0:01:07 > 0:01:12You might think that the lights above my head are stars.
0:01:12 > 0:01:16But they can't be, because I'm in a cave.
0:01:16 > 0:01:19Each one of those tiny lights is produced
0:01:19 > 0:01:24by the larva of a small insect called a fungus gnat,
0:01:24 > 0:01:27as a way of attracting its prey.
0:01:29 > 0:01:33The result is a display that must surely rank as one
0:01:33 > 0:01:38of the most magical illuminations in the whole of the natural world.
0:01:42 > 0:01:46But shine a light sideways across the ceiling...
0:01:48 > 0:01:52..then you can see that each little blue lamp is surrounded by
0:01:52 > 0:01:55a curtain of glistening, beaded filaments -
0:01:55 > 0:01:58curtains that are invisible at other times.
0:02:02 > 0:02:04They're lures
0:02:04 > 0:02:07and they can be lethal.
0:02:07 > 0:02:10Insects, hatching in the water below,
0:02:10 > 0:02:14fly up towards these tiny lights
0:02:14 > 0:02:20and here they are trapped by threads of this extraordinary material
0:02:20 > 0:02:24that is the unique possession of the invertebrates.
0:02:24 > 0:02:26This is silk.
0:02:39 > 0:02:44This astonishing cave is near the small town of Waitomo
0:02:44 > 0:02:46in New Zealand.
0:02:46 > 0:02:49Each light comes from the back end of a larva
0:02:49 > 0:02:53as it lies in a transparent tube of mucus,
0:02:53 > 0:02:56slung from the ceiling by silken threads.
0:02:56 > 0:02:59And it's produced by phosphorescent chemicals
0:02:59 > 0:03:03in a special compartment opening from the side of its intestine.
0:03:05 > 0:03:11The silk comes from glands at the other end, inside the larva's mouth.
0:03:12 > 0:03:14The larvae move around.
0:03:14 > 0:03:16They fix a silk thread to the rock
0:03:16 > 0:03:21and slowly inch their way over the ceiling along a network of threads.
0:03:23 > 0:03:27Arriving in a new position, the larva produces more silk
0:03:27 > 0:03:32but this time, it allows the thread to dangle downwards.
0:03:37 > 0:03:40As each section emerges from its mouth,
0:03:40 > 0:03:43the larva, with a gulp, adds a blob of glue.
0:03:51 > 0:03:55Eventually, a single strand may be a metre long.
0:03:59 > 0:04:02There can be several hundred larvae
0:04:02 > 0:04:05in a single square metre of cave roof.
0:04:05 > 0:04:09And they all work hard, producing strand after strand.
0:04:09 > 0:04:13The more they make, the greater their chances of catching something.
0:04:15 > 0:04:19Below, mayfly are hatching from the stream that runs through the cave.
0:04:19 > 0:04:24They've been carried in here by the current from outside - as larvae.
0:04:24 > 0:04:26Now, they must look for a mate.
0:04:29 > 0:04:33But they find the blue lights above irresistible.
0:04:33 > 0:04:35And they're caught.
0:04:43 > 0:04:46The fungus gnat detects its victim's struggles
0:04:46 > 0:04:49from lines that run between the threads.
0:04:49 > 0:04:53Having made a capture, it turns off its light.
0:04:53 > 0:04:55That saves energy.
0:04:55 > 0:04:57Laboriously, it makes its way across
0:04:57 > 0:05:00to the thread from which the vibrations are coming.
0:05:00 > 0:05:04It hauls it up and eats what's hanging on the end.
0:05:04 > 0:05:06It also eats the filament.
0:05:06 > 0:05:08That saves silk.
0:05:14 > 0:05:17This wonderful hunting technique
0:05:17 > 0:05:20is just one of an enormous number
0:05:20 > 0:05:23of varied ways in which animals use silk.
0:05:23 > 0:05:27Silk really is an extraordinary material.
0:05:27 > 0:05:31It's stronger than a steel thread of the same diameter
0:05:31 > 0:05:34and unlike steel, it's elastic.
0:05:34 > 0:05:37It can stretch up to twice its length.
0:05:43 > 0:05:45The inhabitants of the undergrowth
0:05:45 > 0:05:49developed the ability to produce this marvellous material
0:05:49 > 0:05:54very early in their evolutionary history, over 300 million years ago.
0:05:54 > 0:05:59At first, it seems, they used it in a very simple way,
0:05:59 > 0:06:03as an adhesive. And lacewings still do,
0:06:03 > 0:06:06though for them, it's an adhesive with a difference.
0:06:09 > 0:06:10This is a female.
0:06:10 > 0:06:15She is looking for a safe place to deposit her eggs.
0:06:15 > 0:06:20Silk will provide it, but not exactly in the way you might think.
0:06:22 > 0:06:28She will lay up to 300 eggs, almost twice her body weight.
0:06:28 > 0:06:31However, there are plenty of other insects around
0:06:31 > 0:06:34that will eat those eggs if they find them.
0:06:38 > 0:06:43So, she doesn't glue them directly on the plant stem.
0:06:43 > 0:06:46First, she produces a little drop of sticky silk.
0:06:46 > 0:06:50Then, at the end of that, the egg.
0:06:58 > 0:07:02It's suspended safely in mid-air.
0:07:05 > 0:07:10The silk is produced by glands in her abdomen in liquid form.
0:07:10 > 0:07:12It's the very act of pulling it out
0:07:12 > 0:07:15that changes it from liquid to solid.
0:07:15 > 0:07:18And that is true for all invertebrate silk.
0:07:24 > 0:07:31She will lay up to 30 eggs a day, each on its own stalk.
0:07:35 > 0:07:38That silken thread is so incredibly fine
0:07:38 > 0:07:41that insect predators like these ants
0:07:41 > 0:07:44walk right by the eggs without realising
0:07:44 > 0:07:48that there's a tasty meal within millimetres of them.
0:07:49 > 0:07:53So, despite regular ant patrols in search of food,
0:07:53 > 0:07:57the lacewing's eggs remain undiscovered.
0:07:57 > 0:08:00After three days, they begin to hatch.
0:08:04 > 0:08:07Now, at least, if danger threatens,
0:08:07 > 0:08:10her offspring will be able to help themselves
0:08:10 > 0:08:12by running away.
0:08:17 > 0:08:20In the lush rainforest of Trinidad,
0:08:20 > 0:08:24you can find sheets of silk, wrapped around trees.
0:08:25 > 0:08:28Here it's also used for protection
0:08:28 > 0:08:32but by a quite different creature in a quite different way.
0:08:35 > 0:08:37The manufacturers,
0:08:37 > 0:08:40a little-known group of insects called web-spinners,
0:08:40 > 0:08:42live beneath.
0:08:50 > 0:08:53They graze on algae and lichens,
0:08:53 > 0:08:57hidden by the sheets immediately above them.
0:08:57 > 0:09:00They produce their silk, not from their abdomens
0:09:00 > 0:09:04or their mouths, but from glands in their forelegs.
0:09:19 > 0:09:23Each leg has about 150 tiny silk ejectors
0:09:23 > 0:09:27which between them create a thin silken tissue.
0:09:53 > 0:09:58An ant in search of prey strolls over the surface
0:09:58 > 0:09:59of the web-spinners' marquee.
0:09:59 > 0:10:03But the silk sheet, thin though it is, is impervious to smells
0:10:03 > 0:10:07and, as long as the web-spinner doesn't move too much,
0:10:07 > 0:10:10the ant will be unaware of it, a millimetre beneath its feet.
0:10:12 > 0:10:15RUMBLING THUNDER
0:10:15 > 0:10:18THUNDERCLAP
0:10:20 > 0:10:24And the tent, like any decent tent, is waterproof.
0:10:24 > 0:10:27In fact, the tent is so waterproof
0:10:27 > 0:10:32that the web-spinners beneath are in danger of not getting enough water.
0:10:32 > 0:10:35So, after the storm's over, they bite holes in places
0:10:35 > 0:10:40where a little rain has accumulated and drink the tiny puddle dry.
0:10:54 > 0:10:57Of course, the hole has to be repaired after a drink
0:10:57 > 0:10:59but that's easy enough when you have
0:10:59 > 0:11:02an almost limitless supply of silk in your legs.
0:11:05 > 0:11:10Of all the inhabitants of the undergrowth to have exploited silk,
0:11:10 > 0:11:14none have done so with more variety and skill than the spiders,
0:11:14 > 0:11:19and this is almost certainly the first way in which THEY used it.
0:11:20 > 0:11:24Here on a bank in the Malaysian rain forest, there are strands of silk
0:11:24 > 0:11:29radiating from this little patch in the middle.
0:11:29 > 0:11:32Watch what happens if I touch one of them.
0:11:40 > 0:11:42There! Oh...
0:11:42 > 0:11:45I... I can't help jumping.
0:11:45 > 0:11:51That was a trapdoor spider and it was so swift that you hardly saw it.
0:11:51 > 0:11:53Let me see if I can get it to do it again.
0:11:59 > 0:12:04The spider, when hungry, sits close behind the trapdoor.
0:12:04 > 0:12:06The strands outside are all connected
0:12:06 > 0:12:10to a silken collar that surrounds the mouth of the hole.
0:12:11 > 0:12:14Each of her feet is in contact with it.
0:12:14 > 0:12:17The slightest twitch is enough to tell her
0:12:17 > 0:12:19that something's moving around outside.
0:12:23 > 0:12:27A single twitch will produce no reaction.
0:12:27 > 0:12:30That could be caused by a falling leaf or a drop of water.
0:12:30 > 0:12:32But a repeated vibration,
0:12:32 > 0:12:37especially if it moves from one strand to another, could mean prey.
0:12:37 > 0:12:39Prey like this beetle.
0:12:51 > 0:12:54Got it! Now, it will kill it.
0:12:55 > 0:12:59This is the most ancient of living spiders.
0:12:59 > 0:13:03The fact that it has, uniquely, segmented plates on its back
0:13:03 > 0:13:06shows that it's more closely-related than any other
0:13:06 > 0:13:09to those pioneer hunters, the scorpions.
0:13:09 > 0:13:12And like them, it has a powerful venom.
0:13:12 > 0:13:16Once bitten, its victim has little chance.
0:13:20 > 0:13:24Trip-lines were one of the earliest of the spiders' hunting techniques
0:13:24 > 0:13:27but other, later spiders used silk
0:13:27 > 0:13:30to build much more sophisticated structures.
0:13:34 > 0:13:36Orb webs are so familiar to us
0:13:36 > 0:13:40that we tend to forget what complex structures they are.
0:13:40 > 0:13:44A single one can contain up to 60 metres of silk,
0:13:44 > 0:13:50of up to six different kinds, and involve 3,000 separate attachments.
0:13:50 > 0:13:53And what's more, some orb-web spiders spin
0:13:53 > 0:13:56a different one every night.
0:13:57 > 0:14:01The biggest and best webs are made, in most species, by the female.
0:14:01 > 0:14:06She has to start by bridging the gap across which she's to hang her web.
0:14:06 > 0:14:10The faintest breeze will catch a filament as she spins it
0:14:10 > 0:14:12and carry it away into space.
0:14:12 > 0:14:16With luck, it will catch on a suitable anchor point.
0:14:16 > 0:14:18There!
0:14:20 > 0:14:22She runs across the filament,
0:14:22 > 0:14:26trailing a line of much thicker, stronger silk, and ties it off.
0:14:27 > 0:14:31Then, she goes back to the middle of this line
0:14:31 > 0:14:33and drops down another.
0:14:35 > 0:14:37And she tightens it.
0:14:49 > 0:14:53The junction at the top becomes the hub of the web
0:14:53 > 0:14:56to which she will attach radiating spokes.
0:14:56 > 0:14:58These must be particularly strong,
0:14:58 > 0:15:01for the shape of the whole structure depends on them.
0:15:15 > 0:15:17Once they're complete,
0:15:17 > 0:15:21she adds a spiral working from the middle outwards.
0:15:25 > 0:15:30This first spiral is quite widely spaced because it's only temporary.
0:15:30 > 0:15:33It will serve as scaffolding along which she runs to add a stronger,
0:15:33 > 0:15:37stickier and more closely-spaced spiral.
0:15:38 > 0:15:39That's this one.
0:15:42 > 0:15:46As the filament for this emerges from her spinneret,
0:15:46 > 0:15:50she coats it with glue from separate glands in her abdomen.
0:15:52 > 0:15:56After completing one section, she eats the scaffolding line.
0:15:56 > 0:15:59It has no further use and it saves valuable silk.
0:16:01 > 0:16:05At first, this glue is evenly spread,
0:16:05 > 0:16:08but each time she fixes a section, she twangs it with her leg
0:16:08 > 0:16:12so that it breaks up and forms a line of droplets.
0:16:25 > 0:16:27She can complete the whole, intricate,
0:16:27 > 0:16:31elegantly symmetrical structure in about an hour.
0:16:57 > 0:17:01When an insect strikes the web, the capture spiral stretches
0:17:01 > 0:17:04and then retracts to its former size
0:17:04 > 0:17:08without distorting its shape and without such a severe recoil
0:17:08 > 0:17:11that the insect might be catapulted off again.
0:17:11 > 0:17:14The beads of glue are the key.
0:17:14 > 0:17:21Tension on the surface of a droplet hauls any slack into it.
0:17:21 > 0:17:23When the insect hits,
0:17:23 > 0:17:26it pulls out the coils of thread in each droplet,
0:17:26 > 0:17:30slowing the insect to a momentary standstill.
0:17:32 > 0:17:38And then the surface tension pulls the silk back into each drop.
0:17:40 > 0:17:42So, the spiral thread doesn't break
0:17:42 > 0:17:46and the web, as a whole, regains its symmetry.
0:17:48 > 0:17:53The spider sits with her legs resting on the spokes.
0:17:54 > 0:17:57Any vibration on them will travel up her leg
0:17:57 > 0:18:02and be received by a small sense organ in the joint.
0:18:02 > 0:18:05This is covered with microscopic slits
0:18:05 > 0:18:08which are distorted by the slightest movement.
0:18:08 > 0:18:12So the spider is immediately aware of the tiniest tremor.
0:18:12 > 0:18:16Once alerted, she pulls on neighbouring spokes of the web
0:18:16 > 0:18:18to assess exactly in what direction
0:18:18 > 0:18:21and how far away the signals originate.
0:18:33 > 0:18:37The fly is on the verge of breaking loose.
0:18:37 > 0:18:39Here she comes.
0:18:39 > 0:18:42She isn't hindered by the glue she put on the capture spiral
0:18:42 > 0:18:45because her feet are coated with a special oil.
0:19:02 > 0:19:07Once her victim is in her grasp, she produces yet another kind of silk.
0:19:07 > 0:19:11It emerges as a sheet from a group of minute spigots.
0:19:11 > 0:19:13This is a fuzzy silk used for wrapping
0:19:13 > 0:19:17and, at moments like this, as a shroud.
0:19:32 > 0:19:36The biggest and strongest webs are those made by Nephila -
0:19:36 > 0:19:40the golden orb-web spider of the tropics.
0:19:40 > 0:19:42They may be several metres across
0:19:42 > 0:19:45and they're strong enough to catch small birds.
0:19:48 > 0:19:50This time, only a moth.
0:19:52 > 0:19:57After a killing bite, she returns to the hub of her web to wrap it up.
0:19:59 > 0:20:01But big webs bring problems.
0:20:01 > 0:20:05It's not easy to control what happens on their outer regions.
0:20:07 > 0:20:09This is Argyrodes.
0:20:09 > 0:20:12She's only one-hundredth the weight of Nephila
0:20:12 > 0:20:17so she can move across this huge web undetected. And she's a thief.
0:20:20 > 0:20:23A fly has arrived not far from her.
0:20:23 > 0:20:25She has a chance to steal it.
0:20:33 > 0:20:37But Nephila has also detected its arrival...
0:20:39 > 0:20:42..and claims it without much difficulty.
0:20:52 > 0:20:56Another fly is caught in the web.
0:20:56 > 0:20:59Argyrodes now stands a better chance,
0:20:59 > 0:21:02since Nephila is busy feeding.
0:21:02 > 0:21:06She cuts the filaments between the fly and Nephila,
0:21:06 > 0:21:09so that vibrations made by its struggles won't reach her.
0:21:31 > 0:21:34Nephila, sitting at the hub of the web,
0:21:34 > 0:21:38seems quite unaware of what is going on at its outer margin.
0:21:40 > 0:21:43The fly is now hanging from a single thread.
0:21:43 > 0:21:45But it's five times the weight of Argyrodes
0:21:45 > 0:21:47and too heavy for her to carry.
0:21:47 > 0:21:49She has to be clever.
0:21:49 > 0:21:54She attaches a thread to it and runs it up to a twig outside the web.
0:22:03 > 0:22:06Nephila is still occupied with her meal.
0:22:08 > 0:22:11Another line, just to make sure.
0:22:25 > 0:22:30Now she can snip the last filaments of the web and haul it away.
0:22:45 > 0:22:48Safely off Nephila's web at last,
0:22:48 > 0:22:52Argyrodes can enjoy her stolen meal in safety.
0:22:56 > 0:22:58BELL TOLLS
0:22:58 > 0:23:00For all its complexity,
0:23:00 > 0:23:05the orb web was one of the first kind of silken traps
0:23:05 > 0:23:07devised by spiders.
0:23:07 > 0:23:13Subsequently, other species modified it in some quite extraordinary ways.
0:23:13 > 0:23:17There's a web in this yew tree that's triangular -
0:23:17 > 0:23:19a slice, as it were, from an orb.
0:23:19 > 0:23:21It's made by Hyptiotes
0:23:21 > 0:23:25and her body forms an essential link in its mooring cable.
0:23:25 > 0:23:30To be effective, the web has to be very taut.
0:23:32 > 0:23:34Hyptiotes ratchets up the tension
0:23:34 > 0:23:38by hauling in the main cable and coiling it above her body.
0:23:44 > 0:23:45Tighter.
0:23:49 > 0:23:51Tighter.
0:23:58 > 0:24:01And that's about as tight as it'll go.
0:24:02 > 0:24:05Now she has to wait.
0:24:05 > 0:24:08Flies can sometimes disentangle themselves from a web,
0:24:08 > 0:24:11if the spider doesn't grab them quickly.
0:24:11 > 0:24:16But a fly hitting this web won't get that chance.
0:24:18 > 0:24:21A strike triggers an instant reaction.
0:24:22 > 0:24:25In slow motion, you can see what happens.
0:24:25 > 0:24:31Hyptiotes immediately lets go of the coil she was holding over her back.
0:24:31 > 0:24:34That causes her web to collapse
0:24:34 > 0:24:37and almost instantaneously entangle the prey.
0:24:42 > 0:24:47Few flies that hit a Hyptiotes' web manage to escape.
0:24:51 > 0:24:55The gladiator spider makes her web
0:24:55 > 0:24:58from a very special kind of multi-strand silk
0:24:58 > 0:25:01which she backcombs to make fuzzy.
0:25:01 > 0:25:04She carefully attaches this to a framework of ordinary,
0:25:04 > 0:25:06un-fuzzy filaments.
0:25:14 > 0:25:20The fuzzy silk doesn't have glue on it, but will entangle hairy legs.
0:25:23 > 0:25:28And it's also extremely elastic, which is crucially important.
0:25:43 > 0:25:45It's finished.
0:25:45 > 0:25:49She reaches down with her forelegs to check how far away
0:25:49 > 0:25:50she is from the ground.
0:25:52 > 0:25:56Then she snips most of the framework threads
0:25:56 > 0:26:00and holds the fuzzy rectangle between her four front legs.
0:26:00 > 0:26:02She's ready.
0:26:04 > 0:26:09Her enormous eyes are so sensitive she can hunt in near-darkness.
0:26:11 > 0:26:14A bush cricket would make a rich meal, but it's very powerful
0:26:14 > 0:26:17and it could put up a good fight.
0:26:39 > 0:26:44Now it must be parcelled up and the fuzzy silk makes excellent wrapping,
0:26:44 > 0:26:46just as it does for Hyptiotes.
0:26:51 > 0:26:54In Australia, there's a species of spider
0:26:54 > 0:26:57that has taken web construction a stage further still.
0:26:57 > 0:27:02It builds not just in two dimensions but three.
0:27:02 > 0:27:04It regularly takes up residence
0:27:04 > 0:27:07in people's back yards and on their verandas.
0:27:07 > 0:27:09There's one under this plant holder.
0:27:09 > 0:27:13It's the notorious and very venomous redback.
0:27:13 > 0:27:15What's brought it here
0:27:15 > 0:27:18is the extraordinary way in which it uses silk.
0:27:20 > 0:27:22The female usually builds at night
0:27:22 > 0:27:25and constructs this very elaborate web.
0:27:25 > 0:27:29It's not just wide, it's deep.
0:27:29 > 0:27:33To make it, she needs two flat surfaces, one beneath the other.
0:27:33 > 0:27:37And that's what she has found underneath the plant holder.
0:27:37 > 0:27:41First, she drops down, pulling a thread behind her.
0:27:43 > 0:27:46She sticks the end to the veranda floor.
0:27:50 > 0:27:54Then she goes back up again, trailing a second line,
0:27:54 > 0:27:58which she sticks to the first, so strengthening it.
0:27:59 > 0:28:02Then she pulls the line tight.
0:28:02 > 0:28:05That is a crucial element in the construction.
0:28:13 > 0:28:15Down she goes again.
0:28:38 > 0:28:40By the time she has finished,
0:28:40 > 0:28:45she will have fixed several dozen of these sticky, taut, vertical lines.
0:28:47 > 0:28:50An ant is approaching in the distance.
0:28:50 > 0:28:53An orb-web would never catch one of these.
0:28:55 > 0:28:58It's a scout, leading an exploring party.
0:28:58 > 0:29:00Searching beneath the plant holder,
0:29:00 > 0:29:04it's almost bound to blunder into one of the redback's lines.
0:29:07 > 0:29:10It struggles and so seals its fate.
0:29:13 > 0:29:16And its followers go the same way.
0:29:26 > 0:29:31The threads carry the vibrations back to the redback, waiting above.
0:29:31 > 0:29:34She has no need to hurry.
0:29:34 > 0:29:38Her meals are suspended in mid-air. Escape is impossible.
0:29:38 > 0:29:42She hauls them up in her own good time.
0:29:49 > 0:29:52The redback's trap is certainly economical with silk.
0:29:52 > 0:29:57But one North American spider hunts with just a single filament.
0:29:58 > 0:30:01This may look like a bird dropping
0:30:01 > 0:30:05but that's just a disguise to fool anything that might want to eat it.
0:30:05 > 0:30:08In fact, it's a spider,
0:30:08 > 0:30:12and one with an even more extraordinary hunting technique.
0:30:12 > 0:30:14It's a bolas spider.
0:30:14 > 0:30:17Throughout the day, she remains motionless.
0:30:17 > 0:30:21But when evening comes, she prepares for action.
0:30:26 > 0:30:31She abandons her disguise and starts to move.
0:30:35 > 0:30:40Slowly, she makes her way down to the underside of the leaf.
0:30:44 > 0:30:48There, she hangs from a horizontal thread.
0:30:51 > 0:30:55Next, she starts to spin a single strong line,
0:30:55 > 0:30:58pulling it out of her spinneret with her back legs.
0:31:06 > 0:31:11And at the end, there is a sticky globule.
0:31:11 > 0:31:15This is her bolas. It's all she needs.
0:31:15 > 0:31:18She climbs back up to her leaf
0:31:18 > 0:31:21and takes up her position on the horizontal thread
0:31:21 > 0:31:24with her weighted filament dangling from one of her front legs.
0:31:40 > 0:31:42A moth.
0:31:42 > 0:31:45She whirls her bolas but misses.
0:31:46 > 0:31:49But she has ways of enticing the moth back.
0:31:49 > 0:31:53She can produce a pheromone, a chemical perfume,
0:31:53 > 0:31:55that the moth finds irresistible.
0:31:55 > 0:31:59What is more, she can change it to suit the particular species of moth
0:31:59 > 0:32:01that happens to be around.
0:32:03 > 0:32:05The moth comes back.
0:32:10 > 0:32:12This time, she's got it.
0:32:22 > 0:32:25Now, she starts to wrap it.
0:32:32 > 0:32:35But she's not finished yet.
0:32:39 > 0:32:44Different moths and a different pheromone.
0:32:53 > 0:32:56Silk can do other things as well.
0:32:56 > 0:33:00It can totally change a spider's lifestyle
0:33:00 > 0:33:04and turn a solitary killer into a creature that hunts in great packs.
0:33:04 > 0:33:10This enormous web above me contains thousands of spiders.
0:33:10 > 0:33:13They're all tiny, but because they work together,
0:33:13 > 0:33:17they can kill prey many times their own size.
0:33:19 > 0:33:24Any spider sitting on its web might be expected to react aggressively
0:33:24 > 0:33:26towards another that approaches it.
0:33:26 > 0:33:28But not these.
0:33:32 > 0:33:37These tiny, ant-sized spiders seem totally relaxed in one another's presence.
0:33:37 > 0:33:40More than that, they cooperate with one another,
0:33:40 > 0:33:45working together to repair and extend their huge, silken palace.
0:33:45 > 0:33:49There are tens of thousands of them in this one
0:33:49 > 0:33:52and they are constantly at work.
0:34:03 > 0:34:08Their home can rise 15, 20 metres up towards the canopy.
0:34:09 > 0:34:14It's so big, it's a major obstacle in the airways of the forest.
0:34:17 > 0:34:19This cricket weighs several hundred times
0:34:19 > 0:34:21as much as one of these spiders.
0:34:21 > 0:34:25However, the slightest attempt to free itself
0:34:25 > 0:34:29only serves to attract lots of them from all over the giant web.
0:34:46 > 0:34:50Soon, it's surrounded by hundreds.
0:34:50 > 0:34:53They squirt glue from their spinnerets,
0:34:53 > 0:34:55immobilising the cricket, limb by limb.
0:35:06 > 0:35:10They sink their tiny jaws into its most vulnerable places -
0:35:10 > 0:35:13its joints - and inject their venom.
0:35:34 > 0:35:37Before long, the cricket is dead
0:35:37 > 0:35:41and the horde of tiny victors share their vast meal.
0:35:57 > 0:36:01On occasion, even a solitary spider must meet another spider.
0:36:01 > 0:36:04Male, after all, must meet female.
0:36:04 > 0:36:08This is a male Argiope and he's looking for a mate.
0:36:10 > 0:36:14But she is huge, ten times bigger than he is.
0:36:14 > 0:36:16He has to be very careful
0:36:16 > 0:36:20if he's not to be mistaken for prey and eaten.
0:36:27 > 0:36:28Once he reaches her,
0:36:28 > 0:36:32he starts stroking her body, nibbling her toes.
0:36:32 > 0:36:36From their taste, he can tell whether the female is a virgin.
0:36:36 > 0:36:39If she is, she will be less likely to eat him.
0:36:42 > 0:36:45To confirm that her taste is encouraging,
0:36:45 > 0:36:47he wipes his feet across his mouth.
0:36:49 > 0:36:53Apparently he's reassured, for he starts to snip
0:36:53 > 0:36:57some of the strands of her web to create an open space in it.
0:37:00 > 0:37:04He runs a line across it, down towards her.
0:37:10 > 0:37:14And now he plucks it, like a guitar string.
0:37:25 > 0:37:26He's doing very well.
0:37:26 > 0:37:28She's not attacked him - yet.
0:37:38 > 0:37:41She spreads her eight legs.
0:37:41 > 0:37:43It's a clear invitation to mate.
0:37:46 > 0:37:51He checks the taste on his legs again and decides to go further.
0:37:53 > 0:37:55He pauses. After mating he has,
0:37:55 > 0:38:00at best, a 50-50 chance of staying alive.
0:38:00 > 0:38:02But nothing ventured, nothing gained.
0:38:02 > 0:38:06He moves in and delivers his sperm.
0:38:09 > 0:38:11But his luck runs out.
0:38:11 > 0:38:13Virgin she may be,
0:38:13 > 0:38:18but with mating completed, she grabs him and binds him in silk.
0:38:37 > 0:38:40She will eat him later.
0:38:44 > 0:38:48Some spiders don't spin webs of any kind,
0:38:48 > 0:38:52but they still need silk to help them find a mate.
0:38:52 > 0:38:54And there's one such just here.
0:38:55 > 0:38:59It's a female wolf spider, a solitary wandering hunter.
0:39:03 > 0:39:08Like all spiders, she trails a drag-line of silk behind her wherever she goes.
0:39:08 > 0:39:11It's a safety line, in case she falls, or is blown away
0:39:11 > 0:39:15or needs to drop out of sight in a hurry.
0:39:15 > 0:39:17And here's a male.
0:39:22 > 0:39:24He's noticed her drag-line.
0:39:24 > 0:39:28The taste of a silk line is very informative for him too.
0:39:28 > 0:39:33It tells him that it comes from a female. So he follows.
0:39:35 > 0:39:40His black palps are covered in hairs which are extremely sensitive.
0:39:43 > 0:39:46Each hair contains a nerve, which can detect
0:39:46 > 0:39:51even minute quantities of female pheromone.
0:39:51 > 0:39:54Now, he's within sight of her.
0:39:54 > 0:39:57Being active hunters, wolf spiders have excellent eyesight.
0:39:57 > 0:40:02So he uses his black palps to send visual signals to her.
0:40:12 > 0:40:15This display is not slowed down.
0:40:15 > 0:40:17This is how he does it. It takes a lot of energy
0:40:17 > 0:40:22and while he's performing, his heartbeat triples.
0:40:38 > 0:40:42She encourages him by tapping her legs.
0:40:53 > 0:40:57He's now within striking distance. The palps he's waving,
0:40:57 > 0:41:01like those of all male spiders, are loaded with sperm.
0:41:08 > 0:41:10He leans over, inserts one of them
0:41:10 > 0:41:14into her abdomen and pumps his sperm into her.
0:41:31 > 0:41:34Then he does the same with the other.
0:41:37 > 0:41:41And that's that - at any rate, as far as HE is concerned.
0:41:43 > 0:41:48Three weeks pass and the female's ovaries start to produce eggs.
0:41:48 > 0:41:51The male's sperm, that the female has been holding within her
0:41:51 > 0:41:55for all this time, is now released and fertilises them.
0:41:55 > 0:41:57At last, she's ready to lay.
0:41:59 > 0:42:03But she needs a safe place in which to do so.
0:42:03 > 0:42:08And once again, silk provides a solution to her problems.
0:42:08 > 0:42:12She starts by spinning a silken sheet,
0:42:12 > 0:42:15stretched between fragments of the leaf-litter.
0:42:22 > 0:42:27She uses that fuzzy silk that comes from multiple nozzles.
0:42:27 > 0:42:30It will provide a soft padding to protect her eggs.
0:42:50 > 0:42:54She expels a drop of liquid onto the sheet.
0:42:59 > 0:43:04And into the liquid, she injects her fertilised eggs.
0:43:27 > 0:43:30There may be several dozen of them.
0:43:30 > 0:43:34She checks that the drop has dried.
0:43:42 > 0:43:46She adds more fuzzy silk to protect it and its vulnerable contents
0:43:46 > 0:43:48from knocks and bumps.
0:43:52 > 0:43:54Then she changes silk
0:43:54 > 0:43:58and starts to spin a tougher kind to cover the whole capsule.
0:44:21 > 0:44:23She cuts the platform free from its attachments
0:44:23 > 0:44:28and goes round it, pinching the cut edges firmly together.
0:44:39 > 0:44:43Finally, she covers the whole parcel with a waterproof silken wrapping.
0:44:58 > 0:45:03She now carries her precious package with her wherever she goes.
0:45:07 > 0:45:10She seeks out patches of sunlight
0:45:10 > 0:45:14so that she can warm it and speed the development of the eggs within.
0:45:16 > 0:45:21It's a long process that may last several weeks.
0:45:26 > 0:45:28And, then, at last,
0:45:28 > 0:45:33her babies are sufficiently developed to leave their nursery.
0:45:54 > 0:45:57But even now, she doesn't abandon them.
0:45:59 > 0:46:02They climb up her legs and onto her back.
0:46:03 > 0:46:08The egg capsule is now empty and can be discarded.
0:46:10 > 0:46:13And away they go.
0:46:13 > 0:46:17It's a somewhat rough ride, but the babies, even at this early stage
0:46:17 > 0:46:22in their lives, know how silk can keep them out of trouble.
0:46:22 > 0:46:25They use it to tie themselves to their mother's back.
0:46:35 > 0:46:37And then they use it for yet another purpose,
0:46:37 > 0:46:40and produce it in such abundance,
0:46:40 > 0:46:43that in some seasons of the year,
0:46:43 > 0:46:46it covers great areas of the open countryside.
0:46:53 > 0:46:59This wonderful, shimmering carpet of gossamer,
0:46:59 > 0:47:05strands of the finest silk, is the creation of a million baby spiders.
0:47:05 > 0:47:11It's autumn in England and time for spiderlings to leave their mothers.
0:47:16 > 0:47:18The youngsters climb up the threads they have spun
0:47:18 > 0:47:22to reach the topmost twigs of the bushes.
0:47:25 > 0:47:28They tip their abdomens into the air
0:47:28 > 0:47:30and the gentle breeze catches the filaments
0:47:30 > 0:47:33as they issue from the spinnerets.
0:47:36 > 0:47:39Some filaments drift down and become entangled in the bushes.
0:47:52 > 0:47:55But when conditions are right,
0:47:55 > 0:47:58the threads rise vertically upwards.
0:48:01 > 0:48:04And away the spiderlings go.
0:48:18 > 0:48:22On a calm day, they may only travel a few metres,
0:48:22 > 0:48:25but if there's a breeze, as there is now,
0:48:25 > 0:48:28they can be swept up high into the sky.
0:48:29 > 0:48:33Spiderlings have been recorded thousands of feet up
0:48:33 > 0:48:36and can travel for hundreds of miles.
0:48:42 > 0:48:45So, silk can be used for transport
0:48:45 > 0:48:47as well as looking after the young,
0:48:47 > 0:48:51courtship and, of course, catching prey.
0:48:51 > 0:48:55In an area of heath, like this around me, it's been estimated
0:48:55 > 0:48:59that there's probably 14,000 miles of silk -
0:48:59 > 0:49:03enough to stretch from here in England to Australia.
0:49:03 > 0:49:06Ingenious though we are, we've not yet been able to invent
0:49:06 > 0:49:12anything as strong, as light or as elastic as silk.
0:49:27 > 0:49:30In the next programme of Life In The Undergrowth,
0:49:30 > 0:49:33we explore a fascinating world of intimate relations.
0:49:37 > 0:49:41Ants with aphids, aphids with plants,
0:49:41 > 0:49:45ladybirds with ants, aphids and plants.
0:49:45 > 0:49:49Because invertebrates have existed for such a long time,
0:49:49 > 0:49:53they have evolved relationships so complex they're almost unbelievable.
0:49:53 > 0:49:55For us,
0:49:55 > 0:49:59'trying to unravel the true story of what's going on was a real problem.
0:49:59 > 0:50:05'First, we went to the foremost scientists and got the very latest chapter in their research.'
0:50:05 > 0:50:09And then it was a question of developing the technology
0:50:09 > 0:50:13to try and capture such intricate behaviour.
0:50:13 > 0:50:16To get really close to our subjects,
0:50:16 > 0:50:19but without disturbing what they're doing.
0:50:19 > 0:50:23Martin Dohrn is fine-tuning
0:50:23 > 0:50:28a portable motion-control rig that can position tiny lenses to within millimetres,
0:50:28 > 0:50:33so we can spy into the world of intimate relations from a distance.
0:50:35 > 0:50:40You get a lovely sense of motion, a sense of tracking.
0:50:40 > 0:50:46I'm flying in a helicopter at the moment just over these extraordinary, gigantic plants.
0:50:46 > 0:50:51It's just given me and hopefully others a new perspective on the world.
0:50:55 > 0:51:01And it was exactly that new perspective that we needed in Peru
0:51:01 > 0:51:05to look into a bizarre relationship between an ant and a plant.
0:51:08 > 0:51:11For two years, scientist Megan Frederickson
0:51:11 > 0:51:15has been investigating some strange clearings in the forest.
0:51:15 > 0:51:18She's discovered that they are made by ants -
0:51:18 > 0:51:22tiny ants which diligently weed out all types of plants
0:51:22 > 0:51:26except this one, which they protect and, in return,
0:51:26 > 0:51:28get a home.
0:51:29 > 0:51:34What Megan doesn't know is exactly how the ants are killing the rival plants.
0:51:34 > 0:51:38And that's what Martin is hoping to help her find out.
0:51:38 > 0:51:41So he sets up his rig in the clearing.
0:51:48 > 0:51:50It's not a case that the ants are going out
0:51:50 > 0:51:53and they're actually cutting off the leaves
0:51:53 > 0:51:56of the plants that are trying to grow here.
0:51:56 > 0:51:58They seem to be attacking the leaves
0:51:58 > 0:52:01using some kind of chemical that they produce.
0:52:01 > 0:52:05Megan thinks the ants are poisoning the plants they don't want,
0:52:05 > 0:52:08but because they're so small, she can't see how they're managing it.
0:52:11 > 0:52:15But with Martin's array of microscopic lenses,
0:52:15 > 0:52:19she can look into the ants' tiny world in detail.
0:52:22 > 0:52:26And what she sees is quite remarkable.
0:52:26 > 0:52:29They may be small but that doesn't stop them.
0:52:29 > 0:52:33Their first action is to cut through the plant's protective bark.
0:52:33 > 0:52:36Then they swing their abdomens under their legs
0:52:36 > 0:52:39and squirt a drop of lethal formic acid into the wound.
0:52:45 > 0:52:47It's a minuscule amount
0:52:47 > 0:52:51but then there may be over five million ants in one clearing,
0:52:51 > 0:52:53so it soon adds up.
0:52:53 > 0:52:59At last, Megan can see how her ants are poisoning plants...
0:53:01 > 0:53:04..and we get some quite extraordinary footage.
0:53:10 > 0:53:14Just as we've used technology to get us that insect-eye view,
0:53:14 > 0:53:16Chris Watson has used all kinds of devices
0:53:16 > 0:53:20to eavesdrop on the extraordinary sounds of the undergrowth.
0:53:27 > 0:53:30RUSTLING CLICKS
0:53:35 > 0:53:37And there's one sound in particular
0:53:37 > 0:53:42that could play a key role in another very startling relationship.
0:53:43 > 0:53:48Jeremy Thomas has spent over 30 years studying one caterpillar -
0:53:48 > 0:53:53the larva of the blue butterfly, which starts life on a plant.
0:53:53 > 0:53:57The caterpillar feeds for about two or three weeks on the plant
0:53:57 > 0:54:00and when it's still very tiny, it falls off the plant.
0:54:00 > 0:54:03At that stage, it produces some secretions that mimic
0:54:03 > 0:54:07one species of red ant's chemicals
0:54:07 > 0:54:10and so it's so similar to the ant's own grub
0:54:10 > 0:54:14that the ant mistakes it for its ant grub, picks it up in its jaws,
0:54:14 > 0:54:17runs underground into its nest
0:54:17 > 0:54:21and actually places the caterpillar with its own ant brood.
0:54:21 > 0:54:25And from then on, the caterpillar actually induces the ants to feed it
0:54:25 > 0:54:28instead of their own grubs.
0:54:28 > 0:54:30But that's not the end of it,
0:54:30 > 0:54:33because it has recently been discovered
0:54:33 > 0:54:36that the caterpillars also produce sounds,
0:54:36 > 0:54:38and Jeremy thinks they are important.
0:54:38 > 0:54:41FAINT CLICKS
0:54:41 > 0:54:44The problem is that they are incredibly quiet,
0:54:44 > 0:54:48which is where Chris comes in, with a new bit of kit.
0:54:48 > 0:54:53We're using very special microphones, particle velocity microphones
0:54:53 > 0:54:58which have been built into this soundproof container.
0:54:58 > 0:55:00And the caterpillars are introduced
0:55:00 > 0:55:04onto the very element of this special microphone.
0:55:04 > 0:55:10And we've had to come into a BBC Radio studio here simply because...
0:55:10 > 0:55:15we need somewhere with very little, if any, background sound.
0:55:15 > 0:55:18Because the sounds produced are so quiet, so minimal,
0:55:18 > 0:55:21that we would never be able to hear them,
0:55:21 > 0:55:23let alone record them, outside.
0:55:24 > 0:55:26I'm trying not to breathe too much.
0:55:32 > 0:55:34FAINT RHYTHMIC POPPING
0:55:37 > 0:55:39It's doing it now!
0:55:42 > 0:55:48REGULAR BEATING CALL
0:55:50 > 0:55:52STRANGE REVERBERATING CALL
0:55:57 > 0:56:02Some of these sounds have been heard before, though never this clearly,
0:56:02 > 0:56:04but others are completely new.
0:56:04 > 0:56:07Such a revelation, it's really...
0:56:07 > 0:56:11really an amazing thing to hear it for the first time!
0:56:13 > 0:56:15It's probably the quietest sound
0:56:15 > 0:56:17I've ever had the opportunity of recording.
0:56:28 > 0:56:31Apart from the alarm sounds of the caterpillar -
0:56:31 > 0:56:36which arouses the ants and brings them scurrying to the caterpillar,
0:56:36 > 0:56:42presumably to protect it - we simply don't know what the other noises do.
0:56:42 > 0:56:46So what we're going to do is take the tapes from the BBC and play them
0:56:46 > 0:56:51back to the ants and just watch what the response of the ants is.
0:56:51 > 0:56:53Does it make them fight?
0:56:53 > 0:56:56Does it make them run away? Do they run towards the sound?
0:56:56 > 0:56:59Do they bring food to it? Anything is possible.
0:56:59 > 0:57:01We simply don't know until we try.
0:57:07 > 0:57:09Whatever happens,
0:57:09 > 0:57:13we know that this caterpillar is the most extraordinary survivor.
0:57:13 > 0:57:16Nurtured and protected by the ants for up to two years,
0:57:16 > 0:57:18the grub that made those strange noises emerges
0:57:18 > 0:57:21as a beautiful adult butterfly.
0:58:11 > 0:58:14Subtitles by Red Bee Media Ltd