:23:26,260 --> 00:23:27,780
we must head to
a meadow
333
00:23:27,780 --> 00:23:29,260
in the Swiss Alps.
334
00:23:34,340 --> 00:23:38,020
This is a female
Alcon Blue butterfly.
335
00:23:40,780 --> 00:23:43,260
She lays her eggs
on the gentian plant,
336
00:23:43,260 --> 00:23:45,740
and when they've hatched
into caterpillars
337
00:23:45,740 --> 00:23:48,540
and eaten their fill,
they drop onto the ground.
338
00:23:50,340 --> 00:23:52,580
Surrounded by foraging ants,
339
00:23:52,580 --> 00:23:55,060
you'd think they'd be at risk,
340
00:23:55,060 --> 00:23:56,940
but the ants dutifully collect them
341
00:23:56,940 --> 00:23:59,220
and transport them
back to their nest,
342
00:23:59,220 --> 00:24:01,740
where they're cleaned and fed.
343
00:24:07,660 --> 00:24:10,460
The Alcon Blue caterpillars
have bewitched the ants.
344
00:24:12,140 --> 00:24:14,180
They emit chemicals
that convince the ants
345
00:24:14,180 --> 00:24:16,620
they're one of their own larvae,
346
00:24:16,620 --> 00:24:18,420
even producing noises
347
00:24:18,420 --> 00:24:20,940
similar to those
made by the queen ant,
348
00:24:20,940 --> 00:24:22,860
to ensure the royal treatment.
349
00:24:24,420 --> 00:24:28,020
They've checked in
to a five-star ant hotel.
350
00:24:31,780 --> 00:24:34,980
But the ingenuity of this
butterfly species
351
00:24:34,980 --> 00:24:39,140
presents an opportunity to any other
species canny enough to exploit it.
352
00:24:44,260 --> 00:24:46,900
This is an ichneumon wasp.
353
00:24:49,340 --> 00:24:51,660
Out of hundreds of ants' nests,
354
00:24:51,660 --> 00:24:53,340
she's able to detect the one
355
00:24:53,340 --> 00:24:56,300
that contains
an Alcon butterfly caterpillar.
356
00:25:00,940 --> 00:25:03,620
The ants' response
to any invader is to attack...
357
00:25:06,660 --> 00:25:08,300
..but she releases a pheromone
358
00:25:08,300 --> 00:25:09,820
that deranges the ants.
359
00:25:12,620 --> 00:25:14,580
They begin to attack each other...
360
00:25:17,380 --> 00:25:20,300
..and this buys her
the time she needs.
361
00:25:26,620 --> 00:25:28,300
The wasp makes a beeline for
362
00:25:28,300 --> 00:25:30,140
the butterfly larvae,
363
00:25:30,140 --> 00:25:31,940
where she will lay her eggs.
364
00:25:42,620 --> 00:25:45,180
Job complete, she leaves the nest.
365
00:25:49,220 --> 00:25:52,580
The ants' nest,
released from the wasp's spell,
366
00:25:52,580 --> 00:25:54,060
returns to normal.
367
00:26:00,700 --> 00:26:03,020
The caterpillars remain in the nest
368
00:26:03,020 --> 00:26:04,660
until the following summer,
369
00:26:04,660 --> 00:26:06,300
when they transform into a pupa,
370
00:26:06,300 --> 00:26:07,500
and after about a month,
371
00:26:07,500 --> 00:26:09,780
the adult butterflies emerge.
372
00:26:20,300 --> 00:26:22,580
But not every pupa contains
373
00:26:22,580 --> 00:26:24,380
an Alcon Blue adult.
374
00:26:26,620 --> 00:26:28,420
A young ichneumon wasp.
375
00:26:40,980 --> 00:26:44,180
The exploiter
has become the exploited.
376
00:26:49,300 --> 00:26:52,820
Diversity has presented opportunity.
377
00:26:57,420 --> 00:27:00,820
The wasp has evolved
to exploit the butterfly...
378
00:27:02,540 --> 00:27:05,100
..who exploits the ants.
379
00:27:05,100 --> 00:27:07,420
Their lives are intricately linked.
380
00:27:07,420 --> 00:27:10,620
They have coevolved.
381
00:27:12,300 --> 00:27:14,900
And this is the final vital key
382
00:27:14,900 --> 00:27:16,380
to the abundance
383
00:27:16,380 --> 00:27:18,380
and diversity of the arthropods.
384
00:27:18,380 --> 00:27:21,380
The process of coevolution.
385
00:27:23,020 --> 00:27:24,300
All over the world,
386
00:27:24,300 --> 00:27:27,500
the opportunities
presented by other arthropods
387
00:27:27,500 --> 00:27:30,300
are creating the pressure
to innovate and evolve.
388
00:27:32,580 --> 00:27:34,660
No doubt this diversity
wouldn't exist
389
00:27:34,660 --> 00:27:38,100
if it weren't for
that winning body plan,
390
00:27:38,100 --> 00:27:40,340
small size,
391
00:27:40,340 --> 00:27:42,900
ability to reproduce,
392
00:27:42,900 --> 00:27:44,780
work cooperatively,
393
00:27:44,780 --> 00:27:48,100
and finally,
predate and exploit each other.
394
00:27:51,540 --> 00:27:53,540
But it's this snowballing effect
395
00:27:53,540 --> 00:27:54,860
of diversity itself
396
00:27:54,860 --> 00:27:56,620
that raises arthropods onto
397
00:27:56,620 --> 00:27:58,340
a whole new level.
398
00:28:02,180 --> 00:28:03,620
The arthropods have lived,
399
00:28:03,620 --> 00:28:06,340
survived and thrived on Earth
400
00:28:06,340 --> 00:28:08,980
for 400 million years...
401
00:28:08,980 --> 00:28:11,300
THUNDER BREAKS
402
00:28:12,900 --> 00:28:15,020
..through extreme climate change,
403
00:28:15,020 --> 00:28:17,700
volcanic events,
404
00:28:17,700 --> 00:28:21,500
ice ages,
405
00:28:21,500 --> 00:28:23,540
and the extinction of the dinosaurs.
406
00:28:25,820 --> 00:28:30,700
In fact,
arthropods have survived and thrived
407
00:28:30,700 --> 00:28:33,300
when 99.9% of all species
408
00:28:33,300 --> 00:28:36,380
that ever lived have become extinct.
409
00:28:38,460 --> 00:28:40,460
They are the most successful
410
00:28:40,460 --> 00:28:43,820
and diverse group of animals
to have ever lived.
411
00:29:06,620 --> 00:29:10,300
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