1 00:00:38,887 --> 00:00:41,959 An eye from another world, 2 00:00:47,287 --> 00:00:50,996 A smell-detector, investigating the path ahead. 3 00:00:53,647 --> 00:00:56,286 We don't often see a snail that way. 4 00:00:57,367 --> 00:01:02,964 And that's because we've only recently had the tiny lenses and electronic cameras 5 00:01:03,047 --> 00:01:06,084 that we need to explore this miniature world. 6 00:01:10,567 --> 00:01:13,923 But when we meet its inhabitants face to face, 7 00:01:14,007 --> 00:01:18,717 we suddenly realise that their behaviour can be just as meaningful to us 8 00:01:18,807 --> 00:01:22,356 as the behaviour of many animals more our own size. 9 00:01:24,407 --> 00:01:26,363 Look at this, for example. 10 00:01:27,167 --> 00:01:29,283 It's an earwig, yes. 11 00:01:30,567 --> 00:01:33,604 But it's also a female and a mother. 12 00:01:33,687 --> 00:01:37,362 And like so many mothers, she's guarding her young. 13 00:01:41,407 --> 00:01:45,480 These two ants are not quite sure whether they like one another. 14 00:01:45,567 --> 00:01:50,960 Stroking antennae is the equivalent of a cautious chat over the garden fence. 15 00:01:53,607 --> 00:01:56,724 When big animals go courting, they show off. 16 00:01:56,807 --> 00:01:58,684 And so do damselflies. 17 00:02:03,047 --> 00:02:07,484 Courtship signals for a male wolf spider are rather more frantic 18 00:02:07,567 --> 00:02:12,004 because if his female doesn't understand why he's approaching her, 19 00:02:12,087 --> 00:02:13,964 she'll eat him. 20 00:02:15,647 --> 00:02:17,797 This ant is a farmer 21 00:02:17,887 --> 00:02:23,359 and these aphids, the cows which it milks for a drink of honeydew every day. 22 00:02:26,327 --> 00:02:29,205 Other ants are eternally on the march. 23 00:02:30,927 --> 00:02:35,000 Powerfully armed soldiers guard the flanks of their column as they travel, 24 00:02:35,087 --> 00:02:38,841 protecting the workers who are carrying their helpless young. 25 00:02:43,927 --> 00:02:48,364 When it comes to craftsmanship, few can beat this wasp. 26 00:02:48,447 --> 00:02:53,316 Using mud to construct an elegant jar in which to store her eggs. 27 00:02:55,127 --> 00:02:58,005 Mud is also used by termites. 28 00:02:58,087 --> 00:03:01,238 They build tower blocks that, in proportion to their size, 29 00:03:01,327 --> 00:03:03,716 are taller than New York skyscrapers. 30 00:03:04,167 --> 00:03:10,117 These two worlds, ours and theirs, influence one another to an extraordinary degree. 31 00:03:10,847 --> 00:03:15,921 If we and the rest of the backboned animals were to disappear overnight, 32 00:03:16,007 --> 00:03:18,760 the rest of the world would get on pretty well. 33 00:03:19,607 --> 00:03:21,962 But if they were to disappear, 34 00:03:22,047 --> 00:03:25,039 the land's ecosystems would collapse. 35 00:03:26,167 --> 00:03:29,443 For the fact is, they were the pioneers. 36 00:03:29,527 --> 00:03:34,396 The first animals of any kind to colonise the lands of the earth. 37 00:03:35,567 --> 00:03:39,196 To tell their story we must go back to a time 38 00:03:39,287 --> 00:03:41,801 when the world was a very different place. 39 00:03:59,647 --> 00:04:03,117 Some 400 million years ago, 40 00:04:03,207 --> 00:04:07,678 the lands of planet earth were totally without life. 41 00:04:07,767 --> 00:04:10,486 They were bare, naked rock, 42 00:04:10,567 --> 00:04:14,958 roasted by sun during the day, freezing cold at night 43 00:04:15,047 --> 00:04:17,845 and swept by terrible storms. 44 00:04:18,367 --> 00:04:23,157 But in the waters of the world conditions were much more stable. 45 00:04:24,727 --> 00:04:29,164 Life had begun there some 2,000 million years earlier still. 46 00:04:29,727 --> 00:04:34,847 For a long time it remained microscopic, but eventually larger animals appeared. 47 00:04:35,247 --> 00:04:38,922 Jellyfish and corals, starfish and snails, 48 00:04:39,007 --> 00:04:41,567 and animals with segmented bodies. 49 00:04:44,687 --> 00:04:46,518 All needed food. 50 00:04:46,607 --> 00:04:50,316 Many would have eaten unguarded eggs given the chance. 51 00:04:51,647 --> 00:04:56,323 And then, around 400 million years ago, some enterprising creatures 52 00:04:56,407 --> 00:05:01,162 found it safer to lay their eggs out of the sea, up on the beach. 53 00:05:02,287 --> 00:05:03,879 They still do. 54 00:05:16,447 --> 00:05:21,646 Every spring, on a few special nights along the Atlantic coast of North America, 55 00:05:21,727 --> 00:05:25,515 thousands of horseshoe crabs emerge from the sea. 56 00:05:32,247 --> 00:05:35,637 And here in the wet sand, they spawn. 57 00:05:36,327 --> 00:05:39,683 They may only stay for a few minutes or hours, 58 00:05:39,767 --> 00:05:43,237 but animals like these may well have been the first of any kind 59 00:05:43,327 --> 00:05:46,478 to leave the sea and venture onto land. 60 00:06:03,647 --> 00:06:07,765 Although these creatures spend virtually all their lives at sea, 61 00:06:07,847 --> 00:06:11,317 they can survive surprisingly well on land. 62 00:06:11,407 --> 00:06:14,479 It's almost as if they were pre-adapted. 63 00:06:14,567 --> 00:06:17,445 They have shells, external skeletons, 64 00:06:17,527 --> 00:06:22,885 and that means that their legs are rigid and jointed. 65 00:06:22,967 --> 00:06:28,724 And at the back they have a series of plates, called book lungs, 66 00:06:28,807 --> 00:06:32,277 which extract oxygen from seawater, 67 00:06:32,367 --> 00:06:37,361 but can also do the same thing, if they're kept reasonably moist, from the air. 68 00:06:37,807 --> 00:06:43,165 So creatures like this can in fact spend about a week on land. 69 00:06:43,687 --> 00:06:47,441 And it only requires minimum modifications 70 00:06:47,527 --> 00:06:50,360 to enable them to live up there permanently. 71 00:06:54,807 --> 00:06:59,676 It was difficult to abandon the sea altogether until the land became green. 72 00:06:59,767 --> 00:07:02,156 But eventually it did. 73 00:07:02,247 --> 00:07:06,035 Simple plants, algae and then mosses and liverworts 74 00:07:06,127 --> 00:07:10,245 began to advance over the mud and rock to clothe the earth. 75 00:07:10,327 --> 00:07:15,765 And into these first green tangles came animals looking for food. 76 00:07:28,487 --> 00:07:30,318 Some had armour 77 00:07:30,407 --> 00:07:34,082 for that, in the sea, had protected them from their enemies. 78 00:07:34,167 --> 00:07:37,045 Now it would help them conserve moisture. 79 00:07:52,207 --> 00:07:55,836 They were the ancestors of today's millipedes. 80 00:08:00,247 --> 00:08:03,956 Small holes had developed along the underside of their bodies 81 00:08:04,047 --> 00:08:09,041 that led to internal tubes with which they could absorb oxygen from the air. 82 00:08:13,127 --> 00:08:16,483 Their rigid,jointed legs, however, were largely unchanged 83 00:08:16,567 --> 00:08:20,480 and worked very well on land even without the support of water. 84 00:08:27,007 --> 00:08:31,364 Battering-ram heads enabled them to bulldoze their way through the vegetation 85 00:08:31,447 --> 00:08:34,245 to collect the rotting plants on which they fed. 86 00:08:42,247 --> 00:08:46,035 They grew big, increasing the number of segments in their bodies. 87 00:08:46,127 --> 00:08:50,245 Some had over 300, each with two pairs of legs. 88 00:08:53,367 --> 00:08:59,158 Some that didn't curl up, reinforced their armour with plates along their backs. 89 00:09:11,367 --> 00:09:14,439 Crustaceans like shrimps came, too. 90 00:09:16,927 --> 00:09:19,805 They were the ancestors of woodlice. 91 00:09:33,007 --> 00:09:37,080 So, today there is a huge and varied population of animals 92 00:09:37,167 --> 00:09:41,797 living on the land with bodies that are little different from those of their ancestors 93 00:09:41,887 --> 00:09:44,196 who lived in the sea so long ago. 94 00:09:47,567 --> 00:09:50,684 And they are extraordinarily successful. 95 00:09:50,767 --> 00:09:54,521 Some are the most numerous of all land-living species. 96 00:09:55,447 --> 00:10:00,521 But we seldom see them. This pin will give you an idea of why. 97 00:10:02,767 --> 00:10:04,280 They're tiny. 98 00:10:04,367 --> 00:10:07,996 This minute little creature is a springtail. 99 00:10:08,087 --> 00:10:12,797 It's less than a half a millimetre long. The size of a full stop. 100 00:10:14,567 --> 00:10:19,118 In one square metre of soil, there may be over 10,000 of them. 101 00:10:21,167 --> 00:10:23,806 Drying out is a very real danger for them. 102 00:10:23,887 --> 00:10:29,803 And some waterproof themselves regularly with a droplet of special grooming fluid. 103 00:10:31,487 --> 00:10:36,436 You might even say that they have turned bathing into an art form. 104 00:10:42,327 --> 00:10:45,205 They even have two inflatable tubes 105 00:10:45,287 --> 00:10:49,166 that enable them to get to those hard-to-reach places. 106 00:10:52,167 --> 00:10:54,635 To help them get around through the leaf litter, 107 00:10:54,727 --> 00:11:00,279 these springtails, as their name suggests, have a rather novel way of jumping. 108 00:11:07,447 --> 00:11:11,679 They have a tiny two-pronged lever beneath their abdomen. 109 00:11:11,767 --> 00:11:15,077 One small flick from it can catapult them six inches, 110 00:11:15,167 --> 00:11:17,681 some 15 centimetres, into the air. 111 00:11:23,487 --> 00:11:27,799 It's the equivalent of a human being jumping over the Eiffel Tower. 112 00:11:31,487 --> 00:11:35,162 And if they happen to land upside down, 113 00:11:35,247 --> 00:11:38,796 well, they have a special way of righting themselves. 114 00:11:39,287 --> 00:11:43,405 They use their grooming fluid dispenser to stick onto the ground 115 00:11:43,487 --> 00:11:46,206 so that they can pull themselves back onto their feet. 116 00:11:53,647 --> 00:11:59,199 So the foundations were laid for the ecosystems that now flourish on earth 117 00:11:59,287 --> 00:12:01,721 and on which we ourselves depend. 118 00:12:06,007 --> 00:12:07,725 It has to be said, however, 119 00:12:07,807 --> 00:12:11,959 that sometimes some of us regard a few of these pioneers 120 00:12:12,047 --> 00:12:15,198 more as our enemies than our friends. 121 00:12:19,607 --> 00:12:26,046 Many of the molluscs in the sea develop shells to protect themselves from predators. 122 00:12:26,607 --> 00:12:32,762 But on land those shells serve just as well to keep the occupant nice and moist. 123 00:12:32,927 --> 00:12:39,321 So without any major change to their anatomy, molluscs were able to creep up out of the water 124 00:12:39,407 --> 00:12:44,686 and graze in the forests of algae and mosses that were then spreading over the land. 125 00:12:45,167 --> 00:12:48,284 And given the right conditions, they still do. 126 00:12:59,967 --> 00:13:05,325 With rain and the coming of night a secret army comes out of hiding. 127 00:13:12,687 --> 00:13:15,804 These are the conditions they like best. 128 00:13:15,887 --> 00:13:19,197 Dark and, best of all, wet. 129 00:13:39,087 --> 00:13:41,476 Gliding along a carpet of slime 130 00:13:41,567 --> 00:13:45,321 works just as well on land as it does underwater. 131 00:13:46,727 --> 00:13:51,562 And a rasping tongue scrapes algae off rocks wherever they are. 132 00:13:57,327 --> 00:14:02,003 In times of drought, snails may be unable to move around for months on end. 133 00:14:02,527 --> 00:14:07,317 So when conditions are right, they eagerly set off to find food. 134 00:14:11,767 --> 00:14:16,045 Their upper pair of tentacles carry those eyes with which they look around. 135 00:14:17,527 --> 00:14:20,485 The lower ones smell what's beneath. 136 00:14:30,647 --> 00:14:33,605 They breed by means of a small pouch 137 00:14:33,687 --> 00:14:36,918 on the right hand side of their body just within the shell 138 00:14:37,007 --> 00:14:41,125 which, because it's permanently moist, is able to absorb oxygen. 139 00:14:49,047 --> 00:14:53,165 This is what they're seeking, a succulent green leaf. 140 00:14:53,247 --> 00:14:54,965 No time to be lost. 141 00:15:05,207 --> 00:15:08,165 Dawn will bring a change in conditions. 142 00:15:10,047 --> 00:15:14,120 So they have to return to their shelters and clamp down their shells once more 143 00:15:14,207 --> 00:15:16,402 so that they retain their moisture. 144 00:15:23,607 --> 00:15:27,646 The ancient forests were colonised by all kinds of plant eaters 145 00:15:27,727 --> 00:15:33,279 long before there were any frogs or lizards, birds or insect-eating mammals. 146 00:15:33,767 --> 00:15:38,363 But there were, nonetheless, hunters prowling through the vegetation. 147 00:15:40,367 --> 00:15:42,642 This was one of the first. 148 00:15:43,527 --> 00:15:49,557 Fossils very like it have been found in rocks that are 540 million years old. 149 00:15:49,647 --> 00:15:51,717 This is a velvet worm. 150 00:15:53,047 --> 00:15:57,279 It, too, has scarcely changed since it lived in the sea 151 00:15:57,367 --> 00:16:01,042 and today it's only found in wet, humid forests. 152 00:16:02,767 --> 00:16:07,318 It usually hunts at night, but infrared cameras can reveal it in action. 153 00:16:09,447 --> 00:16:13,486 Soft, stumpy legs enable it to move in total silence 154 00:16:13,567 --> 00:16:16,877 and it finds its way with long, sensitive feelers. 155 00:16:18,207 --> 00:16:20,277 It's a master of stealth. 156 00:16:22,327 --> 00:16:27,082 This cricket has huge eyes but it's difficult to see what's going on around it. 157 00:16:34,647 --> 00:16:36,922 Though the velvet worm has fangs, 158 00:16:37,007 --> 00:16:41,046 it will attack its prey when it finds it with a very special weapon. 159 00:17:05,967 --> 00:17:09,721 Anxiously, the cricket probes around in the darkness 160 00:17:09,807 --> 00:17:11,604 with its long antennae. 161 00:17:15,847 --> 00:17:20,238 The velvet worm will only know if it's found its prey when it touches it. 162 00:17:20,327 --> 00:17:23,478 So when it does, it has to react immediately. 163 00:17:27,407 --> 00:17:28,726 There! 164 00:17:32,967 --> 00:17:35,083 And the cricket is trapped. 165 00:17:36,847 --> 00:17:42,365 A slow motion camera shows the remarkable way in which the velvet worm attacks. 166 00:17:42,447 --> 00:17:47,123 Two nozzles beneath its feelers squirt twin streams of glue. 167 00:17:55,767 --> 00:17:59,919 The more the cricket struggles, the more it becomes entangled. 168 00:18:03,327 --> 00:18:08,720 With the prey immobilised, the velvet worm reclaims its glue by eating it. 169 00:18:08,807 --> 00:18:11,241 And then it starts on the cricket. 170 00:18:21,287 --> 00:18:24,677 There were other hunters, too, in the ancient forests, 171 00:18:24,767 --> 00:18:26,917 relatives of the horseshoe crabs, 172 00:18:27,007 --> 00:18:29,362 and they were even more formidable. 173 00:18:35,367 --> 00:18:37,722 This is a whip spider. 174 00:18:38,327 --> 00:18:41,956 Like its ancestors, it has a hard external skeleton. 175 00:18:44,447 --> 00:18:49,840 Two of its limbs have been turned into highly mobile, sensitive feelers. 176 00:18:50,127 --> 00:18:54,279 It uses them to probe around delicately both in front and behind. 177 00:18:54,887 --> 00:18:58,516 Any prey within a foot of it will be immediately detected. 178 00:19:04,287 --> 00:19:09,600 It's extremely territorial and it has no hesitation in attacking one of its own kind. 179 00:19:15,207 --> 00:19:18,722 The width of its claws is a good indicator of strength 180 00:19:18,807 --> 00:19:21,605 and a smaller animal will quickly back down. 181 00:19:25,207 --> 00:19:29,246 But these two are equally matched and they will fight. 182 00:19:46,407 --> 00:19:48,159 The loser retreats. 183 00:19:51,687 --> 00:19:56,317 But even whip spiders were not the most formidable hunters in these forests. 184 00:19:56,407 --> 00:19:59,763 There were others with an even more venomous weaponry. 185 00:20:05,687 --> 00:20:10,807 This centipede has powerful jaws, poison fangs 186 00:20:10,887 --> 00:20:13,003 and is very, very fast. 187 00:20:13,087 --> 00:20:15,078 It's a very good hunter. 188 00:20:15,607 --> 00:20:19,236 But it's only half as long as my little finger. 189 00:20:20,207 --> 00:20:25,565 There are centipedes in the world, however, that are as big as my forearm. 190 00:20:27,447 --> 00:20:30,723 This is one of these alarming giants. 191 00:20:31,007 --> 00:20:34,124 It's over 13 inches, 35 centimetres, long 192 00:20:34,207 --> 00:20:37,404 and with the muscular strength of a small snake. 193 00:20:37,967 --> 00:20:41,676 And the poison in its black-tipped fangs is lethal. 194 00:20:44,327 --> 00:20:47,956 It hunts in the dark, bat-haunted caves of Venezuela. 195 00:20:55,367 --> 00:20:58,165 Like the whip spider and the velvet worm, 196 00:20:58,247 --> 00:21:01,603 it uses its antennae to feel for its victims. 197 00:21:02,847 --> 00:21:07,079 The beetles that swarm on the rocky floor of the cave are of no interest to it. 198 00:21:07,167 --> 00:21:09,078 It's after bigger prey. 199 00:21:11,247 --> 00:21:14,478 And it knows it can find that by climbing. 200 00:21:20,487 --> 00:21:24,639 Its many legs give it a secure hold on the vertical rocks. 201 00:21:24,727 --> 00:21:26,922 It's heading for the ceiling. 202 00:21:38,687 --> 00:21:42,362 Now in the darkness, it can sense bats flying past it. 203 00:21:46,447 --> 00:21:50,486 Holding on with its hind legs, it reaches out into their flight path 204 00:21:50,567 --> 00:21:53,798 and almost immediately it has one. 205 00:22:00,047 --> 00:22:04,962 An injection of venom from its fangs kills the bat almost instantaneously. 206 00:22:09,007 --> 00:22:13,478 It will take it an hour or so, but it will eat all the bat's flesh. 207 00:22:21,607 --> 00:22:24,246 So all these animals, having left the sea, 208 00:22:24,327 --> 00:22:27,603 solved the problems of moving around and breathing air 209 00:22:27,687 --> 00:22:29,564 in their own differing ways. 210 00:22:29,727 --> 00:22:32,878 But there was another difficulty, mating. 211 00:22:32,967 --> 00:22:36,323 In the sea, animals need only release their eggs and sperm 212 00:22:36,407 --> 00:22:38,637 and the water mixed the two together. 213 00:22:38,727 --> 00:22:43,642 On dry land that couldn't happen, even for the most moisture-loving of creatures. 214 00:22:45,887 --> 00:22:49,846 An individual slug carries both male and female organs. 215 00:22:49,927 --> 00:22:54,876 But even then, that was of no help. Each had to both give and receive. 216 00:22:56,207 --> 00:23:00,723 Somehow or other, pairs of individuals had to get together 217 00:23:00,807 --> 00:23:06,086 and the ways they have evolved in which to do so are quite extraordinary. 218 00:23:06,167 --> 00:23:10,240 Indeed, some of them are almost beyond imagining. 219 00:23:12,047 --> 00:23:15,926 The leopard slug, you might think, has the simplest of habits. 220 00:23:16,367 --> 00:23:19,837 Maybe, but not when it comes to mating. 221 00:23:21,247 --> 00:23:23,442 When an individual is looking for a partner, 222 00:23:23,527 --> 00:23:27,805 it gives its trail of slime a special taste that advertises the fact. 223 00:23:29,047 --> 00:23:33,837 Another, if it feels the same way, will detect the invitation and start to follow. 224 00:23:35,647 --> 00:23:39,845 The pursuer, to confirm that it's there and ready to mate, 225 00:23:39,927 --> 00:23:42,043 gives the pursued a nibble. 226 00:23:46,967 --> 00:23:49,083 The leader heads upwards. 227 00:23:49,567 --> 00:23:51,797 An overhang is what's needed. 228 00:24:05,247 --> 00:24:08,557 The underside of a branch will do very nicely. 229 00:24:08,927 --> 00:24:12,476 The two start to circle one another more and more closely 230 00:24:12,567 --> 00:24:14,683 until they entwine. 231 00:24:18,407 --> 00:24:22,525 For an hour or so they continue to wind themselves around one another. 232 00:24:34,927 --> 00:24:38,886 Then, suddenly, the pair release their hold on the branch 233 00:24:38,967 --> 00:24:42,846 and start to slide downwards on a rope of mucus. 234 00:24:59,447 --> 00:25:03,998 Now, in midair, they move to the next stage in their pairing. 235 00:25:05,207 --> 00:25:09,200 Each everts its male organ from just behind its head. 236 00:25:19,407 --> 00:25:21,967 These grow longer and longer. 237 00:25:30,447 --> 00:25:33,996 Then they, too, begin to entwine. 238 00:25:56,647 --> 00:26:01,675 They fan out to form a translucent, flower-like globe. 239 00:26:09,927 --> 00:26:14,921 And now, at last, sperm passes from one slug to the other. 240 00:26:19,047 --> 00:26:22,926 The transfer is complete. Each has been fertilized. 241 00:26:32,927 --> 00:26:37,284 Finally, their strange, balletic relationship comes to an end... 242 00:26:39,647 --> 00:26:41,080 with a bump. 243 00:26:43,727 --> 00:26:48,039 A millipede, unlike a slug, is either a male or a female. 244 00:26:49,047 --> 00:26:52,357 In southern Africa, where there are many different species, 245 00:26:52,447 --> 00:26:55,166 both sexes spend the winter in hibernation, 246 00:26:55,247 --> 00:26:58,284 curled up in the leaf litter or beneath the bark. 247 00:26:59,647 --> 00:27:02,320 As the temperatures rise with the coming of spring, 248 00:27:02,407 --> 00:27:05,956 they all unwind themselves and set off to look for a mate. 249 00:27:06,687 --> 00:27:09,997 Finding one in the tangled undergrowth is not easy. 250 00:27:11,927 --> 00:27:14,964 This male forest millipede 251 00:27:15,047 --> 00:27:20,201 knows that he can increase his chances if he heads upwards 252 00:27:20,287 --> 00:27:21,879 into the trees. 253 00:27:26,807 --> 00:27:30,277 Leaving the safety of the undergrowth may seem a risky thing to do, 254 00:27:30,367 --> 00:27:33,996 but these millipedes secrete a poison from pores in their armour 255 00:27:34,087 --> 00:27:38,638 and their conspicuous red and black colours warn predators to leave them alone. 256 00:27:39,527 --> 00:27:41,677 They emerge in thousands. 257 00:27:45,927 --> 00:27:49,556 Surprisingly perhaps, a male, when he does find a female, 258 00:27:49,647 --> 00:27:51,842 is not met with a friendly greeting. 259 00:27:51,927 --> 00:27:54,282 Quite the reverse. She coils up. 260 00:27:59,287 --> 00:28:02,757 This is her way of sorting out the men from the boys. 261 00:28:02,847 --> 00:28:08,126 Only the strongest and fittest male will have the strength to force her coils apart. 262 00:28:09,607 --> 00:28:14,158 To help him do so, he has white suction pads on the bottom of his feet 263 00:28:14,247 --> 00:28:16,078 which give him a good grip. 264 00:28:18,007 --> 00:28:21,522 Eventually she relaxes and he lifts her up 265 00:28:21,607 --> 00:28:26,635 so he can extend two specially modified legs with which he inseminates her. 266 00:28:32,287 --> 00:28:37,645 Once inserted, these legs swell so that the partners become fastened together. 267 00:28:37,727 --> 00:28:39,285 And that's important 268 00:28:39,367 --> 00:28:43,326 because it will take him a couple of hours to transfer his sperm. 269 00:28:45,807 --> 00:28:50,756 But there are lots of males around and before long another one turns up. 270 00:28:54,927 --> 00:28:58,317 The new arrival checks out the pair with his antennae. 271 00:28:58,407 --> 00:29:02,685 If they're not tightly bound together, he may have a chance of taking over. 272 00:29:06,927 --> 00:29:09,885 He pushes between them, levering them apart. 273 00:29:10,967 --> 00:29:14,277 Gradually, he manages to unzip their legs. 274 00:29:18,207 --> 00:29:21,483 The first male's white mating legs are dragged out. 275 00:29:22,927 --> 00:29:24,280 He's been defeated. 276 00:29:24,367 --> 00:29:28,440 It will be the second, stronger male who fertilizes her eggs. 277 00:29:32,247 --> 00:29:37,196 So, mating on land isn't as random as it had been for so many in the sea. 278 00:29:37,287 --> 00:29:39,323 Now it's selective. 279 00:29:39,407 --> 00:29:42,922 But brute force isn't the only basis on which to select. 280 00:29:44,087 --> 00:29:46,885 A female springtail is bigger than a male 281 00:29:46,967 --> 00:29:51,836 and she prefers a partner who can give her a sustained head-to-head push. 282 00:29:54,647 --> 00:29:59,926 Other males are eager to try their luck but butting her sides won't get them anywhere. 283 00:30:14,447 --> 00:30:17,245 She seems unimpressed by any of them, 284 00:30:17,327 --> 00:30:20,558 but one is determined to stay as her dance partner. 285 00:30:33,607 --> 00:30:35,837 She simply can't get rid of him. 286 00:30:42,287 --> 00:30:47,236 He confidently signals victory with a couple of fancy twirls. 287 00:30:49,847 --> 00:30:53,442 Then he deposits a droplet of sperm onto the leaf 288 00:30:53,527 --> 00:30:56,121 and she graciously takes it onboard. 289 00:31:06,247 --> 00:31:10,035 One group of colonists were of particular importance 290 00:31:10,127 --> 00:31:12,322 for they changed the nature of the soil 291 00:31:12,407 --> 00:31:16,844 and thus made it possible for new kinds of plants and animals to evolve. 292 00:31:17,647 --> 00:31:21,560 They outweigh all other animals in any given area of the forest. 293 00:31:21,647 --> 00:31:25,242 A single hectare may be home to eight million of them. 294 00:31:26,927 --> 00:31:30,078 They spend nearly all their time below ground. 295 00:31:32,167 --> 00:31:33,566 Worms. 296 00:31:33,647 --> 00:31:38,038 They eat their way through the earth extracting edible vegetable material 297 00:31:38,127 --> 00:31:40,482 and making it suitable for plants. 298 00:31:42,367 --> 00:31:46,758 And at night, they come to the surface and collect dead leaves. 299 00:31:59,887 --> 00:32:03,004 They also take the opportunity to call on their neighbours, 300 00:32:03,087 --> 00:32:05,920 poking their heads into next-door burrows. 301 00:32:09,407 --> 00:32:11,398 They're looking for partners. 302 00:32:11,487 --> 00:32:16,038 Like slugs, they're hermaphrodite, each individual both male and female. 303 00:32:17,887 --> 00:32:20,879 They mate by lying alongside one another. 304 00:32:20,967 --> 00:32:24,277 Two narrow grooves form between their two bodies. 305 00:32:24,447 --> 00:32:28,804 These are the conduits that carry sperm from one partner to the other. 306 00:32:32,087 --> 00:32:36,763 Their bodies slowly pulse as sperm travels along the space between them. 307 00:32:37,207 --> 00:32:39,357 But the process is a long one 308 00:32:39,447 --> 00:32:42,564 and it may be three hours or so before they separate, 309 00:32:42,647 --> 00:32:45,320 each carrying the other's sperm. 310 00:32:52,007 --> 00:32:54,965 Like so many of the inhabitants of the undergrowth, 311 00:32:55,047 --> 00:32:57,686 earthworms can only live in a moist environment. 312 00:32:57,767 --> 00:33:02,318 But they are found in soils of every continent except Antarctica. 313 00:33:03,327 --> 00:33:06,478 This small valley in southern Australia 314 00:33:06,567 --> 00:33:10,276 is home to one of the rarest and the most extraordinary 315 00:33:10,367 --> 00:33:12,198 of all earthworms. 316 00:33:12,287 --> 00:33:13,845 And I know they're around... 317 00:33:13,927 --> 00:33:15,280 (GURGLING) 318 00:33:15,367 --> 00:33:16,800 ...because I can hear them. 319 00:33:20,767 --> 00:33:23,725 Those gurgling noises, believe it or not, 320 00:33:23,807 --> 00:33:29,245 are being made by giant earthworms as they squelch along their water-filled burrows. 321 00:33:36,727 --> 00:33:41,118 The vibrations of my footsteps are enough to stir them into activity. 322 00:33:45,127 --> 00:33:47,083 They never come to the surface, 323 00:33:47,167 --> 00:33:51,922 but in places where there's been a small landslip, you can sometimes find their burrows. 324 00:33:59,767 --> 00:34:02,156 They are over an inch in diameter 325 00:34:02,247 --> 00:34:06,957 and in them, if you're very lucky, you may occasionally find one of these. 326 00:34:08,487 --> 00:34:09,806 This... 327 00:34:10,927 --> 00:34:15,284 is one of their egg cocoons and it's enormous. 328 00:34:15,887 --> 00:34:21,086 If I hold it up against the light, I can see the young worm inside wriggling. 329 00:34:21,207 --> 00:34:25,519 It'll take a year for this to develop 330 00:34:25,607 --> 00:34:28,724 and when the young one finally does break free, 331 00:34:28,807 --> 00:34:32,117 it's already 20 centimetres long. 332 00:34:32,727 --> 00:34:34,126 Huge. 333 00:34:36,047 --> 00:34:39,642 It will take a further five years to reach full size 334 00:34:39,727 --> 00:34:42,400 and become this remarkable creature. 335 00:34:43,367 --> 00:34:48,122 So the question is, how long is a giant earthworm? 336 00:34:48,207 --> 00:34:50,641 Well, it's not an easy question to answer. 337 00:34:50,727 --> 00:34:55,642 The fact of the matter is they're rather delicate creatures and they break. 338 00:34:56,327 --> 00:35:00,559 If I were so unfeeling as to try and stretch it, 339 00:35:00,647 --> 00:35:04,401 well, I guess it might stretch to a couple of metres, 340 00:35:04,487 --> 00:35:06,682 almost six feet long. 341 00:35:10,127 --> 00:35:14,803 How long they live, well, some say up to 20 years but we really don't know. 342 00:35:15,687 --> 00:35:19,885 And we certainly don't know how they manage to mate deep underground 343 00:35:19,967 --> 00:35:23,437 as they squelch their way through their lonely tunnels. 344 00:35:28,527 --> 00:35:31,246 The land may have been a safe place for eggs 345 00:35:31,327 --> 00:35:34,637 when horseshoe crabs first laid theirs up on the beach. 346 00:35:34,727 --> 00:35:37,082 But as new kinds of animals appeared, 347 00:35:37,167 --> 00:35:41,206 so it became increasingly important for animals to protect their eggs. 348 00:35:41,807 --> 00:35:46,403 Most creatures just hid them, but a few now actively defend them. 349 00:35:48,207 --> 00:35:52,758 The builder of this circular mud wall in the Central American rainforest is one. 350 00:35:53,327 --> 00:35:56,160 During the day he conceals himself, 351 00:35:56,247 --> 00:36:01,037 but when night comes, he emerges to inspect his collection of eggs. 352 00:36:04,887 --> 00:36:07,924 His body is smaller than a grain of wheat. 353 00:36:08,007 --> 00:36:11,682 He's a relative of the spiders, a harvestman. 354 00:36:13,927 --> 00:36:18,523 His eggs, up to 100 of them, are half buried in the floor of his nest 355 00:36:18,607 --> 00:36:21,360 and he regularly inspects each one of them. 356 00:36:27,527 --> 00:36:33,602 If it has a fungus on it, he carefully cleans it before putting it back into its moist bed. 357 00:36:37,367 --> 00:36:41,406 He also continually repairs and improves his nest 358 00:36:41,487 --> 00:36:46,766 for females will only call on those who have well-built and well-kept ones. 359 00:36:49,287 --> 00:36:52,723 Some males, however, follow a different policy. 360 00:36:52,807 --> 00:36:58,643 They don't bother to build a nest for themselves, they try to take over an existing one. 361 00:37:00,087 --> 00:37:03,363 A nest holder has to leave sometimes to feed 362 00:37:03,447 --> 00:37:06,007 and that gives an intruder his chance. 363 00:37:14,167 --> 00:37:17,045 But the owner is back almost immediately... 364 00:37:18,447 --> 00:37:22,486 and they fight, trying to bite one another in the weakest point of their armour, 365 00:37:22,567 --> 00:37:24,478 the joints of the legs. 366 00:37:41,567 --> 00:37:46,004 The intruder retreats and the nest owner checks his eggs. 367 00:37:51,687 --> 00:37:53,200 No damage done. 368 00:37:57,967 --> 00:38:01,596 And then another, more welcome visitor arrives. 369 00:38:07,527 --> 00:38:09,199 This is a female. 370 00:38:09,287 --> 00:38:13,644 She's bigger than he is and she's touring all the nests in the neighbourhood 371 00:38:13,727 --> 00:38:17,163 to choose the one where her eggs will be best cared for. 372 00:38:22,647 --> 00:38:25,957 She seems to approve of the standard of his housekeeping. 373 00:38:27,207 --> 00:38:31,758 So now, face to face through the tangle of legs, she mates with him. 374 00:38:35,047 --> 00:38:38,483 He has a rod with which he injects his sperm. 375 00:38:45,367 --> 00:38:48,723 He withdraws and she's been fertilized. 376 00:38:56,007 --> 00:39:00,319 Half an hour later, she lowers her white, tubular ovipositor 377 00:39:00,407 --> 00:39:03,001 feeling for a suitable place for her egg. 378 00:39:05,967 --> 00:39:11,325 She thrusts the egg into the floor of the nest and then covers it with a thin blanket of mud. 379 00:39:30,327 --> 00:39:31,726 She leaves. 380 00:39:31,807 --> 00:39:35,880 He will now tend and guard the egg with the rest of his collection 381 00:39:35,967 --> 00:39:38,356 for the month that it will take to hatch. 382 00:39:41,647 --> 00:39:44,605 His nest is clearly one of the best in the neighbourhood 383 00:39:44,687 --> 00:39:48,236 for throughout the night a succession of females call on him. 384 00:39:50,207 --> 00:39:54,723 But not all have come to lay. Life is not that simple. 385 00:39:56,607 --> 00:40:00,646 This one starts, as usual, with a routine inspection 386 00:40:00,727 --> 00:40:03,764 and then, without more ado, she mates. 387 00:40:07,807 --> 00:40:12,756 He waits for her to produce a new egg, but nothing appears. 388 00:40:20,687 --> 00:40:24,362 He constantly checks the nest floor with his feelers 389 00:40:24,447 --> 00:40:27,007 but there are no signs of a new egg. 390 00:40:31,967 --> 00:40:36,563 And then she grabs one from his collection. She wants to eat one. 391 00:40:39,367 --> 00:40:41,039 She grabs again. 392 00:40:41,727 --> 00:40:44,958 He bites her leg joints and tries to pull her away. 393 00:40:51,047 --> 00:40:54,437 She's had enough and he has rescued his egg. 394 00:40:58,967 --> 00:41:04,200 He checks it over, cleans it with great care and then takes it away to rebury it. 395 00:41:09,407 --> 00:41:13,685 A month after the eggs were laid, his young begin to emerge. 396 00:41:28,487 --> 00:41:32,526 The skins from which they hatched provide them with their first meal. 397 00:41:34,287 --> 00:41:38,758 He will now guard his young for a couple of days until they leave the nest. 398 00:41:41,967 --> 00:41:45,721 Excellently adapted though harvestmen are to a life on land, 399 00:41:45,807 --> 00:41:50,039 they cannot survive for very long away from this damp undergrowth. 400 00:41:53,247 --> 00:41:58,526 In fact, most of the direct descendants of those early colonists that came from the sea 401 00:41:58,607 --> 00:42:01,599 are still trapped in a world of moisture. 402 00:42:10,327 --> 00:42:16,038 Those with no external skeletons are always in imminent danger of death by drought. 403 00:42:20,767 --> 00:42:24,157 Even those with exoskeletons are not safe 404 00:42:24,247 --> 00:42:27,557 for most have armour that is not totally watertight 405 00:42:27,647 --> 00:42:33,244 and will eventually dry out and die if they leave the dank shelter of the undergrowth. 406 00:42:42,607 --> 00:42:45,201 But beyond the reach of the forests, 407 00:42:45,287 --> 00:42:48,757 in the centre of continents where little or no rain falls, 408 00:42:48,847 --> 00:42:51,236 there is a very different territory. 409 00:42:51,327 --> 00:42:53,318 Empty and hostile. 410 00:42:59,767 --> 00:43:03,203 Here there is little shelter from the scorching sun. 411 00:43:03,887 --> 00:43:07,800 Temperatures rise above 70 degrees centigrade 412 00:43:07,887 --> 00:43:11,357 and there may be no rain whatsoever for years on end. 413 00:43:20,007 --> 00:43:24,603 Deserts, like this one in the southwest of the United States, 414 00:43:24,687 --> 00:43:28,646 represented the ultimate challenge for those ancient creatures 415 00:43:28,727 --> 00:43:31,924 whose ancestors first left the sea. 416 00:43:32,007 --> 00:43:35,443 Here there's virtually no water at all. 417 00:43:36,127 --> 00:43:41,884 And yet those early creatures, the very first to walk on land, 418 00:43:41,967 --> 00:43:43,798 reached even here. 419 00:43:44,367 --> 00:43:46,358 And they're still around. 420 00:43:47,567 --> 00:43:51,446 In order to survive the ferocious heat of the day, 421 00:43:51,527 --> 00:43:55,236 they take refuge in little burrows like this, 422 00:43:55,327 --> 00:43:58,603 which go quite a long way down into the ground. 423 00:43:59,327 --> 00:44:03,161 But I can use this special optical probe 424 00:44:03,247 --> 00:44:05,556 to see whether anyone's at home. 425 00:44:21,647 --> 00:44:23,842 And there it is. 426 00:44:23,967 --> 00:44:25,764 It's a scorpion. 427 00:44:25,847 --> 00:44:28,407 They won't come out for the rest of the day. 428 00:44:28,847 --> 00:44:34,604 But at night, when it gets cool, scorpions all over the desert will be emerging. 429 00:44:36,127 --> 00:44:39,836 And then, we have a very special way of finding them. 430 00:44:42,607 --> 00:44:46,680 In ultraviolet light scorpions are magically transformed. 431 00:44:46,767 --> 00:44:49,076 They glow with fluorescence. 432 00:44:50,367 --> 00:44:53,279 So with an ultraviolet torch 433 00:44:53,367 --> 00:44:57,042 you can get a better idea of just how abundant scorpions are, 434 00:44:57,127 --> 00:45:00,358 even in this arid wilderness. 435 00:45:03,087 --> 00:45:09,196 That's because they have managed to develop external skeletons that are virtually watertight. 436 00:45:11,807 --> 00:45:14,526 They also have powerful stings and pincers, 437 00:45:14,607 --> 00:45:17,326 so getting together to mate could be dangerous. 438 00:45:19,807 --> 00:45:23,083 A male looking for a female must be careful. 439 00:45:26,407 --> 00:45:29,763 She is powerful enough to kill and eat him. 440 00:45:34,047 --> 00:45:36,959 So he begins to dance. 441 00:45:44,927 --> 00:45:46,724 Is she impressed? 442 00:45:57,127 --> 00:46:01,518 Apparently so. And his solo becomes a pas de deux. 443 00:46:17,527 --> 00:46:21,600 But stings are still held high, ready to strike. 444 00:46:42,847 --> 00:46:44,803 She tries to sting him. 445 00:46:49,287 --> 00:46:53,803 His response is to give her a dose of her own medicine with a quick jab. 446 00:46:58,927 --> 00:47:02,476 But it's so slight, it merely makes her a little drowsy. 447 00:47:05,727 --> 00:47:08,321 At last she seems more amenable. 448 00:47:08,407 --> 00:47:12,195 He pulls her to a part of the dance ground that is smooth and level. 449 00:47:12,647 --> 00:47:17,198 He has extruded a small packet of sperm on a stalk glued to the ground. 450 00:47:18,807 --> 00:47:21,321 He manoeuvres her so that as she dances, 451 00:47:21,407 --> 00:47:25,559 she goes over the stalk and takes the sperm packet up into her body. 452 00:47:29,727 --> 00:47:31,922 The nuptial dance is over. 453 00:47:35,967 --> 00:47:39,755 Her fertilized eggs stay within a special chamber in her body 454 00:47:39,847 --> 00:47:42,566 for more than a year while they slowly develop. 455 00:47:44,687 --> 00:47:48,885 And then, in her burrow deep underground, she gives birth. 456 00:47:52,327 --> 00:47:55,046 She has produced up to 50 young ones. 457 00:47:55,127 --> 00:47:58,324 They cling tightly to her back for a few weeks after birth, 458 00:47:58,407 --> 00:48:01,763 each sustained by a small blob of yolk in its stomach. 459 00:48:05,687 --> 00:48:11,080 And then at last, they're all ready to venture into the open desert for themselves. 460 00:48:17,487 --> 00:48:21,366 By colonising this, the most hostile of environments, 461 00:48:21,447 --> 00:48:26,282 the first animals to walk on land finally broke their link with open water. 462 00:48:27,007 --> 00:48:31,364 And they did that about 300 million years ago 463 00:48:31,447 --> 00:48:36,521 at a time when the animals with backbones, including our own ancestors, 464 00:48:36,607 --> 00:48:38,916 were still swimming in the seas.