0:00:02 > 0:00:08Animals can sometimes reach such numbers they defy our understanding.
0:00:08 > 0:00:12They seem to rise up and invade our lives.
0:00:16 > 0:00:17They become super swarms,
0:00:17 > 0:00:21so immense they're impossible to ignore.
0:00:24 > 0:00:28From the outside, they can be the stuff of nightmares.
0:00:28 > 0:00:31From the inside, they're a thing of beauty.
0:00:32 > 0:00:34As we enter the heart of the swarm,
0:00:34 > 0:00:38we'll discover what happens when they meet our world.
0:00:49 > 0:00:54It's 1995 at a football match between Costa Rica and Italy.
0:00:54 > 0:00:58Killer bees stage a pitch invasion of a different kind.
0:01:00 > 0:01:02The players' panic is understandable.
0:01:02 > 0:01:04These bees have been known to kill.
0:01:04 > 0:01:09To understand what the swarm was thinking, we recreated the encounter
0:01:09 > 0:01:12and slowed it down to match the bees' reaction times.
0:01:24 > 0:01:28In Costa Rica, pitch invasions by killer bees are surprisingly common.
0:01:28 > 0:01:31Swarms like to make a beeline across open areas,
0:01:31 > 0:01:35which can bring them right into the field of play.
0:01:51 > 0:01:53By showing the action in slow motion,
0:01:53 > 0:01:55it becomes clear that the footballers
0:01:55 > 0:01:57are making the situation worse.
0:01:58 > 0:02:02Swiping at the bees is a complete waste of time.
0:02:02 > 0:02:06Their reactions are so fast, they can easily swerve out of the way.
0:02:06 > 0:02:10It's also a very bad idea.
0:02:10 > 0:02:12Movement not only makes them angrier,
0:02:12 > 0:02:14it gives them a target to aim for.
0:02:23 > 0:02:25The bees' lives depend on teamwork.
0:02:25 > 0:02:29Retaliation is swift and devastating.
0:02:36 > 0:02:39Although each bee can sting only once,
0:02:39 > 0:02:42the sting left behind pumps out a pheromone,
0:02:42 > 0:02:45that whips nearby bees into a fighting frenzy.
0:02:53 > 0:02:59They join the attack and each new sting brings in more angry bees.
0:03:04 > 0:03:07Unlike honey bees, killer bees rarely give up a fight.
0:03:07 > 0:03:10But this swarm has other things on its mind.
0:03:10 > 0:03:12It's house-hunting.
0:03:15 > 0:03:18Swarms arise when a hive gets too big.
0:03:18 > 0:03:21They view the camera as a possible new home.
0:03:21 > 0:03:24Bees are sensitive to electrical fields.
0:03:24 > 0:03:27That seems to make the camera even more desirable.
0:03:54 > 0:03:57A killer bee sting is no worse than that of a honey bee,
0:03:57 > 0:03:59but hundreds will sting at any one time
0:03:59 > 0:04:03and it's this behaviour that makes them dangerous.
0:04:15 > 0:04:17Lying down is a surprisingly good idea.
0:04:17 > 0:04:19The players merge with the ground
0:04:19 > 0:04:23and the bees simply fly over them, unable to find a target.
0:04:27 > 0:04:31SIRENS WAIL
0:04:31 > 0:04:34Killer bees were accidentally created in Brazil in 1957
0:04:34 > 0:04:38by crossing honey bees with aggressive African bees.
0:04:38 > 0:04:40Since then, they've spread northwards
0:04:40 > 0:04:43as far as the southern states of America.
0:04:47 > 0:04:50But there is a swarm that has created far more devastation
0:04:50 > 0:04:52than any killer bees.
0:04:56 > 0:04:59Morocco, December 2004.
0:04:59 > 0:05:04One of the oldest swarms on the planet is on the move.
0:05:04 > 0:05:08One of the ten plagues of Egypt, they still invade one fifth
0:05:08 > 0:05:10of the world's land surface
0:05:10 > 0:05:13and affect one tenth of the human population.
0:05:14 > 0:05:17Even today, meeting a swarm is unnerving
0:05:17 > 0:05:20as this home video footage shows.
0:05:20 > 0:05:22Eurgh!
0:05:22 > 0:05:24- That's crazy.- They're grasshoppers.
0:05:24 > 0:05:26Why just now?
0:05:27 > 0:05:29- Mommy!- This is crazy.
0:05:32 > 0:05:36Oh, my God, there are going to be millions of these and they're all flying off.
0:05:36 > 0:05:38There's so many of them just swarming over here.
0:05:38 > 0:05:42They're still just coming, across the road on both sides of the car.
0:05:42 > 0:05:45- Oh, my God!- ..Complete infestation...
0:05:45 > 0:05:47Argh! Mommy! Mommy!
0:05:47 > 0:05:49Swarm locusts start life as hoppers,
0:05:49 > 0:05:52the offspring of a solitary locust,
0:05:52 > 0:05:55forced together by a shortage of food.
0:06:00 > 0:06:04The hoppers crowd together to create an insect army.
0:06:11 > 0:06:16It's an army that marches on its stomach, eating everything in its path.
0:06:28 > 0:06:31When they run out of food, they march some more,
0:06:31 > 0:06:33crossing open ground in vast battalions,
0:06:33 > 0:06:37travelling up to 1.5 miles each day.
0:06:43 > 0:06:46They stay as a ground army for four weeks,
0:06:46 > 0:06:49before changing into an air force.
0:06:52 > 0:06:56Now, they really chew up the miles. An average swarm contains billions
0:06:56 > 0:07:00of locusts and the biggest outbreaks can infest half a continent.
0:07:05 > 0:07:07They're almost impossible to control.
0:07:11 > 0:07:16Seen for the first time from inside the swarm, it's easy to see why.
0:07:22 > 0:07:25As they fly, they synchronise their wing beats.
0:07:25 > 0:07:28This reduces turbulence, making their flying more efficient.
0:07:39 > 0:07:43Like fighter pilots, they fly in perfect formation.
0:07:43 > 0:07:45But here, one billion locusts
0:07:45 > 0:07:48are covering an area the size of Heathrow.
0:07:51 > 0:07:53It would be an air traffic controller's nightmare,
0:07:53 > 0:07:55but the locusts maintain formation
0:07:55 > 0:07:58by keeping a set distance from each other.
0:07:58 > 0:08:01They even have anti-crash detectors, to avoid mid-air collisions.
0:08:02 > 0:08:05They react six times faster than a human pilot,
0:08:05 > 0:08:07so even in such crowded air space,
0:08:07 > 0:08:10split-second manoeuvres can get them out of trouble.
0:08:22 > 0:08:27Such state of the art flight systems make locusts a force to be reckoned with.
0:08:44 > 0:08:46When they reach the human world,
0:08:46 > 0:08:48they're almost impossible to control.
0:08:53 > 0:08:56They eat their own weight in food in a day.
0:08:56 > 0:09:03A large swarm can get through 200,000 tonnes, enough to feed half a billion people.
0:09:14 > 0:09:18Locusts aren't the only ancient swarm that still plagues us today.
0:09:29 > 0:09:33The house mouse originated in Asia and spread with human agriculture.
0:09:33 > 0:09:37When conditions are right, they can quickly become plagues.
0:09:43 > 0:09:44In South Australia,
0:09:44 > 0:09:48mice outbreaks happen on average once every ten years.
0:10:00 > 0:10:03They erupt after a period of wet weather creates bumper harvests.
0:10:03 > 0:10:07By the time they're discovered, it's usually too late.
0:10:15 > 0:10:19It's every farmer's living nightmare.
0:10:35 > 0:10:41One unfortunate farmer filmed the actual moment she realised her pest problem was out of control.
0:10:53 > 0:10:55Oh! They're in my boots! Eugh!
0:10:55 > 0:10:57Concerned for the safety of her pigs,
0:10:57 > 0:10:59she throws caution to the wind.
0:11:06 > 0:11:08Yuk! Oh!
0:11:09 > 0:11:14SHE SCREAMS
0:11:22 > 0:11:27Among the pens, the mice found a limitless supply of food and safety from predators.
0:11:27 > 0:11:31It didn't take long for their numbers to go through the roof.
0:11:53 > 0:11:56The secret of their success is a rampant sex life
0:11:56 > 0:11:59that turns breeding into a weapon.
0:11:59 > 0:12:02A pair of mice can have babies every three weeks.
0:12:02 > 0:12:07These, in turn, can breed just five weeks later and their offspring, five weeks after that.
0:12:07 > 0:12:11Unchecked, the numbers just keep growing.
0:12:17 > 0:12:20In theory, one mouse and her offspring
0:12:20 > 0:12:22can produce 3,000 mice a year.
0:12:22 > 0:12:27With few predators to control them, theory rapidly comes fact.
0:12:34 > 0:12:38In one outbreak, 35 million mice were killed in just one month.
0:12:38 > 0:12:42Even this hardly dented their numbers.
0:12:48 > 0:12:52Eventually, stress and disease take their toll and the population crashes.
0:12:52 > 0:12:57For every 1,000 mice, just two will remain.
0:12:57 > 0:12:59But they're breeding machines,
0:12:59 > 0:13:02ready to surprise us at any time in the future.
0:13:05 > 0:13:09Some swarms are more predictable.
0:13:16 > 0:13:19This eerie invasion happens like clockwork
0:13:19 > 0:13:22once a year in the Midwest of America.
0:13:22 > 0:13:26It can be so dense, it slows traffic to a crawl.
0:13:29 > 0:13:33The swarms can smother an area of 4,500 square miles
0:13:33 > 0:13:35and even show up on radar.
0:13:44 > 0:13:45These are mayflies.
0:13:45 > 0:13:48They have emerged from the Mississippi River,
0:13:48 > 0:13:51where they lived as larvae for the past year.
0:13:51 > 0:13:54They will exist as adults for only a day.
0:14:01 > 0:14:06Their cue is a water temperature of exactly 17.5 degrees centigrade.
0:14:06 > 0:14:10They hardly have a chance to try out their new body and wings
0:14:10 > 0:14:12before it's all change again.
0:14:14 > 0:14:16For some unknown reason, the body splits
0:14:16 > 0:14:19and an almost identical mayfly emerges.
0:14:19 > 0:14:22The previous form may have existed for less than an hour.
0:14:28 > 0:14:32They soon spread their second set of wings, this time in a mating flight.
0:14:35 > 0:14:38By emerging together, there's safety in numbers.
0:14:38 > 0:14:40Nothing could eat this many.
0:14:40 > 0:14:44They soon return to the Mississippi to lay their eggs.
0:14:48 > 0:14:53Each female lays up to 8,000, making up for the billions of mayflies
0:14:53 > 0:14:55that never make it back to the river.
0:14:59 > 0:15:04It's easy to get lost. They navigate using the moon as a reference point.
0:15:04 > 0:15:05But near towns,
0:15:05 > 0:15:09this ancient navigational system is next to useless.
0:15:11 > 0:15:15Artificial moons are everywhere and these beckoning lights cause
0:15:15 > 0:15:17mayflies to become hopelessly confused.
0:15:23 > 0:15:26The road also reflects light like the river,
0:15:26 > 0:15:29attracting the mayflies down to lay their eggs.
0:15:35 > 0:15:41As the mayflies are lured further into town, they spread their own haze of confusion.
0:15:44 > 0:15:49This gas station is being attacked by something.
0:15:53 > 0:15:56I don't know what those bugs could possibly be.
0:16:00 > 0:16:03And that lady won't even get out of her car.
0:16:03 > 0:16:05That is really creepy.
0:16:07 > 0:16:10But not everyone's so mystified.
0:16:10 > 0:16:13The locals have seen it all before.
0:16:13 > 0:16:16It's mayflies' breeding day...
0:16:16 > 0:16:18in South Ballston Virginia.
0:16:18 > 0:16:23I don't know if you can see this, but this is mayflies, and they come out in the gajillions!
0:16:23 > 0:16:26And these things, they come out like this
0:16:26 > 0:16:29on certain days, all at once
0:16:29 > 0:16:32and they cover the streets of downtown South Ballston,
0:16:32 > 0:16:36and probably just about anywhere where the damn river runs through.
0:16:36 > 0:16:39It sounds like the wind is blowing out here, because there's so many of them.
0:16:39 > 0:16:42It's just amazing, you just wouldn't believe it.
0:16:42 > 0:16:46There are certainly huge numbers,
0:16:46 > 0:16:50an estimated 18 trillion emerge for one night only.
0:16:50 > 0:16:533,000 times the world's human population.
0:16:53 > 0:16:57Many fly straight to the bright lights of the city,
0:16:57 > 0:17:0110,000 may cover a single neon sign.
0:17:04 > 0:17:09By extraordinary coincidence, the invasion often coincides
0:17:09 > 0:17:12with one of America's most important celebrations -
0:17:12 > 0:17:14Independence Day.
0:17:16 > 0:17:20As crowds fill the streets of La Crosse, Wisconsin,
0:17:20 > 0:17:25they are outnumbered by mayflies 25 million to one.
0:17:29 > 0:17:33Stand too close to a light and you certainly know it's mayfly day.
0:17:40 > 0:17:43Fortunately, the mayflies are totally harmless.
0:17:43 > 0:17:47They have no mouth parts, so they can't even feed, let alone bite.
0:17:47 > 0:17:49To us, they seem out of control,
0:17:49 > 0:17:52but inside the swarm, it's a different picture.
0:17:55 > 0:17:57These angelic forms easily avoid each other,
0:17:57 > 0:18:00even in the thickest gathering.
0:18:14 > 0:18:18As America celebrates its independence,
0:18:18 > 0:18:21the mayflies enjoy the last day of their life.
0:18:43 > 0:18:49Bird swarms invade our cities too, a fact which many find unnerving.
0:18:49 > 0:18:51There's some of them. Oh, my God!
0:18:51 > 0:18:55It's like The Birds, it's like the invasion of The Birds over here.
0:18:55 > 0:18:57Look at this!
0:18:57 > 0:19:00We're driving through them.
0:19:00 > 0:19:04I have never in my life experienced this.
0:19:04 > 0:19:07- Let's get out. - I don't know, Daddy can get out.
0:19:07 > 0:19:11The birds are everywhere.
0:19:11 > 0:19:13Is this a phenomenon, or...? Ooh, my gosh!
0:19:15 > 0:19:17I'm very shocked at this.
0:19:17 > 0:19:20I don't understand what's going on here.
0:19:20 > 0:19:23I think this is like something that happens
0:19:23 > 0:19:26before like a big earthquake or something.
0:19:36 > 0:19:40In recent years, huge flocks of roosting birds have become a common
0:19:40 > 0:19:46sight in many cities, but nowhere do they reach the numbers seen in Rome.
0:19:46 > 0:19:51Here, they have become a super swarm that gets bigger year on year.
0:19:53 > 0:19:56Ten million starlings flock together
0:19:56 > 0:20:00to create the most astonishing aerial display in nature.
0:20:36 > 0:20:39These spectacular aerial displays are seen
0:20:39 > 0:20:42between October and February,
0:20:42 > 0:20:45when birds pour into Rome from all over northern Europe.
0:20:58 > 0:21:01To create such perfect synchrony,
0:21:01 > 0:21:05it was once thought that the birds might use telepathic communication.
0:21:05 > 0:21:10But in reality, the birds react 13 times faster than any person.
0:21:10 > 0:21:14They turn in exact synchrony with their nearest neighbours,
0:21:14 > 0:21:19but the tiniest delay in their reflexes help create these mesmerising waves in the sky.
0:21:32 > 0:21:36Until recently, the spectacle could only be seen from the ground.
0:21:36 > 0:21:41Now it's possible to take cameras into the swarms and fly among them.
0:22:09 > 0:22:14This aerial ballet acts as a beacon to others still entering the city.
0:22:14 > 0:22:18Smaller parties join the bigger flocks, increasing their numbers.
0:22:18 > 0:22:23As they swirl in the sky, they collectively make decisions on where they should spend the night.
0:22:31 > 0:22:35After an hour of pirouetting, the birds dive downwards,
0:22:35 > 0:22:38having decided together where they will roost.
0:22:44 > 0:22:49Birds are drawn to cities because they're warmer than the surrounding countryside.
0:22:49 > 0:22:51Rome, with its balmy Mediterranean climate,
0:22:51 > 0:22:54is an especially popular destination.
0:22:57 > 0:23:02But the city pays a heavy price for hosting this incredible air show.
0:23:07 > 0:23:11Each night, ten million birds produce
0:23:11 > 0:23:15seven tons of excrement, playing havoc with the paintwork of cars.
0:23:17 > 0:23:20As the birds settle down for the night,
0:23:20 > 0:23:23there is a constant rain falling below.
0:23:27 > 0:23:30Slippery roads and pavements cause accidents too,
0:23:30 > 0:23:32it's a health and safety nightmare.
0:23:34 > 0:23:37But the starlings don't have it all their own way.
0:23:53 > 0:23:57They may look like extras from a sci-fi B-movie,
0:23:57 > 0:24:00but these people really are wearing their work clothes.
0:24:03 > 0:24:09They are starling busters, hired to clear the streets of this urban menace.
0:24:11 > 0:24:14Their weapon of choice is a loudspeaker.
0:24:14 > 0:24:19BIRDS SCREECHING
0:24:19 > 0:24:24It plays back a starling's alarm call, amplified 100 times.
0:24:31 > 0:24:34For these birds, there is simply nothing scarier.
0:24:34 > 0:24:41This roost of 100,000 starlings will have to up sticks and find somewhere else to roost.
0:24:41 > 0:24:45This bizarre experiment has been running for the last 12 years.
0:24:45 > 0:24:47It certainly moves the starlings,
0:24:47 > 0:24:50but Rome offers plenty of alternative accommodation
0:24:50 > 0:24:53and the number of winter visitors keeps rising.
0:24:56 > 0:25:00Some swarms only invade our cities once in a generation.
0:25:04 > 0:25:08It's July 2008, and in the suburbs of Cincinnati,
0:25:08 > 0:25:12an alien invasion is about to begin.
0:25:27 > 0:25:31These periodic cicadas have lived deep underground
0:25:31 > 0:25:37for the last 17 years, but now, under the cover of darkness,
0:25:37 > 0:25:39they start to emerge.
0:25:39 > 0:25:43Across vast areas of the state of Ohio,
0:25:43 > 0:25:47ten billion cicadas join the alien invasion.
0:25:54 > 0:25:57They have spent their subterranean lives sucking the sap
0:25:57 > 0:25:59from the roots of trees.
0:26:03 > 0:26:06Now, in the 17th year, when the ground warms
0:26:06 > 0:26:10to 18 degrees centigrade, they all come to the surface.
0:26:12 > 0:26:17They make their way towards the nearest tree and immediately start to climb.
0:26:25 > 0:26:29Their marching orders are genetically programmed
0:26:29 > 0:26:31and so is their sense of timing.
0:26:35 > 0:26:39But genetics couldn't predict the changes made by those living in the world above.
0:26:43 > 0:26:46These cicadas are the lucky ones.
0:26:46 > 0:26:51Each year, more of the surface becomes concreted over, leaving millions trapped beneath.
0:26:55 > 0:26:57It's Memorial Day.
0:26:57 > 0:27:00In the 17 years the cicadas have spent underground,
0:27:00 > 0:27:04America has fought three wars and had three Presidents.
0:27:04 > 0:27:07And the human world has been transformed.
0:27:12 > 0:27:14With people absorbed in the commemorations,
0:27:14 > 0:27:17the cicadas go mostly unnoticed.
0:27:19 > 0:27:22But all this is about to change.
0:27:29 > 0:27:32The cicada nymphs break out of their old body,
0:27:32 > 0:27:36a miracle that can take as little as ten minutes.
0:27:53 > 0:27:58Soon, all the cicadas undergo the same extraordinary transformation.
0:28:11 > 0:28:15But before these crumpled wings can fly,
0:28:15 > 0:28:19they must be pumped full of blood, a marvel of insect hydraulics.
0:28:30 > 0:28:32Their gossamer wings need time to harden.
0:28:32 > 0:28:37While they wait, they're often noticed by their human neighbours for the first time.
0:28:37 > 0:28:40See that tree right there?
0:28:40 > 0:28:43It has lots and lots of cicadas on it.
0:28:43 > 0:28:46There's, like, a bunch of cicadas on the floor too.
0:28:46 > 0:28:49So I don't exactly wanna, like, step on any of them.
0:28:49 > 0:28:52Tons of them, like, tons.
0:28:52 > 0:28:56On her flowers. Cicadas everywhere.
0:28:58 > 0:28:59Cicada mania.
0:29:04 > 0:29:09While some enjoy the novelty, others are driven crazy by the noise.
0:29:11 > 0:29:16I am putting myself out here in the middle of this disgusting invasion of insects.
0:29:16 > 0:29:18It's like I'm a journalist in a war-zone.
0:29:20 > 0:29:24I can hear these things inside, with air-conditioning on high
0:29:24 > 0:29:27and Metallica blasting. I kid you not.
0:29:31 > 0:29:38The combined racket reaches 100 decibels, far louder than a jet flying overhead.
0:29:38 > 0:29:43The male makes the sound by vibrating a drum-like organ hidden in his body.
0:29:56 > 0:29:59It's not a good time for those with sensitive hearing.
0:30:03 > 0:30:09Adults have seen and heard it all before, but for kids it's a totally new experience.
0:30:09 > 0:30:12One they won't see again for 17 years.
0:30:21 > 0:30:26By appearing all together, cicadas swamp predators with too much food to eat,
0:30:26 > 0:30:31but their trick of using such an odd intervals of years is a masterstroke.
0:30:31 > 0:30:35It stops any predator breeding to exploit the feast.
0:30:35 > 0:30:41They count their 17 years below ground by measuring seasonal changes in the pressure of tree sap.
0:30:43 > 0:30:48Although millions are eaten, the majority manage to find each other and mate.
0:31:00 > 0:31:04After mating, they use their egg-laying tube as a saw
0:31:04 > 0:31:08to cut a slit under the bark, laying 30 eggs in each incision.
0:31:10 > 0:31:15Job done, they drop dead in their millions.
0:31:25 > 0:31:30Having spent the span of a human childhood underground,
0:31:30 > 0:31:32their adult life is over in just three weeks.
0:31:53 > 0:31:57Cincinnati is left with a huge clean-up operation.
0:31:57 > 0:32:00Hundreds of tons of dead and dying carcasses
0:32:00 > 0:32:02from this single emergence alone.
0:32:07 > 0:32:1117 years is the longest cycle of any breeding swarm.
0:32:11 > 0:32:14Most occur once every year.
0:32:19 > 0:32:22These Cuban land crabs have spent the last 11 months
0:32:22 > 0:32:24hidden in the rainforest.
0:32:24 > 0:32:29Today, they begin a six-mile march back to the sea.
0:32:29 > 0:32:31Crossing open ground is dangerous.
0:32:31 > 0:32:38They can soon become cooked in the sun, but civilisation soon stops them in their tracks.
0:32:40 > 0:32:44For the crabs, this is a shady paradise.
0:32:44 > 0:32:50Cool air flowing through the shutters makes thousands cluster together to avoid drying out.
0:32:59 > 0:33:05But it can only be a temporary pit stop, as the urge to march soon drives them on.
0:33:11 > 0:33:14The crustacean army is 100 million strong
0:33:14 > 0:33:18and their cue to migrate is triggered by the spring rains.
0:33:20 > 0:33:22The hordes have to cross a coast road
0:33:22 > 0:33:25that forms a barrier between land and sea.
0:33:25 > 0:33:29It's hardly a motorway, but for the crabs it's an accident black spot.
0:33:45 > 0:33:49For unsuspecting drivers, coming across the invasion
0:33:49 > 0:33:54is a memorable experience, making some reach for the camera.
0:33:56 > 0:34:01Traffic comes to a standstill, as cars try to find a way through.
0:34:01 > 0:34:04TOOTING
0:34:10 > 0:34:14Those that ignore the crabs usually pay the price.
0:34:14 > 0:34:17One garage mends 100 punctures a day.
0:34:27 > 0:34:31Despite the carnage, the crabs are distinctly unsentimental.
0:34:31 > 0:34:35Seizing the chance for a roadside snack.
0:34:43 > 0:34:46Fortunately for the crabs' ultimate survival,
0:34:46 > 0:34:49most of the millions make it to the other side,
0:34:49 > 0:34:52then it's an easy sprint to the sea.
0:34:55 > 0:34:58Land crabs came from the ocean.
0:34:58 > 0:35:01They need to return here to lay the eggs
0:35:01 > 0:35:04that will secure the next generation.
0:35:04 > 0:35:11Crabs only swarm in the breeding season, but some animals live their entire life as a swarm.
0:35:15 > 0:35:18It's May 2008 and the rains have arrived
0:35:18 > 0:35:20in the mountains of East Africa.
0:35:23 > 0:35:27One of the most organised swarms on Earth has appeared above ground.
0:35:29 > 0:35:34They're driver ants, an insect with a fearsome reputation.
0:35:39 > 0:35:44In the rainy season they form these extraordinary hunting trails.
0:35:47 > 0:35:50Large soldier ants line the trail,
0:35:50 > 0:35:52protecting the smaller workers inside.
0:35:52 > 0:35:56Their massive jaws create an impregnable barricade.
0:35:59 > 0:36:04The soldiers create these protective tunnels whenever they cross open ground.
0:36:13 > 0:36:17The trails radiate from the nest in every direction,
0:36:17 > 0:36:19some stretch 100 metres.
0:36:19 > 0:36:23A worker running at full pelt takes 45 minutes to run its length.
0:36:32 > 0:36:36The soldiers protect the trail at any cost.
0:36:42 > 0:36:45Although blind, they're highly sensitive
0:36:45 > 0:36:47to vibrations and air currents
0:36:47 > 0:36:50and become instantly defensive when under attack.
0:36:57 > 0:37:00Stress pheromones put the whole task force on alert.
0:37:00 > 0:37:04They're ready to take on anything, including people.
0:37:10 > 0:37:15They're programmed to keep the trail moving or die trying.
0:37:22 > 0:37:26Driver ants are the stuff of legend.
0:37:26 > 0:37:29It is said that they kill everything that crosses their path.
0:37:29 > 0:37:32It is said that no animal is safe when they're on the warpath.
0:37:32 > 0:37:36It is even said that they'll enter huts to attack people
0:37:36 > 0:37:39or kill babies left unattended in their cots.
0:37:40 > 0:37:46BABY CRIES
0:37:46 > 0:37:49The truth is somewhat different.
0:37:49 > 0:37:53Although the bites are painful, against people they're purely defensive.
0:37:53 > 0:37:57The jaws may slice through human flesh like butter, but it's simply a warning.
0:38:04 > 0:38:08Despite the myths, driver ants are still ruthless killers,
0:38:08 > 0:38:11but in a way that often benefits the villagers.
0:38:11 > 0:38:15Dangerous pests like scorpions are quickly set upon by the ants.
0:38:18 > 0:38:21Even the scorpion's deadly sting is powerless
0:38:21 > 0:38:23against this invincible army.
0:38:23 > 0:38:30Both workers and soldiers join the attack, dividing up their roles according to their size.
0:38:36 > 0:38:42As some look for a chink in its leg armour, others prise open its body plates like a tin opener.
0:38:48 > 0:38:52With its sting immobilised and faced by such overwhelming odds,
0:38:52 > 0:38:56the scorpion eventually gives up the fight.
0:38:58 > 0:39:03The dismembered body is hauled back down the trail to feed the nest.
0:39:15 > 0:39:20The farmers' fields provide even more opportunities for the attack force.
0:39:23 > 0:39:29Pests disturbed by digging are soon dispatched by hundreds of razor-sharp jaws.
0:39:31 > 0:39:33The ants make a clean sweep,
0:39:33 > 0:39:39capturing up to 100,000 insects in a single raid.
0:39:44 > 0:39:46Despite the ants' formidable reputation,
0:39:46 > 0:39:50most farmers value their role as pest controllers.
0:39:51 > 0:39:55Their feelings about another African swarm are equally surprising.
0:40:03 > 0:40:06Once a month throughout the rainy season,
0:40:06 > 0:40:11mysterious clouds rise over Lake Victoria, Africa's largest lake.
0:40:24 > 0:40:29This smoke on the water is not quite what it seems.
0:40:29 > 0:40:31It's actually a living entity
0:40:31 > 0:40:34comprised of trillions of individuals.
0:40:54 > 0:40:58They're lake flies, a kind of midge that emerges from the water
0:40:58 > 0:41:02in astronomical numbers at the time of the new moon.
0:41:13 > 0:41:15There have always been flies here,
0:41:15 > 0:41:18but increased pollution from fertilisers and sewage,
0:41:18 > 0:41:20while damaging the lake,
0:41:20 > 0:41:23is encouraging these huge columns of flies.
0:41:25 > 0:41:30In terms of sheer numbers, these are now the biggest swarms on earth.
0:41:32 > 0:41:34Eventually they make landfall
0:41:34 > 0:41:38and people encounter the flies for the first time.
0:41:46 > 0:41:50On land, countless numbers swirl around every physical feature
0:41:50 > 0:41:52as they shimmy in courtship flights.
0:41:58 > 0:42:02Although the air is choked with billions of flies,
0:42:02 > 0:42:03daily life continues.
0:42:05 > 0:42:09It's rumoured that the flies can suffocate people by sheer number.
0:42:09 > 0:42:12In reality the lake villagers are surprisingly blase
0:42:12 > 0:42:14about this living smog.
0:42:18 > 0:42:22Eventually the flies take a rest, covering every available surface
0:42:22 > 0:42:26as they break off from their mating rituals.
0:42:29 > 0:42:33Like many swarms, the midges emerge together to swamp predators
0:42:33 > 0:42:36with too much food to eat at any one time.
0:42:36 > 0:42:38Birds even migrate to the lake
0:42:38 > 0:42:41to make the most of this monthly free lunch.
0:42:56 > 0:42:57But it's not only birds
0:42:57 > 0:43:01that can't bear to see so much protein go to waste.
0:43:18 > 0:43:23Such incredible quantity of biomass is equivalent to huge herds of game,
0:43:23 > 0:43:25but far easier to catch.
0:43:37 > 0:43:41A frying pan makes the perfect weapon.
0:43:44 > 0:43:49The flies are so valuable the whole village joins in the hunt.
0:43:49 > 0:43:54The saucepans have been dampened first to make the flies stick and then clump together.
0:43:58 > 0:43:59These midges are so nutritious,
0:43:59 > 0:44:02they're valued as a supplement to the local diet.
0:44:15 > 0:44:18Each patty contains half a million flies
0:44:18 > 0:44:22and has seven times more protein than the average beefburger.
0:44:26 > 0:44:29For people who have little protein in their diet,
0:44:29 > 0:44:32this burger bar is a health food centre.
0:44:35 > 0:44:37Each village has its own recipes,
0:44:37 > 0:44:40but fly burgers always go like hot cakes.
0:44:44 > 0:44:51In times of famine, this traditional method of dealing with the monthly invasion becomes ever more relevant.
0:44:57 > 0:45:01Both ants and flies are swarms with a positive side,
0:45:01 > 0:45:05but one African plague has no such redeeming features.
0:45:05 > 0:45:08It can be heard long before it arrives.
0:45:13 > 0:45:17These are the most numerous birds on Earth with a growing population
0:45:17 > 0:45:20that may now be more than ten billion.
0:45:25 > 0:45:29The largest flock on record consisted of over 40 million birds
0:45:29 > 0:45:33and took five hours to pass.
0:45:36 > 0:45:39Sometimes known as the locust bird,
0:45:39 > 0:45:42these are quelea, a type of weaver.
0:45:46 > 0:45:49They follow the seasonal rains in an endless search for food.
0:45:49 > 0:45:52Water holes draw huge numbers together.
0:45:57 > 0:45:59They need to drink twice a day
0:45:59 > 0:46:03and usually stay no more than ten miles from the nearest water.
0:46:14 > 0:46:17Being sociable allows them to share information.
0:46:17 > 0:46:21Each bird acts as a scout and those that have found food let their neighbours know.
0:46:21 > 0:46:26This guarantees that the departing flocks set off in the most promising direction.
0:46:37 > 0:46:41They eat grass seeds but, unfortunately for African farmers,
0:46:41 > 0:46:45they prefer the more succulent domesticated varieties,
0:46:45 > 0:46:48getting through 10,000 tonnes a day.
0:46:48 > 0:46:54Smaller parties break off from the main flock to make sneaky raids on the farmers' fields.
0:46:56 > 0:46:59Even a relatively small flock will demolish
0:46:59 > 0:47:01an acre of rice in less than an hour.
0:47:01 > 0:47:05SCREAMING
0:47:05 > 0:47:07Subsistence farmers suffer the most.
0:47:07 > 0:47:10It's a full-time occupation keeping the crop raiders at bay.
0:47:10 > 0:47:12HE SHOUTS
0:47:15 > 0:47:17It's like trying to dam an ever-flowing river.
0:47:17 > 0:47:20As the flocks separate and reform,
0:47:20 > 0:47:24predicting where they might land is anyone's guess.
0:47:24 > 0:47:26With their livelihood at stake,
0:47:26 > 0:47:28the whole village creates a taskforce
0:47:28 > 0:47:30to try to stop them settling.
0:47:30 > 0:47:32HE BANGS DRUM AND SHOUTS
0:47:37 > 0:47:41A flock of one million quelea consume 60 tonnes of food a day
0:47:41 > 0:47:43but, as they're always on the move,
0:47:43 > 0:47:46they're almost impossible to control.
0:47:47 > 0:47:51Only when the birds roost do the farmers get some respite.
0:47:54 > 0:47:58Eventually the birds have to stop their endless travelling.
0:47:58 > 0:48:01Nesting is their most vulnerable time.
0:48:03 > 0:48:06The red-billed males do the main work,
0:48:06 > 0:48:09weaving up to 20 of these intricate,
0:48:09 > 0:48:12bowl-shaped nests to attract different females.
0:48:21 > 0:48:25He takes the greatest of care, as his quality of work is being judged.
0:48:32 > 0:48:34Only the finest weaving passes the test.
0:48:36 > 0:48:39If she approves, she moves inside.
0:48:47 > 0:48:51But stopping to breed has made them vulnerable.
0:49:07 > 0:49:12A colony may cover four square miles and hold 300,000 birds,
0:49:12 > 0:49:16all breeding a new generation of crop raiders.
0:49:16 > 0:49:19As sunset draws the birds back to the nest,
0:49:19 > 0:49:24they pay the ultimate price for feeding off our success.
0:49:24 > 0:49:27COUNTDOWN IS SHOUTED
0:49:44 > 0:49:48In the birds' world, this is Armageddon.
0:49:49 > 0:49:53Over 100,000 annihilated in a single blast.
0:49:57 > 0:50:01The inferno brings temporary relief to nearby farmers but does nothing
0:50:01 > 0:50:06to stop the ever-expanding growth of Africa's quelea population.
0:50:06 > 0:50:10We may unintentionally encourage swarms
0:50:10 > 0:50:12but we cannot control them.
0:50:17 > 0:50:19It's August 2008.
0:50:19 > 0:50:22In the Mississippi and its many tributaries,
0:50:22 > 0:50:25a purely man-made swarm is on the rise.
0:50:37 > 0:50:40In the 1970s, silver carp were accidentally
0:50:40 > 0:50:44introduced into these waters after escaping from a fish farm.
0:50:50 > 0:50:54They now outnumber local fish by 10-1.
0:50:57 > 0:51:00They have some truly bizarre behaviour.
0:51:08 > 0:51:11Some can clear three metres in a single leap.
0:51:26 > 0:51:31They say when fish are leaping, it's good for anglers.
0:51:31 > 0:51:33It's certainly the case in this part of the river.
0:51:39 > 0:51:42Here, fishermen don't even need a rod and line.
0:51:42 > 0:51:45Fish are only too willing to do the job for them.
0:51:50 > 0:51:52It's a phenomenon that takes some getting used to.
0:51:54 > 0:51:56Whoa!
0:51:56 > 0:51:58- I got him too!- There's a guy.
0:51:58 > 0:52:00Look out! SQUEALING
0:52:00 > 0:52:02Hey! LAUGHTER
0:52:07 > 0:52:08That could be cut bait.
0:52:08 > 0:52:10That could be cut bait.
0:52:12 > 0:52:16Hey! We don't even have to get the fishing pole wet! Look out!
0:52:19 > 0:52:22Damn it, let's eat that thing, see what it tastes like!
0:52:22 > 0:52:26But this kind of angling has its dangers.
0:52:26 > 0:52:30Oh! No!
0:52:30 > 0:52:32Right in the mummy-daddy button!
0:52:34 > 0:52:36It's not just anglers who are affected.
0:52:36 > 0:52:40Pleasure craft have to run the gauntlet too.
0:52:42 > 0:52:44Did you see that?
0:52:44 > 0:52:46That would hurt!
0:52:46 > 0:52:49My goodness! Holy toledo!
0:52:49 > 0:52:51There's massive fish flying
0:52:51 > 0:52:54out of the water behind them. I mean, large.
0:52:54 > 0:52:56That's insane!
0:52:58 > 0:53:00Get us out of here!
0:53:07 > 0:53:10The secret of this extraordinary behaviour lies in the boats' motors.
0:53:12 > 0:53:15The fish mistake pressure waves from the propeller
0:53:15 > 0:53:17for the movement of predators
0:53:17 > 0:53:19and literally jump with fright.
0:53:27 > 0:53:30They react in alarm to every passing boat.
0:53:41 > 0:53:43With numbers close to bursting point,
0:53:43 > 0:53:45each leaping fish scares its neighbour,
0:53:45 > 0:53:47creating a dangerous chain reaction.
0:53:51 > 0:53:56As some weigh over 40 pounds, collisions can be deadly.
0:54:03 > 0:54:07In some parts of the river, 200 fish missiles can launch at any one time.
0:54:07 > 0:54:10They've already caused serious injuries and,
0:54:10 > 0:54:14as they spread to other rivers, the danger increases by the day.
0:54:23 > 0:54:27Silver carp are a swarm that we accidentally created,
0:54:27 > 0:54:31living proof that, as we interfere with the balance of nature,
0:54:31 > 0:54:33we create more swarms.
0:54:35 > 0:54:38But there is one swarm that we have been deliberately encouraging
0:54:38 > 0:54:39for thousands of years.
0:54:41 > 0:54:45The honey bee is the only swarm we have ever domesticated,
0:54:45 > 0:54:47but we're still unravelling the mysteries
0:54:47 > 0:54:49of much of their behaviour.
0:54:55 > 0:54:58For those who understand the swarm's inner mind,
0:54:58 > 0:55:02it's possible to control them in a way that is quite extraordinary.
0:55:13 > 0:55:17The bees are being summoned by a secret signal,
0:55:17 > 0:55:20one that causes them to act with a single mind.
0:55:25 > 0:55:28Swarms are made from thousands of organisms
0:55:28 > 0:55:30but they work by acting as one.
0:55:37 > 0:55:40By understanding the swarm's secret language,
0:55:40 > 0:55:44it's possible to make them do the most extraordinary things.
0:56:11 > 0:56:16100,000 bees cover his body, creating a living suit.
0:56:26 > 0:56:31He has such control over the bees he rarely suffers a single sting.
0:56:36 > 0:56:39He is a professor of entomology,
0:56:39 > 0:56:43and has been studying bees and their communication all his life.
0:56:46 > 0:56:49By covering his body with the pheromone that the queen bee
0:56:49 > 0:56:53uses to control her hive, he has them at his command.
0:56:53 > 0:56:56They react by protecting him with their bodies.
0:57:03 > 0:57:07To them, he is the queen bee.
0:57:12 > 0:57:15Using science, he has learnt to control what most of us dread,
0:57:15 > 0:57:18and the result is quite disturbing.
0:57:23 > 0:57:26As he brings a swarm into our world,
0:57:26 > 0:57:28people are forced to confront their fears.
0:57:32 > 0:57:35They also see that, with knowledge,
0:57:35 > 0:57:39such an incredible force of nature can be controlled.
0:57:40 > 0:57:43When swarms invade, we are inevitably separated
0:57:43 > 0:57:45by an invisible barrier created
0:57:45 > 0:57:48by our different senses and perceptions.
0:57:50 > 0:57:54Swarms may share our world but the gulf between us couldn't be greater.
0:58:00 > 0:58:06To control the swarm, we need to understand how they see the world.
0:58:07 > 0:58:12But swarms not only experience things differently from us,
0:58:12 > 0:58:15they think in a different way too.
0:58:22 > 0:58:28The next programme will explore the extraordinary alien intelligence
0:58:28 > 0:58:30that controls the swarm.
0:58:45 > 0:58:48Subtitles by Red Bee Media Ltd
0:58:48 > 0:58:51E-mail subtitling@bbc.co.uk