The Silk Spinners Life in the Undergrowth

You might think that the lights above my head are stars.

But they can't be, because I'm in a cave.

Each one of those tiny lights is produced

by the larva of a small insect called a fungus gnat,

as a way of attracting its prey.

The result is a display that must surely rank as one

of the most magical illuminations in the whole of the natural world.

But shine a light sideways across the ceiling...

..then you can see that each little blue lamp is surrounded by

a curtain of glistening, beaded filaments -

curtains that are invisible at other times.

They're lures

and they can be lethal.

Insects, hatching in the water below,

fly up towards these tiny lights

and here they are trapped by threads of this extraordinary material

that is the unique possession of the invertebrates.

This is silk.

This astonishing cave is near the small town of Waitomo

in New Zealand.

Each light comes from the back end of a larva

as it lies in a transparent tube of mucus,

slung from the ceiling by silken threads.

And it's produced by phosphorescent chemicals

in a special compartment opening from the side of its intestine.

The silk comes from glands at the other end, inside the larva's mouth.

The larvae move around.

They fix a silk thread to the rock

and slowly inch their way over the ceiling along a network of threads.

Arriving in a new position, the larva produces more silk

but this time, it allows the thread to dangle downwards.

As each section emerges from its mouth,

the larva, with a gulp, adds a blob of glue.

Eventually, a single strand may be a metre long.

There can be several hundred larvae

in a single square metre of cave roof.

And they all work hard, producing strand after strand.

The more they make, the greater their chances of catching something.

Below, mayfly are hatching from the stream that runs through the cave.

They've been carried in here by the current from outside - as larvae.

Now, they must look for a mate.

But they find the blue lights above irresistible.

And they're caught.

The fungus gnat detects its victim's struggles

from lines that run between the threads.

Having made a capture, it turns off its light.

That saves energy.

Laboriously, it makes its way across

to the thread from which the vibrations are coming.

It hauls it up and eats what's hanging on the end.

It also eats the filament.

That saves silk.

This wonderful hunting technique

is just one of an enormous number

of varied ways in which animals use silk.

Silk really is an extraordinary material.

It's stronger than a steel thread of the same diameter

and unlike steel, it's elastic.

It can stretch up to twice its length.

The inhabitants of the undergrowth

developed the ability to produce this marvellous material

very early in their evolutionary history, over 300 million years ago.

At first, it seems, they used it in a very simple way,

as an adhesive. And lacewings still do,

though for them, it's an adhesive with a difference.

This is a female.

She is looking for a safe place to deposit her eggs.

Silk will provide it, but not exactly in the way you might think.

She will lay up to 300 eggs, almost twice her body weight.

However, there are plenty of other insects around

that will eat those eggs if they find them.

So, she doesn't glue them directly on the plant stem.

First, she produces a little drop of sticky silk.

Then, at the end of that, the egg.

It's suspended safely in mid-air.

The silk is produced by glands in her abdomen in liquid form.

It's the very act of pulling it out

that changes it from liquid to solid.

And that is true for all invertebrate silk.

She will lay up to 30 eggs a day, each on its own stalk.

That silken thread is so incredibly fine

that insect predators like these ants

walk right by the eggs without realising

that there's a tasty meal within millimetres of them.

So, despite regular ant patrols in search of food,

the lacewing's eggs remain undiscovered.

After three days, they begin to hatch.

Now, at least, if danger threatens,

her offspring will be able to help themselves

by running away.

In the lush rainforest of Trinidad,

you can find sheets of silk, wrapped around trees.

Here it's also used for protection

but by a quite different creature in a quite different way.

The manufacturers,

a little-known group of insects called web-spinners,

live beneath.

They graze on algae and lichens,

hidden by the sheets immediately above them.

They produce their silk, not from their abdomens

or their mouths, but from glands in their forelegs.

Each leg has about 150 tiny silk ejectors

which between them create a thin silken tissue.

An ant in search of prey strolls over the surface

of the web-spinners' marquee.

But the silk sheet, thin though it is, is impervious to smells

and, as long as the web-spinner doesn't move too much,

the ant will be unaware of it, a millimetre beneath its feet.

RUMBLING THUNDER

THUNDERCLAP

And the tent, like any decent tent, is waterproof.

In fact, the tent is so waterproof

that the web-spinners beneath are in danger of not getting enough water.

So, after the storm's over, they bite holes in places

where a little rain has accumulated and drink the tiny puddle dry.

Of course, the hole has to be repaired after a drink

but that's easy enough when you have

an almost limitless supply of silk in your legs.

Of all the inhabitants of the undergrowth to have exploited silk,

none have done so with more variety and skill than the spiders,

and this is almost certainly the first way in which THEY used it.

Here on a bank in the Malaysian rain forest, there are strands of silk

radiating from this little patch in the middle.

Watch what happens if I touch one of them.

There! Oh...

I... I can't help jumping.

That was a trapdoor spider and it was so swift that you hardly saw it.

Let me see if I can get it to do it again.

The spider, when hungry, sits close behind the trapdoor.

The strands outside are all connected

to a silken collar that surrounds the mouth of the hole.

Each of her feet is in contact with it.

The slightest twitch is enough to tell her

that something's moving around outside.

A single twitch will produce no reaction.

That could be caused by a falling leaf or a drop of water.

But a repeated vibration,

especially if it moves from one strand to another, could mean prey.

Prey like this beetle.

Got it! Now, it will kill it.

This is the most ancient of living spiders.

The fact that it has, uniquely, segmented plates on its back

shows that it's more closely-related than any other

to those pioneer hunters, the scorpions.

And like them, it has a powerful venom.

Once bitten, its victim has little chance.

Trip-lines were one of the earliest of the spiders' hunting techniques

but other, later spiders used silk

to build much more sophisticated structures.

Orb webs are so familiar to us

that we tend to forget what complex structures they are.

A single one can contain up to 60 metres of silk,

of up to six different kinds, and involve 3,000 separate attachments.

And what's more, some orb-web spiders spin

a different one every night.

The biggest and best webs are made, in most species, by the female.

She has to start by bridging the gap across which she's to hang her web.

The faintest breeze will catch a filament as she spins it

and carry it away into space.

With luck, it will catch on a suitable anchor point.

There!

She runs across the filament,

trailing a line of much thicker, stronger silk, and ties it off.

Then, she goes back to the middle of this line

and drops down another.

And she tightens it.

The junction at the top becomes the hub of the web

to which she will attach radiating spokes.

These must be particularly strong,

for the shape of the whole structure depends on them.

Once they're complete,

she adds a spiral working from the middle outwards.

This first spiral is quite widely spaced because it's only temporary.

It will serve as scaffolding along which she runs to add a stronger,

stickier and more closely-spaced spiral.

That's this one.

As the filament for this emerges from her spinneret,

she coats it with glue from separate glands in her abdomen.

After completing one section, she eats the scaffolding line.

It has no further use and it saves valuable silk.

At first, this glue is evenly spread,

but each time she fixes a section, she twangs it with her leg

so that it breaks up and forms a line of droplets.

She can complete the whole, intricate,

elegantly symmetrical structure in about an hour.

When an insect strikes the web, the capture spiral stretches

and then retracts to its former size

without distorting its shape and without such a severe recoil

that the insect might be catapulted off again.

The beads of glue are the key.

Tension on the surface of a droplet hauls any slack into it.

When the insect hits,

it pulls out the coils of thread in each droplet,

slowing the insect to a momentary standstill.

And then the surface tension pulls the silk back into each drop.

So, the spiral thread doesn't break

and the web, as a whole, regains its symmetry.

The spider sits with her legs resting on the spokes.

Any vibration on them will travel up her leg

and be received by a small sense organ in the joint.

This is covered with microscopic slits

which are distorted by the slightest movement.

So the spider is immediately aware of the tiniest tremor.

Once alerted, she pulls on neighbouring spokes of the web

to assess exactly in what direction

and how far away the signals originate.

The fly is on the verge of breaking loose.

Here she comes.

She isn't hindered by the glue she put on the capture spiral

because her feet are coated with a special oil.

Once her victim is in her grasp, she produces yet another kind of silk.

It emerges as a sheet from a group of minute spigots.

This is a fuzzy silk used for wrapping

and, at moments like this, as a shroud.

The biggest and strongest webs are those made by Nephila -

the golden orb-web spider of the tropics.

They may be several metres across

and they're strong enough to catch small birds.

This time, only a moth.

After a killing bite, she returns to the hub of her web to wrap it up.

But big webs bring problems.

It's not easy to control what happens on their outer regions.

This is Argyrodes.

She's only one-hundredth the weight of Nephila

so she can move across this huge web undetected. And she's a thief.

A fly has arrived not far from her.

She has a chance to steal it.

But Nephila has also detected its arrival...

..and claims it without much difficulty.

Another fly is caught in the web.

Argyrodes now stands a better chance,

since Nephila is busy feeding.

She cuts the filaments between the fly and Nephila,

so that vibrations made by its struggles won't reach her.

Nephila, sitting at the hub of the web,

seems quite unaware of what is going on at its outer margin.

The fly is now hanging from a single thread.

But it's five times the weight of Argyrodes

and too heavy for her to carry.

She has to be clever.

She attaches a thread to it and runs it up to a twig outside the web.

Nephila is still occupied with her meal.

Another line, just to make sure.

Now she can snip the last filaments of the web and haul it away.

Safely off Nephila's web at last,

Argyrodes can enjoy her stolen meal in safety.

BELL TOLLS

For all its complexity,

the orb web was one of the first kind of silken traps

devised by spiders.

Subsequently, other species modified it in some quite extraordinary ways.

There's a web in this yew tree that's triangular -

a slice, as it were, from an orb.

It's made by Hyptiotes

and her body forms an essential link in its mooring cable.

To be effective, the web has to be very taut.

Hyptiotes ratchets up the tension

by hauling in the main cable and coiling it above her body.

Tighter.

Tighter.

And that's about as tight as it'll go.

Now she has to wait.

Flies can sometimes disentangle themselves from a web,

if the spider doesn't grab them quickly.

But a fly hitting this web won't get that chance.

A strike triggers an instant reaction.

In slow motion, you can see what happens.

Hyptiotes immediately lets go of the coil she was holding over her back.

That causes her web to collapse

and almost instantaneously entangle the prey.

Few flies that hit a Hyptiotes' web manage to escape.

The gladiator spider makes her web

from a very special kind of multi-strand silk

which she backcombs to make fuzzy.

She carefully attaches this to a framework of ordinary,

un-fuzzy filaments.

The fuzzy silk doesn't have glue on it, but will entangle hairy legs.

And it's also extremely elastic, which is crucially important.

It's finished.

She reaches down with her forelegs to check how far away

she is from the ground.

Then she snips most of the framework threads

and holds the fuzzy rectangle between her four front legs.

She's ready.

Her enormous eyes are so sensitive she can hunt in near-darkness.

A bush cricket would make a rich meal, but it's very powerful

and it could put up a good fight.

Now it must be parcelled up and the fuzzy silk makes excellent wrapping,

just as it does for Hyptiotes.

In Australia, there's a species of spider

that has taken web construction a stage further still.

It builds not just in two dimensions but three.

It regularly takes up residence

in people's back yards and on their verandas.

There's one under this plant holder.

It's the notorious and very venomous redback.

What's brought it here

is the extraordinary way in which it uses silk.

The female usually builds at night

and constructs this very elaborate web.

It's not just wide, it's deep.

To make it, she needs two flat surfaces, one beneath the other.

And that's what she has found underneath the plant holder.

First, she drops down, pulling a thread behind her.

She sticks the end to the veranda floor.

Then she goes back up again, trailing a second line,

which she sticks to the first, so strengthening it.

Then she pulls the line tight.

That is a crucial element in the construction.

Down she goes again.

By the time she has finished,

she will have fixed several dozen of these sticky, taut, vertical lines.

An ant is approaching in the distance.

An orb-web would never catch one of these.

It's a scout, leading an exploring party.

Searching beneath the plant holder,

it's almost bound to blunder into one of the redback's lines.

It struggles and so seals its fate.

And its followers go the same way.

The threads carry the vibrations back to the redback, waiting above.

She has no need to hurry.

Her meals are suspended in mid-air. Escape is impossible.

She hauls them up in her own good time.

The redback's trap is certainly economical with silk.

But one North American spider hunts with just a single filament.

This may look like a bird dropping

but that's just a disguise to fool anything that might want to eat it.

In fact, it's a spider,

and one with an even more extraordinary hunting technique.

It's a bolas spider.

Throughout the day, she remains motionless.

But when evening comes, she prepares for action.

She abandons her disguise and starts to move.

Slowly, she makes her way down to the underside of the leaf.

There, she hangs from a horizontal thread.

Next, she starts to spin a single strong line,

pulling it out of her spinneret with her back legs.

And at the end, there is a sticky globule.

This is her bolas. It's all she needs.

She climbs back up to her leaf

and takes up her position on the horizontal thread

with her weighted filament dangling from one of her front legs.

A moth.

She whirls her bolas but misses.

But she has ways of enticing the moth back.

She can produce a pheromone, a chemical perfume,

that the moth finds irresistible.

What is more, she can change it to suit the particular species of moth

that happens to be around.

The moth comes back.

This time, she's got it.

Now, she starts to wrap it.

But she's not finished yet.

Different moths and a different pheromone.

Silk can do other things as well.

It can totally change a spider's lifestyle

and turn a solitary killer into a creature that hunts in great packs.

This enormous web above me contains thousands of spiders.

They're all tiny, but because they work together,

they can kill prey many times their own size.

Any spider sitting on its web might be expected to react aggressively

towards another that approaches it.

But not these.

These tiny, ant-sized spiders seem totally relaxed in one another's presence.

More than that, they cooperate with one another,

working together to repair and extend their huge, silken palace.

There are tens of thousands of them in this one

and they are constantly at work.

Their home can rise 15, 20 metres up towards the canopy.

It's so big, it's a major obstacle in the airways of the forest.

This cricket weighs several hundred times

as much as one of these spiders.

However, the slightest attempt to free itself

only serves to attract lots of them from all over the giant web.

Soon, it's surrounded by hundreds.

They squirt glue from their spinnerets,

immobilising the cricket, limb by limb.

They sink their tiny jaws into its most vulnerable places -

its joints - and inject their venom.

Before long, the cricket is dead

and the horde of tiny victors share their vast meal.

On occasion, even a solitary spider must meet another spider.

Male, after all, must meet female.

This is a male Argiope and he's looking for a mate.

But she is huge, ten times bigger than he is.

He has to be very careful

if he's not to be mistaken for prey and eaten.

Once he reaches her,

he starts stroking her body, nibbling her toes.

From their taste, he can tell whether the female is a virgin.

If she is, she will be less likely to eat him.

To confirm that her taste is encouraging,

he wipes his feet across his mouth.

Apparently he's reassured, for he starts to snip

some of the strands of her web to create an open space in it.

He runs a line across it, down towards her.

And now he plucks it, like a guitar string.

He's doing very well.

She's not attacked him - yet.

She spreads her eight legs.

It's a clear invitation to mate.

He checks the taste on his legs again and decides to go further.

He pauses. After mating he has,

at best, a 50-50 chance of staying alive.

But nothing ventured, nothing gained.

He moves in and delivers his sperm.

But his luck runs out.

Virgin she may be,

but with mating completed, she grabs him and binds him in silk.

She will eat him later.

Some spiders don't spin webs of any kind,

but they still need silk to help them find a mate.

And there's one such just here.

It's a female wolf spider, a solitary wandering hunter.

Like all spiders, she trails a drag-line of silk behind her wherever she goes.

It's a safety line, in case she falls, or is blown away

or needs to drop out of sight in a hurry.

And here's a male.

He's noticed her drag-line.

The taste of a silk line is very informative for him too.

It tells him that it comes from a female. So he follows.

His black palps are covered in hairs which are extremely sensitive.

Each hair contains a nerve, which can detect

even minute quantities of female pheromone.

Now, he's within sight of her.

Being active hunters, wolf spiders have excellent eyesight.

So he uses his black palps to send visual signals to her.

This display is not slowed down.

This is how he does it. It takes a lot of energy

and while he's performing, his heartbeat triples.

She encourages him by tapping her legs.

He's now within striking distance. The palps he's waving,

like those of all male spiders, are loaded with sperm.

He leans over, inserts one of them

into her abdomen and pumps his sperm into her.

Then he does the same with the other.

And that's that - at any rate, as far as HE is concerned.

Three weeks pass and the female's ovaries start to produce eggs.

The male's sperm, that the female has been holding within her

for all this time, is now released and fertilises them.

At last, she's ready to lay.

But she needs a safe place in which to do so.

And once again, silk provides a solution to her problems.

She starts by spinning a silken sheet,

stretched between fragments of the leaf-litter.

She uses that fuzzy silk that comes from multiple nozzles.

It will provide a soft padding to protect her eggs.

She expels a drop of liquid onto the sheet.

And into the liquid, she injects her fertilised eggs.

There may be several dozen of them.

She checks that the drop has dried.

She adds more fuzzy silk to protect it and its vulnerable contents

from knocks and bumps.

Then she changes silk

and starts to spin a tougher kind to cover the whole capsule.

She cuts the platform free from its attachments

and goes round it, pinching the cut edges firmly together.

Finally, she covers the whole parcel with a waterproof silken wrapping.

She now carries her precious package with her wherever she goes.

She seeks out patches of sunlight

so that she can warm it and speed the development of the eggs within.

It's a long process that may last several weeks.

And, then, at last,

her babies are sufficiently developed to leave their nursery.

But even now, she doesn't abandon them.

They climb up her legs and onto her back.

The egg capsule is now empty and can be discarded.

And away they go.

It's a somewhat rough ride, but the babies, even at this early stage

in their lives, know how silk can keep them out of trouble.

They use it to tie themselves to their mother's back.

And then they use it for yet another purpose,

and produce it in such abundance,

that in some seasons of the year,

it covers great areas of the open countryside.

This wonderful, shimmering carpet of gossamer,

strands of the finest silk, is the creation of a million baby spiders.

It's autumn in England and time for spiderlings to leave their mothers.

The youngsters climb up the threads they have spun

to reach the topmost twigs of the bushes.

They tip their abdomens into the air

and the gentle breeze catches the filaments

as they issue from the spinnerets.

Some filaments drift down and become entangled in the bushes.

But when conditions are right,

the threads rise vertically upwards.

And away the spiderlings go.

On a calm day, they may only travel a few metres,

but if there's a breeze, as there is now,

they can be swept up high into the sky.

Spiderlings have been recorded thousands of feet up

and can travel for hundreds of miles.

So, silk can be used for transport

as well as looking after the young,

courtship and, of course, catching prey.

In an area of heath, like this around me, it's been estimated

that there's probably 14,000 miles of silk -

enough to stretch from here in England to Australia.

Ingenious though we are, we've not yet been able to invent

anything as strong, as light or as elastic as silk.

In the next programme of Life In The Undergrowth,

we explore a fascinating world of intimate relations.

Ants with aphids, aphids with plants,

ladybirds with ants, aphids and plants.

Because invertebrates have existed for such a long time,

they have evolved relationships so complex they're almost unbelievable.

For us,

'trying to unravel the true story of what's going on was a real problem.

'First, we went to the foremost scientists and got the very latest chapter in their research.'

And then it was a question of developing the technology

to try and capture such intricate behaviour.

To get really close to our subjects,

but without disturbing what they're doing.

Martin Dohrn is fine-tuning

a portable motion-control rig that can position tiny lenses to within millimetres,

so we can spy into the world of intimate relations from a distance.

You get a lovely sense of motion, a sense of tracking.

I'm flying in a helicopter at the moment just over these extraordinary, gigantic plants.

It's just given me and hopefully others a new perspective on the world.

And it was exactly that new perspective that we needed in Peru

to look into a bizarre relationship between an ant and a plant.

For two years, scientist Megan Frederickson

has been investigating some strange clearings in the forest.

She's discovered that they are made by ants -

tiny ants which diligently weed out all types of plants

except this one, which they protect and, in return,

get a home.

What Megan doesn't know is exactly how the ants are killing the rival plants.

And that's what Martin is hoping to help her find out.

So he sets up his rig in the clearing.

It's not a case that the ants are going out

and they're actually cutting off the leaves

of the plants that are trying to grow here.

They seem to be attacking the leaves

using some kind of chemical that they produce.

Megan thinks the ants are poisoning the plants they don't want,

but because they're so small, she can't see how they're managing it.

But with Martin's array of microscopic lenses,

she can look into the ants' tiny world in detail.

And what she sees is quite remarkable.

They may be small but that doesn't stop them.

Their first action is to cut through the plant's protective bark.

Then they swing their abdomens under their legs

and squirt a drop of lethal formic acid into the wound.

It's a minuscule amount

but then there may be over five million ants in one clearing,

so it soon adds up.

At last, Megan can see how her ants are poisoning plants...

..and we get some quite extraordinary footage.

Just as we've used technology to get us that insect-eye view,

Chris Watson has used all kinds of devices

to eavesdrop on the extraordinary sounds of the undergrowth.

RUSTLING CLICKS

And there's one sound in particular

that could play a key role in another very startling relationship.

Jeremy Thomas has spent over 30 years studying one caterpillar -

the larva of the blue butterfly, which starts life on a plant.

The caterpillar feeds for about two or three weeks on the plant

and when it's still very tiny, it falls off the plant.

At that stage, it produces some secretions that mimic

one species of red ant's chemicals

and so it's so similar to the ant's own grub

that the ant mistakes it for its ant grub, picks it up in its jaws,

runs underground into its nest

and actually places the caterpillar with its own ant brood.

And from then on, the caterpillar actually induces the ants to feed it

instead of their own grubs.

But that's not the end of it,

because it has recently been discovered

that the caterpillars also produce sounds,

and Jeremy thinks they are important.

FAINT CLICKS

The problem is that they are incredibly quiet,

which is where Chris comes in, with a new bit of kit.

We're using very special microphones, particle velocity microphones

which have been built into this soundproof container.

And the caterpillars are introduced

onto the very element of this special microphone.

And we've had to come into a BBC Radio studio here simply because...

we need somewhere with very little, if any, background sound.

Because the sounds produced are so quiet, so minimal,

that we would never be able to hear them,

let alone record them, outside.

I'm trying not to breathe too much.

FAINT RHYTHMIC POPPING

It's doing it now!

REGULAR BEATING CALL

STRANGE REVERBERATING CALL

Some of these sounds have been heard before, though never this clearly,

but others are completely new.

Such a revelation, it's really...

really an amazing thing to hear it for the first time!

It's probably the quietest sound

I've ever had the opportunity of recording.

Apart from the alarm sounds of the caterpillar -

which arouses the ants and brings them scurrying to the caterpillar,

presumably to protect it - we simply don't know what the other noises do.

So what we're going to do is take the tapes from the BBC and play them

back to the ants and just watch what the response of the ants is.

Does it make them fight?

Does it make them run away? Do they run towards the sound?

Do they bring food to it? Anything is possible.

We simply don't know until we try.

Whatever happens,

we know that this caterpillar is the most extraordinary survivor.

Nurtured and protected by the ants for up to two years,

the grub that made those strange noises emerges

as a beautiful adult butterfly.

Subtitles by Red Bee Media Ltd