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