Читать книгу Life on Earth - David Attenborough, Sir David Attenborough - Страница 8
ОглавлениеThere are few more barren places on earth than the plains surrounding a volcano in the aftermath of its eruption. Black tides of lava lie spilt over its flanks like slag from a furnace. Their momentum has gone but they still creak, and boulders still tumble as the flow settles. Steam hisses between the blocks of lava, caking the mouths of the vents with yellow sulphur. Pools of liquid mud, grey, yellow or blue, boiled by the subsiding heat from far below, bubble creamily. Otherwise all is still. No bush grows to give shelter from the scouring wind; no speck of green relieves the black surface of the empty ash plains.
This desolate landscape has been that of much of the earth for the greater part of its history. The first volcanoes to appear on the surface of the cooling planet erupted on a far greater scale than any that we know today, building entire mountain ranges of lava and ash. Over the millennia, the wind and rain destroyed them. Their rocks weathered and turned to clay and mud. Streams transported the debris, particle by particle, and strewed it over the seafloor beyond the margins of the land. As the deposits accumulated, they compacted into shales and sandstone.
Lava cactus (Brachycereus nesioticus) growing in lava field, coast of Fernandina, Galapagos Islands.
The continents were not stationary. They drifted slowly over the earth’s surface, driven by the convection currents moving deep in the earth’s mantle. When they collided, the sedimentary deposits around them were squeezed and rucked up to form new mountain ranges. As the geological cycles repeated themselves for some three thousand million years, and the volcanoes exploded and spent themselves, the land remained barren. In the sea, however, life burgeoned.
Some marine algae no doubt managed to live on the edges of the seas, rimming the beaches and boulders with green, but they could not have spread far beyond the splash zone, for they would have dried out and died. Then between 450 and 500 million years ago, some forms developed a waxy covering, a cuticle, which warded off desiccation. Even this, however, did not totally emancipate them from water. They could not leave it because their reproductive processes depended on it.
Algae reproduce themselves in two ways – by straightforward asexual division and by the sexual method, which is of great importance in the the evolutionary process. Sex cells will only develop further if they meet each other and fuse in pairs. To make these journeys and achieve these meetings, they need water.
This problem still besets the most primitive land plants living today – both the flat, moist-skinned ones known as liverworts, and the filamentous ones covered with green scales, the mosses. They use these two methods of reproduction, sexual and asexual, in alternate generations. The familiar green moss is the generation which produces the sex cells. Each large egg remains attached to the top of the stem, while the smaller microscopic sperms are released into water and wriggle their way up to fertilise it. The egg then germinates while still attached to the parent plant and produces the next asexual generation – a thin stem with, at its tip, a hollow capsule. In this, great numbers of grain-like spores are produced. When the atmosphere becomes dry, the capsule wall expands until it suddenly snaps apart, throwing the spores into the air to be distributed by the wind. Those that land on a suitably moist site then develop into new plants.
Moss filaments have no rigidity. Some kinds achieve a modest height by packing closely together in cushions and so giving one another support, but their soft, permeable, water-filled cells do not provide enough strength to enable individual stems to stand upright. Plants like these are very likely to have been among the earliest forms to colonise the moist margins of the land, but so far no fossil relics of undoubted mosses have been discovered from this early period.
The first land plants we have indentified, dating from over 400 million years ago, are simple leafless branching strands which occur as filaments of carbon in the rocks of central Wales and in some cherts in Scotland. Like mosses, they had no roots, but when their stems are carefully prepared and examined under the microscope, they are seen to contain structures that no moss possesses – long, thick-walled cells that must have conducted water up the stem. These structures gave them strength and enabled them to stand several centimetres tall. That may not sound very imposing, but it represented a major advance in life’s colonisation of the land.
Apple moss (Bartrimia pomiformis) with spore capsules, Inverness-shire, Scotland, UK.
Endive Pellia liverwort (Pellia endiviifolia) in centre growing through common liverwort (Marchantia polymorpha), the latter bearing cups containing gemmae (used in asexual reproduction). Lathkill Dale, Peak District National Park, Derbyshire, UK.
Such plants, together with primitive mosses and liverworts, formed green tangled carpets, miniature forests that spread inland from the edges of estuaries and rivers, and into these crept the first animal colonists from the sea. They were segmented creatures, ancestors of today’s millipedes, well suited by their chitinous armour to movement on land. At first they doubtless kept close to the edge of the water, but wherever there was moss there was both moisture and vegetable debris and spores to eat. With the land to themselves, these pioneering creatures flourished. Their name millipede, ‘thousand legs’, is something of an overstatement. No species alive today has many more than two hundred legs, and some have as few as eight. Nevertheless, some of them grew to magnificent dimensions. One of them was two metres long and must have had a devastating effect on the plants as it browsed its way through the wet green bogs. It was, after all, as long as a cow.
The external skeleton inherited from their water-living forebears needed few modifications for life on land, but the millipedes did have to acquire a different method of breathing. The feathery gill attached to a stalk alongside the leg that had served their aquatic relatives, the crustaceans, would not work in air. In its place, the millipedes developed a system of breathing tubes, the tracheae. Each tube begins at an opening on the flank of the shell and then branches internally into a fine network that leads ultimately to all the organs and tissues of the body, the tips even entering individual specialised cells called tracheoles that deliver gaseous oxygen to the surrounding tissues and also absorb waste.
Reproduction out of water, however, posed problems for the millipedes. Their marine ancestors had relied, like the algae, on water to enable their sperm to reach their eggs. On land the solution was an obvious one – male and female, being well able to move about, must meet and transfer the sperm directly from one to the other. This is exactly what millipedes do. Both sexes house their reproductive cells in glands close to the base of the second pair of legs. When the male meets the female in the mating season, the two intertwine. The male reaches forward with his seventh leg, collects a bundle of sperm from his sex gland and then clambers alongside the female until the bundle is beside her sexual pouch and she is able to take it in. The process looks rather laborious but at least it is not dangerous. Millipedes are entirely vegetarian. Fiercer invertebrates, which came to the moss jungles to prey on this grazing population of millipedes, could not indulge in such trusting relationships.
Three groups of these predatory creatures still survive today – centipedes, scorpions and spiders. Like their prey, they are members of the segmented group of animals, though the degree to which they have retained divisions in their bodies varies considerably. The centipedes are as clearly and extensively segmented as their close relatives the millipedes. The scorpions show divisions only in their long tails; and most spiders have completely lost all signs of segmentation, except for a few Southeast Asian species which retain clearly recognisable relics of their segmented past.
The scorpions that live today have not only fearsome-looking claws but a large venom gland drooping from the end of a long thin tail with a sharp curving sting. Their copulations cannot be the somewhat hit-and-miss gropings practised by the millipedes. Approaching such an aggressive and powerful creature is a dangerous enterprise even if the move is made by another individual of the same species and its intentions are purely sexual. There is a real risk of it being regarded not as a mate but a meal. So scorpion mating demands, for the first time among the animals that have appeared so far in this history, the ritualised safeguards and placations of courtship.
The male scorpion approaches the female with great wariness. Suddenly he grabs her pincers with his. Thus linked, with her weapons neutralised, the pair begin to dance. Backwards and forwards they move with their tails held upright, sometimes even intertwined. After some time, their shuffling steps have cleared the dancing ground of much of its debris. The male then extrudes a packet of sperm from the genital opening beneath his thorax and deposits it on the ground. Still grasping the female by the claws, he jerks and heaves her forward until her sexual opening, also on her underside, is brought directly above the sperm packet. She takes it up, the partners disengage and then go their separate ways. The eggs eventually hatch inside the mother’s pouch, the young crawl out and clamber up on to her back. There they stay for about a fortnight until they have completed their first moult and can fend for themselves.
Mediterranean/European scorpion (Buthus occitanus) stinging a spider (Amourobius sp.).
Spiders, too, must be extremely cautious in their courtship. Matters are made even more hazardous for the male because he is nearly always smaller than the female. And he prepares for his encounter with his mate long before he meets her. He spins a tiny triangle of silk a few millimetres in length and deposits a drop of sperm on to it from the gland that lies underneath his body. He then sucks it into the hollow first joint of his pedipalp, a special limb at the front of his body. Now he is ready.
The courtships of spiders are beguilingly various and ingenious. Jumping spiders and wolf spiders hunt primarily by sight and have excellent eyes. The courting male, consequently, relies on visual signals to make the female aware of his presence and his purpose. His pedipalps are brightly coloured and patterned, and as soon as he sights a female, he begins to signal with them in a kind of manic semaphore. Nocturnal spiders, on the other hand, depend largely on an extremely delicate sense of touch to find their prey. When they meet one another, they gingerly caress each other’s long legs, and only after a great deal of hesitation do they come to closer quarters. Web-making spiders are sensitive to the vibrations on their silken threads that tell them when a victim has blundered into the web. So when the male of such a species approaches a female hanging, large and menacing, on her web, or lurking hidden beside it, he signals to her by twanging the threads at one side in a special and meaningful way which he trusts the female will recognise. Other species put their faith in bribery. The male catches an insect and carefully parcels it up in silk. Holding this in front of him, he cautiously approaches the female and presents it to her. While she is occupied in examining the gift, he quickly scuttles over her and ties her to the ground with bonds of silk. Only then does he risk an embrace.
All these techniques lead to the same conclusion. The male, having survived every danger, places his pedipalp close to the female’s genital opening, squirts out the sperm and then hastily retreats. It has to be recorded that in spite of all his precautions he sometimes fails to make his getaway in time and the female eats him after all. But in terms of the transmission of his genes, the male’s disaster is of limited consequence: he lost his life after, not before, he had completed his purpose.
While the early segmented animals were perfecting their adaptations for living on land and away from moisture, the plants were also changing. Neither the mosses nor the other early forms had true roots. Their short upright stems sprang from a horizontal one of a similar character lying along the ground or just below it. This construction served well enough in moist surroundings, but in many parts of the world the only permanent water supply lies below the ground. To tap that requires roots that probe deep between the particles of the soil and can absorb the film of water that clings to them in all except the most arid environments. Three groups of plants appeared that possessed such structures, and all three have descendants that have survived without much change: club mosses, which resemble mosses but have stiffer stems; horsetails, which grow in waste patches and ditches and have stems encircled at intervals with rings of needle-like leaves; and ferns.
Wolf spiders (Pardosa sp.), male (right) waving palps in courtship display, Derbyshire, UK.
The ferns, early in their history, had developed a special protein to protect themselves from damage by ultraviolet light, something that had not been a problem for their ancestors since they lived in water where such wavelengths could not reach them. This substance now slowly changed into a material called lignin. This is the basis of wood, and it gave them the rigidity needed to grow tall. So a new kind of competition developed between plants.
All green plants depend on light to power the chemical processes by which they use simple elements to synthesise their body substances. So if a plant does not grow tall, it risks being overshadowed by its neighbours and condemned to shade where, starved of light, it might die. So these early groups used the newly acquired strength of their stems to grow very tall indeed. They became trees. The club mosses and horsetails were still, for the most part, swamp-dwellers, and there they now stood in dense ranks, thirty metres tall, some with woody trunks two metres in diameter. The compacted remains of their stems and leaves today form coal. The great thicknesses of the seams are impressive evidence of the abundance and persistence of the early forests. Other species of both these groups also spread farther inland and there mingled with ferns. These had developed true leaves, large spreading structures with which to collect as much light as possible. They grew tall with curving trunks, like the tree ferns that still thrive in tropical rainforests.
Wood horsetails (Equistetum sylvaticum) Columbia River, Gorge National Scenic Area, Oregon, USA.
The height of these first forests must have caused considerable problems for their animal inhabitants. Once, there had been a superabundance of leaves and spores close to the ground. Now the soaring trunks had raised this source of food high in the sky, creating a dense canopy that cut out much of the light. The floor of these forests was, at best, only sparsely vegetated and great areas may have been entirely without any living leaves. Some of the multi-legged vegetarians found their fodder by clambering up the trunks.
There may have been another factor that induced these creatures to leave the ground. About this time, animals of a completely new kind joined the invertebrates on the land. They had backbones and four legs and wet skins. They were the first amphibians and they too were carnivorous. A description of their origins and fate will have to wait until we have followed the development of the invertebrates to its climax, but their presence at this stage must be mentioned if the scene in these first jungles is not to be misrepresented.
Virtually all of the new-style invertebrate families still survive. Among the most numerous are the bristletails and springtails. Although they are little known and infrequently seen, they are enormously abundant. There is hardly a spadeful of soil or leaf litter anywhere in the world that does not contain some of them. Indeed, the springtails, or collembola, are probably the most abundant arthropods on the planet. Most are only a few millimetres long. Of those new families, only one is commonly noticed – the silverfish that glides smoothly across cellar floors or is occasionally discovered making a meal of the dried glue in the bindings of books. Its body is clearly segmented but it has very many fewer divisions than the millipede. It has a well-defined head with compound eyes and antennae; a thorax bearing three pairs of legs, the result of fusing together three segments; and a segmented abdomen which, while it no longer carries limbs on each segment, retains little stumps as signs that it once possessed them. Three thin filaments trail from its rear end. It breathes like the millipedes by means of tracheae, and it reproduces in a manner reminiscent of those early land invertebrates, the scorpions. The male silverfish deposits a bundle of sperm on the ground and then, one way or another, he entices the female to walk over it. When that happens, she is stimulated to take it up into her own sexual pouch.
There are several thousand different species of bristletails and collembola. They vary considerably in their anatomy and, as is often the case when considering the simpler members of a big group, it is sometimes difficult to decide whether a particular characteristic represents a truly primitive survival or one that has become secondarily reduced to suit a particular way of life. The silverfish, for example, has compound eyes but other members of the group are blind. All lack wings. Some even lack tracheae and breathe through their chitinous skeleton which is particularly thin and permeable. Is this because they never had them or because they have lost them?
Marine springtail/bristletail (Petrobius maritimus) adult resting on stones, Lough Muree, County Clare, Ireland.
Many such debatable questions raised by the anatomy of these creatures still wait universally agreed answers. However, they all have six legs and tripartite bodies and these characteristics clearly link them to that great and varied group of land invertebrates, the insects. They appeared many millions of years after the earlier groups were well established. Geneticists have now shown that collembolla, as well as the insects, including the silverfish, are all closely related to one particular group of water-living crustaceans, the remipedia (the name means ‘oar-foot’), which today are found only in the pools and streams of caves.
The primitive insects must have found some of their food by climbing the trunks of the early tree ferns and horsetails. The ascent was doubtless relatively easy. The climb down, involving long detours over the upward-pointing leaf-bases, may have been very much more laborious and time-consuming. Whether or not the prevalence of such obstacles had anything to do with the next developments, we cannot be sure. It is certain, however, that some of these primitive insects did develop a much swifter and less laborious method of getting down. They flew.
We have no direct evidence of how they achieved flight, but the living silverfish provides a clue. On the back of its thorax it has two flap-like sideways extensions of the chitinous shell that look as though they might be the rudiments of wings. The early wings may not have served initially for flight. Insects, like all animals, are greatly affected by body temperature. The warmer they are, the quicker the energy-producing chemical reactions of their body can proceed and the more active they can be. If their blood were to be circulated through thin flaps extending laterally from the back, they could certainly warm themselves very effectively and quickly in the sunshine. If, furthermore, these flaps had muscles at their base, they could be tilted to face squarely to the sun’s rays. Insect wings do originate as flaps on the back and they do, initially, have blood flowing in their veins, so such a theory seems very plausible.
However this may be, insects with wings appeared some 350 million years ago. The earliest so far discovered are dragonflies. There were several species, most about the size of those living today. But for the dragonflies as for millipedes and other groups that have pioneered a new environment, the absence of competition allowed some early forms to develop to an enormous size, and dragonflies eventually appeared with a wingspan of 70 centimetres, the largest insects ever to exist. When the air became more thickly populated, such extravagant forms disappeared.
Living dragonflies have two pairs of wings which have simple joints to them: they can only move up and down and cannot be folded back. Even so, they are highly accomplished flyers, shooting over the surface of a pond in a blur of gauzy wings at up to 30 kph. At such speeds, they need accurate sense organs if they are to avoid damaging collisions. A tuft of hair on the front of the body helps them to check that their motion through the air is straight, but their primary navigational guidance comes from huge mosaic eyes on either side of the head, which provide superbly accurate and detailed vision.
Because of this dependence on sight, most dragonflies do not fly at night, although there are some that migrate vast distances over the oceans, flying from India to Africa and stopping off at the islands of the Maldives along the way. All are daytime hunters, flying with their six legs crooked in front of them to form a tiny basket in which they catch smaller insects. That fact alone makes it clear that they must have been preceded into the air by other herbivorous forms which, judging from the primitive nature of their anatomy, were probably related to cockroaches, grasshoppers, locusts and crickets.
The presence of these large populations of insects, whirring and buzzing through the air of the ancient forests, was eventually to play an extremely important part in a revolution that was taking place among the plants.
The early trees, like their predecessors, the mosses and liverworts, existed in two alternating forms, a sexual generation and an asexual one. Their greater height posed no problem for spore dispersal: if anything it was a help, since up in the treetops spores were more easily caught by the wind and carried away. The distribution of sex cells, however, was a different matter. Hitherto, it had been achieved by the male cells swimming through water, a process which demanded that the sexual generation be small and live close to the ground. That of ferns, club mosses and horsetails still is. The spores of these plants develop into a thin filmy plant called the thallus which looks not unlike a liverwort and releases its sex cells from its underside where there is permanent moisture. After its eggs have been fertilised, they grow into tall plants like the previous spore-producing generation.
On the ground, the thallus is clearly vulnerable. It is easily grazed by animals; if it dries out it dies; and the very success of the asexual generation with their arching fronds cuts it off from life-giving light. Many advantages would follow if it too could grow tall, but this would require a new technique for getting the male cell to the female.
There were two mechanisms available – the ancient, rather hazardous and capricious method that distributed spores, the wind; and the newly arrived messenger service, the flying insects, which were now regularly moving from tree to tree, feeding on the leaves and the spores. Plants took advantage of both mechanisms. About 350 million years ago, some appeared in which the sexual generation no longer grew flat on the ground, but up in the crowns of the trees. One group among these plants, the cycads, survives today and shows the development at a particularly dramatic stage.
Cycads look superficially like ferns, with long coarse feathery fronds. Some individuals produce tiny spores of the ancient type that can be distributed by the wind. Others develop much larger ones. These are not blown away but remain attached to the parent. There they develop the equivalent of the thallus, a special kind of conical structure within which eggs eventually appear. When a wind-blown spore – which now can be called pollen – lands on an egg-bearing cone, it germinates, not into a filmy thallus for which there is now no need, but into a long tube which burrows its way down into the female cone. The process takes several months. Eventually, when the tube is complete, a sperm cell is produced from the end of the tube. It is a majestic ciliated sphere, the largest known sperm of any organism, plant or animal, so big that a single one is visible to the naked eye. Slowly it makes its way down the tube. When it reaches the bottom, it enters a small drop of water that has been secreted by the surrounding tissues of the cone. There it swims, slowly spinning, driven by its cilia, as it re-enacts in miniature the journeys made through the primordial seas by the sperm cells of its algal ancestors. Only after several days does it fuse with the egg and so complete the long process of fertilisation.
Another group of plants adopting a similar strategy to the cycads arose at about the same time. These were the conifers – pines, larches, cedars, firs and their relations. They too rely on the wind to distribute their pollen. Unlike the cycads, they produce both pollen and egg-bearing cones on the same tree. The process of fertilisation in a pine takes even longer. The pollen tube requires a whole year to grow down and reach the egg, but once there, it contacts the egg cell directly, and the male cell, after descending the tube, does not tarry in a drop of water but fuses directly with the egg. The conifers have at last eliminated water as a transport medium for their sexual processes.
Common hawker dragonfly (Aeshna juncea) on Brackish Moss National Nature Reserve, County Armagh, Northern Ireland.
Cones of the Eastern Cape giant cycad, or bread tree (Encephalartos altensteinii). Present day cycads are survivors of a group dating back 300 million years. Most families died out during the Cretaceous period, 80 million years ago. Cycads are of great evolutionary interest due to their reproductive system, considered to be the forerunner of flowering plants. The cones are the reproductive structures and can be male or female, producing seeds to form new plants.
They have also developed one further refinement. The fertilised egg remains in the cone for one more year. Rich food supplies are laid down within its cells and waterproof coats are wrapped round it. Eventually, more than two years after fertilisation began, the cone dries and becomes woody. Its segments open, and out drop the fertilised fully provisioned eggs – seeds – which if necessary can wait for years before moisture penetrates them and stimulates them to spring to life.
By any standards, the conifers are a great success. Today, they constitute about a third of the forests of the world. The biggest living organism of any kind is a conifer, the giant redwood of California, which grows to 100 metres in height. Another conifer, the bristle-cone pine, which grows in the dry mountains of the southwestern United States, has one of the longest life-spans of any individual organism. The age of trees can easily be calculated if they grow in an environment where there are distinct seasons. In summer, when there is plenty of sunshine and moisture, they grow quickly and produce large wood cells; in winter, when growth is slow, the cells are smaller and the wood consequently more dense. This produces annual rings in the trunk. Counting those in the bristle-cone pine establishes that some of these gnarled and twisted trees germinated over five thousand years ago at a time when people in the Middle East were just beginning to invent writing, and the trees have remained alive throughout the entire duration of civilisation.
Conifers protect their trunks from mechanical damage and insect attack with a special gummy substance, resin. When it first flows from a wound it is runny, but the more liquid part of it, turpentine, quickly evaporates, leaving a sticky lump which seals the wound very effectively. It also, incidentally, acts as a trap. Any insect touching it becomes inextricably stuck and very often buried as more resin flows around it. Such lumps have proved to be the most perfect fossilising medium of all. They survive as pieces of amber and contain ancient insects in their translucent golden depths. When the amber is carefully sectioned, it is possible, through the microscope, to see mouthparts, scales and hairs with as much clarity as if the insect had become entangled in the resin only the day before. Scientists have even been able to distinguish tiny parasitic insects, mites, clinging to the legs of the bigger ones. Extracting the DNA from a blood-sucking arthropod seems likely to remain science fiction, however. Even attempts to do so from insects trapped in copal, the modern equivalent of amber only a few decades old, have all met with failure.
The oldest pieces of amber so far discovered date from around 230 million years ago, a long time after the conifers and the flying insects first appeared, but they contain a huge range of creatures, including representatives of all the major insect groups that we know today. Even in the earliest specimens, each type has already developed its own characteristic way of exploiting that major insect invention, flight.
Ancient bristlecone pine in winter, near Wheeler Peak in Great Basin National Park, Nevada, USA.
The dragonflies beat their two wings synchronously, with the front pair raised while the rear pair are lowered. This, however, creates very considerable physiological complexities. Their wings do not normally come into contact, but even so there are problems when the dragonfly executes sharp turns. Then the fore- and hindwings, bending under the additional stress of the turn, beat against one another, making an audible rattle that you can easily hear as you sit watching them make their circuits over a pond.
The later insect groups seem to have found that flight was more efficiently achieved with just one pair of beating membranes. Bees and wasps, flying ants and sawflies all hitch their fore- and hindwings together with hooks to make, in effect, a single surface. Butterfly wings overlap. Hawkmoths, which are among the swiftest insect flyers, capable of speeds of 50 kph, have reduced their hindwings very considerably in size and latched them on to the long narrow forewings with a curved bristle. Beetles use their forewings for a different purpose altogether. These creatures are the heavy armoured tanks of the insect world and they spend a great deal of their time on the ground, barging their way through the vegetable litter, scrabbling in the soil or gnawing into wood. Such activities could easily damage delicate wings. The beetles protect theirs by turning the front pair into stiff thick covers which fit neatly over the top of the abdomen. The wings are stowed beneath, carefully and ingeniously folded. The wing veins have sprung joints in them. When the wing covers are lifted, the joints unlock and the wings spring open. As the beetle lumbers into the air, the stiff wing covers are usually held out to the side, a posture that inevitably hampers efficient flight. Flower beetles, however, have managed to deal with this problem. They have notches at the sides of the wing covers near the hinges so that the covers can be replaced over the abdomen, leaving the wings extended and beating.
The most accomplished aeronauts of all are the flies. They use only their forewings for flight. The hindwings are reduced to tiny knobs. All flies possess these little structures but they are particularly noticeable in the crane flies, the daddy-long-legs, in which the knobs are placed on the ends of stalks so that they look like the heads of drumsticks. When the fly is in the air, these organs which are jointed to the thorax in the same way as wings, oscillate up and down a hundred or more times a second. They act partly as stabilisers, like gyroscopes, and partly as sense organs presumably telling the fly of the attitude of its body in the air and the direction in which it is moving. Information about its speed comes from its antennae, which vibrate as the air flows over them.
Flies are capable of beating their wings at speeds up to an astonishing 1,000 beats a second. Some flies no longer use muscles directly attached to the bases of the wings. Instead they vibrate the whole thorax, a cylinder constructed of strong pliable chitin, making it click in and out like a bulging metal tin. The thorax is coupled to the wings by an ingenious structure at the wing base, and its contractions cause them to beat up and down.
Longhorn beetle (Cerambycidae) in flight Rookery Wood, Sussex, England, UK, July.
The insects were the first creatures to colonise the air, and for over a 100 million years it was theirs alone. But their lives were not without hazards. Their ancient adversaries, the spiders, never developed wings, but they did not allow their insect prey to escape totally. They set traps of silk across the flyways between the branches and so continued to take toll of the insect population.
Plants now began to turn the flying skills of the insects to their own advantage. Their reliance on the wind for the distribution of their reproductive cells was always haphazard and expensive in biological terms. Spores do not require fertilisation and they will develop wherever they fall, provided the ground is sufficiently moist and fertile. Even so, the vast majority of them, from such a plant as a fern, fail to find the right conditions and die. The chances of survival for a wind-blown pollen grain are very much smaller still, for their requirements are even more precise and restricted. They can only develop and become effective if they happen to land on a female cone. So the pine tree has to produce pollen in gigantic quantities. A single small male cone produces several million grains, and if you tap one in spring, they fall out in such numbers that they create a golden cloud. A whole pine forest produces so much pollen that ponds become covered with curds of it – and all of it wasted.
Insects could provide a much more efficient transport system. If properly encouraged, they could carry the small amount of pollen necessary for fertilisation and place it on the exact spot in the female part of the plant where it was required. This courier service would be most economically operated if both pollen and egg were placed close together on the plant. The insects would then be able to make both deliveries and collections during the same call. And so developed the flower.
Some of the earliest and simplest of these marvellous devices so far identified are those produced by the magnolias. They appeared about a 100 million years ago. The eggs are clustered in the centre, each protected by a green coat with a receptive spike on the top called a stigma, on which the pollen must be placed if the eggs are to be fertilised. Grouped around the eggs are many stamens producing pollen. In order to bring these organs to the notice of the insects, the whole structure is surrounded by brightly coloured modified leaves, the petals.
Beetles had fed on the pollen of cycads and they were among the first to transfer their attentions to the early flowers such as those of magnolias and waterlilies. As they moved from one to another, they collected meals of pollen and paid for them by becoming covered in excess pollen which they involuntarily delivered to the next flower they visited.
Saucer Magnolia (Magnolia x soulangeana) tree in full flower against blue sky. Stourhead gardens, Wiltshire, UK, April.
Meadow in flower, with cork oaks (Quercus suber) in the background, Beja, Portugal.
One danger of having both eggs and pollen in the same structure is that the plant may pollinate itself, thereby preventing cross-fertilisation, the very purpose of all these complexities. This possibility is avoided in the magnolia, as in many plants, by having eggs and pollen that develop at different times. Magnolia stigmas will accept pollen as soon as the flower opens. Its own stamens, however, do not produce their pollen until later, by which time its eggs are likely to have been cross-fertilised by exploring insects.
The appearance of flowers transformed the face of the world. The green forest now flared with colour as the plants advertised the delights and rewards they had on offer. The first flowers were open to all that cared to alight on them. No specialised organs were required in order to reach the centre of the magnolia flower or the waterlily, no particular skill was needed to gather the pollen from the loaded stamens. Such blooms attracted several kinds of insects – bees as well as beetles. But a variety of visitors is not an unmitigated advantage, for they themselves are also likely to visit several kinds of unspecialised flowers. Pollen of one species deposited in flowers of another is pollen wasted. So throughout the evolution of the flowering plants, there has been a tendency for particular flowers and particular insects to develop together, each catering specifically for the other’s requirements and tastes.
Right from the times of the giant horsetails and ferns, insects had been accustomed to visiting the tops of trees to gather spores as food. Pollen was an almost identical diet and it still remains a most important prize. Bees collect it in capacious baskets on their thighs and take it back to their hives for immediate consumption or for turning into pollen bread which is an essential food for their developing young. Some plants, among them species of myrtle, produce two kinds of pollen, one that fertilises their flowers, and another of a particularly tasty kind that has no value except as food.
Other flowers developed a completely new bribe, nectar. The only purpose of this sweet liquid is to please insects so greatly that they devote all possible time during the flowering season to collecting it. With this the plants recruited a whole new regiment of messengers, particularly bees, flies and butterflies.
These prizes of pollen and nectar have to be advertised. The bright colours of flowers make them conspicuous from considerable distances. As the insect approaches, it is provided with markings on the petals which indicate the exact placing of the rewards they seek. Some flowers intensify their colours towards the centre or introduce another shade altogether – as do forget-me-nots, hollyhocks, bindweed. Others are marked with lines and spots like an airfield to show the insect where to land and in which direction to taxi – foxgloves, violets and rhododendrons. There are more of these signals on flowers than we may realise. Many insects can perceive colours of the spectrum that are invisible to us. If we photograph what seem to be plain flowers in ultraviolet light, we can see many more such markings on the petals.
Bee orchid (Ophrys apifera) in flower. Dorset, UK, June.
Scent is also a major lure. In most cases, the perfumes that insects find attractive, such as those produced by lavender, roses, and honeysuckle, please us as well. But this is not always the case. Some flies are attracted to rotting flesh as a food for themselves and their maggots. Flowers that enlist them as pollinators must cater for their tastes and produce a similar smell, and they often do so with an accuracy and pungency far beyond the endurance of the human nose. The maggot-bearing Stapelia from southern Africa has flowers that reek dreadfully of carrion but it also reinforces its appeal to flies with wrinkled brown petals covered with hairs that look like the decaying skin of a dead animal. To complete the illusion, the plant generates heat that mimics the warmth produced by corruption. The whole effect is so convincing that flies transporting Stapelia’s pollen not only visit flower after flower, but even complete the activity for which they visit real carrion – laying their eggs on the flower just as they do in a carcass. When these hatch, the maggots find that they are not provided with a meal of rotting meat but only an inedible petal. They die from starvation, but the Stapelia has been fertilised.
Perhaps the most bizarre imitations of all are those of some orchids that attract insects by sexual impersonation. One produces a flower that closely resembles the form of a female wasp complete with eyes, antennae and wings and even gives off the odour of a female wasp in mating condition. Male wasps, deceived, attempt to copulate with it. As they do so, they deposit a load of pollen within the orchid flower and immediately afterwards receive a fresh batch to carry to the next false female. The extent of this mimicry can be far greater than mere physical resemblance. The orchid’s flowers are covered with waxes that correspond to an extraordinary degree to the sex-specific pheromones that cover the female wasp and which are just as attractive. These orchids produce no nectar. The reward they provide for their insect pollinators is not sex, but its illusion.
Sometimes insects are disinclined to collect pollen, preferring nectar, and will bypass the plant’s strategies and become nectar thieves, biting their way through the flowers from the outside and inserting their proboscis into the nectar source without getting covered in pollen. Then the flowers have to have devices to force their pollen on the insect. Some blooms have become obstacle courses during which their visitors are pummelled by stamens and bombarded with pollen before they are able to leave. Broom flowers are so constructed that if, for example a bee, lands, the stamens, packed under tension inside a sealed capsule of petals, shoot out and strike the underside of the bee, covering its furry abdomen with pollen. The bucket orchid from Central America drugs its visitors. Bees clamber into its throat and sip a nectar so intoxicating that after they have taken only a little they begin to stagger about. The surface of the flower is particularly slippery. The bees lose their foothold and fall into a small bucket of liquid. The only way out of this is up a spout. As the inebriated insect totters up, it has to wriggle beneath an overhanging rod which showers it with pollen.
Rafflesia flower (Rafflesia keithii) in Gunung Gading National Park, Borneo, Sarawak, Malaysia.
Sometimes plant and insect become totally dependent one upon the other. The yucca grows in Central America. It has a rosette of spear-shaped leaves from the centre of which rises a mast bearing cream-coloured flowers. These attract a small moth with a specially curved proboscis that enables it to gather pollen from the yucca stamens. It moulds the pollen into a ball and then carries it off to another yucca flower. First it goes to the bottom of the flower, pierces the base of the ovary with its ovipositor and lays several eggs on some of the ovules that lie within. Then it climbs back up to the top of the stigma rising from the ovary and rams the pollen ball into the top. The plant has now been fertilised and in due course all the ovules in the chamber at the base will swell into seeds. Those that carry the moth’s eggs will grow particularly large and be eaten by the young caterpillars. The rest will propagate the yucca. If the moth were to become extinct, the yuccas would never set seed. If the yuccas disappeared, the moth’s caterpillars could not develop. Each species is inextricably in the debt of the other.
One further debt is clear. Flowers, exquisitely perfumed and graced with a multitude of colours and shapes, bloomed long before humans appeared on the earth. They evolved in order to appeal not to us but to insects. Had butterflies been colour-blind and bees without a delicate sense of smell, we would have been denied some of the greatest delights that the natural world has to offer.
