The plants feel the same. Their Latin name, Orchidaceae, means ‘testicle’ after the unexpected shape of their roots. Orchids advertise their prowess with expensive and often bizarre blooms. So impressive are their carnal powers that the English herbalist Nicholas Culpeper called for caution when they were used as aphrodisiacs. In The Descent of Man and Selection in Relation to Sex Charles Darwin had shown how, in the animal kingdom, the battle to find a mate was as formidable an agent of selection as was the struggle to stay alive. Males, in general, have the potential to have far more offspring than do females - if, that is, they can fight off their rivals and persuade enough members of the opposite sex to play along with their carnal desires. Losers in the conflict reach the end of their evolutionary road for their genes go nowhere. Evolution as played out in the universe of sex is as pitiless as is that in the battle for survival. Sexual selection, as Darwin called it, can lead to rapid change: to the evolution of gigantic antlers, a vivid posterior or - for species interested in such things - gold watches and flashy clothes.

Later in his career, Charles Darwin examined the sexual struggles within the second great realm of life, the plants. He showed how the search for a partner can be as much of a challenge for them as it is for stags or peacocks. Plant reproductive habits were obscure and their mere existence often denied until the seventeenth century, but within a hundred years or so the basic machinery had been worked out. Flowers were both the home of the reproductive organs and an eloquent statement of erotic need. Darwin found that they evolved in rather the same way as an animal’s sexual displays and were subject to the same forces of selection, which often achieved ends equally - or more - bizarre than those found in animals. In addition he discovered (although he found it hard to believe) that for orchids sex was full of dishonesty and discord, with all those involved ready to cheat whenever necessary.

Any botanical marriage is - by definition - more crowded than its animal equivalent, for a third party is needed to consummate it by moving male sex cells to the female. For some species, wind or water step in to help, but most flowers need a flying penis - a pollinator - to carry their DNA to the next individual (Ruskin, with his passion for the beauties of nature, strongly advised his young female readers not to enquire ‘how far flowers invite, or require, flies to interfere in their family affairs’). Darwin himself saw how antagonism between the plant and animal partners is as powerful an agent of selection as is the process of female choice and male competition that gives rise to the peacock’s tail. Flower and pollinator each become trapped into the embrace of the other and enter an evolutionary race that may end with the emergence of structures as unexpected, and tactics as devious, as anything in the animal world.

The interests of those who manufacture the crucial DNA and those who deliver it are quite different. From a female flower’s point of view, or that of the female part of a hermaphrodite plant, one or a few visits by a winged phallus is enough to do the job (although the more callers she gets, the more choice she has of which sex cell to use). To beat its rivals, however, a male is forced to attract the distribution service again and again - and that can be expensive.

In his 1862 volume On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing, Darwin studied the divergence of interests between the two parties. He used the showiest and most diverse of all flowers as an exemplar. He found that ‘the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom’. As well as an exhaustive account of the structure of the orchids themselves (‘I fear, however, that the necessary details will be too minute and complex for any one who has not a strong taste for Natural History’), his work introduced the idea - much developed nine years later, in The Descent of Man, and Selection in Relation to Sex - that large parts of evolution depend on an ancient and endless sexual conflict that crafts the future of all those who are drawn in.

The war between flowers and insects became an overture to a wider world of biological discord. It has led to spectacular bonds between improbable partners, As it does, it reveals many of the details of the mechanism of natural selection, including its uncanny ability to subvert the tactics of any opponent. The orchids and their pollinators were, for Charles Darwin, an introduction to the dishonesty that pervades the world of life.

Pollination has attracted attention since ancient times. Both Aristotle and Virgil were interested in bees, but only because they made honey (or collected the stuff, for the Greeks imagined that it fell from the air: ‘air-born honey, gift of heaven’) rather than because they were essential for reproduction. The Egyptians, in contrast, understood that dates would not grow on cultivated palms unless male flowers were shaken on to the females. They used their slaves as pollinators. A diversity of other creatures has been called in to act as marital aids and quite often that duty drives their own evolution. Two hundred thousand insects (the male malaria mosquito included) are known to transfer pollen and their own vast radiation into a variety of forms began soon after the origin of flowers. From the tropics to the sub-Arctic, hundreds of species of bird are busy shifting genes. Some, such as humming birds, can almost never afford to stop as they need a constant supply of nectar to keep their tiny bodies at a high level of activity. Mammals are also involved, and a certain Ecuadorian bat has a tongue half as long again as its own body - in relative terms the longest of all mammalian tongues. It is coiled up in a special cavity in its chest, except when the animal is feeding. An African tree is even adapted for pollination by the giraffes that browse upon its leaves. In Australia, too, marsupials have taken up the job for the honey possum has lost many of its teeth and gained a long tongue. The sugar glider - a marsupial that floats through the air from flower to flower - is much the same.

Plants and animals make signals of many kinds. They advertise their qualities as a mate, their willingness to fight for territory or food, or their ability to escape from a predator who might as a result be dissuaded from bothering to attack. One surprise is that the signals are so often honest when the reward for dishonesty is so high, be it in the form of sex, food or safety. Some signs are direct and impossible to fake: large tigers make scratch marks higher up a tree trunk than can their smaller rivals and can as a result hold bigger territories. Often, though, the information is indirect. Thus, a black and yellow wasp warns predators about its dangers without the need to sting all of them.

Such secondary signals, too, are sometimes pricey and hard to simulate. Giant antlers, vivid tails or spectacular blooms can be made only by those who can afford them: the healthiest, the sexiest or the super-aggressive. Most of what we interpret as the joys of nature costs a lot, for a stag may die in battle, and a male nightingale loses a tenth of its body weight after a night spent singing in the hope of sex. Testosterone itself, that signifier of masculine identity, is costly in many ways. It suppresses the immune system, so that a red deer male in sexual frenzy is open to attack by parasites - and if he can keep roaring in spite of his tapeworms he might have particularly fine genes. Elephants go even further. Now and again, one falls into a state of ‘musth’, in which its testosterone level goes up by fifty times. The agitated beast becomes very aggressive, and a small animal will fight even to the death against a larger rival.

Flowers, too, are not cheap. Orchid fanciers pay tens of thousands of dollars for prize specimens and the trade as a whole has a worldwide turnover of several billion. The orchids themselves invest far more of their limited capital into sexual display than does the most avid gardener, for if they do not their genetical future is over. The cost of sex to each orchid and to those that market them is manifest in the fact that, in the world of the garden centre, many of the specimens are grown from cells in culture rather than by persuading the plants to go through the expensive ritual of sex.

Orchids and other flowers are, like the peacock’s tail, animated billboards that advertise sexual prowess. For all signals, two parties are involved: those who transmit a message and those who receive it. A system of checks and balances tests whether the information is accurate; that those with the biggest antlers or brightest blooms really are the fiercest or most generous. The system is always under test by potential fraudsters at both ends and sometimes cheats get in. Often, they do well. For insects, black and yellow is no more difficult to manufacture than is brown or blue - and a whole group of harmless flies does just that, with bright stripes that make a false claim of a waspish nature. That costs the wasps a lot when a hungry bird attacks under the assumption that the pattern advertises good taste rather than potential danger.

Such swindlers also flourish in the botanical world - and in orchids most of all. To his considerable surprise, Charles Darwin found that among those elegant flowers dishonesty pays. Many of his specimens had gorgeous displays, but gave no payment to their pollinators. He found it hard to believe that Nature could be so fraudulent or that insects were so foolish as to fall for ‘so gigantic an imposture’ and suggested, wrongly, that his plants had an as yet undiscovered reward. His finding throws light on a question that he posed but failed to solve: how can natural selection favour the dishonest? The orchids give part of the answer.

Many of Darwin’s own observations were made on the ‘Orchis Bank’, close to his home, where he found eleven species of the plants. As he noted with a certain pride, ‘no British county excels Kent in the number of its orchids’, but he also studied specimens sent from all over the world. He soon saw how the conflict between plant and pollinator had led to change. He speaks of an orchid, ‘the Angræcum sesquipedale, of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar’. It had ‘a whip-like green nectary . . . eleven and a half inches long, with only the lower inch and a half filled with very sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length? . . . in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches! . . . As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life . . . those individual plants of the Angræcum which had the longest nectaries . . . and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis.’ In 1903, that long-tongued insect, product of an endless contest with its plant, was at last discovered and named as Morgan’s Sphinx Moth. A long conflict of interests had forced both parties to adapt themselves to each other’s demands.

As the sage of Down House collected orchids from the fields and heaths around his comfortable home and examined the specimens sent to him from afar, he became more and more impressed by the ingenuity of the ways in which as they pass on pollen: ‘Hardly any fact has struck me so much as the endless diversities of structure, - the prodigality of resources, - for gaining the very same end, namely, the fertilisation of one flower by the pollen from another plant.’ He glimpsed but a small part of the game played by all plants as they fulfil their sexual destiny.

As Darwin showed, nine years after the orchid book, in The Descent of Man, and Selection in Relation to Sex, a male peacock’s flashy rear says nothing about the merits of tails, but a lot about his status as a high-quality mate who can afford a gorgeous adornment. The same is true of plants. More food allows them to make more blooms and to proclaim their excellence to a larger audience. To remove a few flowers may also allow them to grow more fruits, as proof of how expensive it is to be attractive. The brightest and most generous individuals get more pollinators and pass on more of their genes, which promotes yet more brightness and generosity in the next generation and, almost as an incidental, leads to an outburst of diversity as the balance of sexual advantage species in different lineages.

Orchids belong to the great subdivision of the flowering plants that generates just a single leaf as the seed germinates. It contains the grasses (crops such as rice included), bananas, tulips and more. The orchids themselves are among the largest families of all for only the group that contains daisies and sunflowers possesses more species. Around twenty-five thousand different kinds are known - about an eighth of all plants with flowers - and no doubt many more remain to be discovered. Britain has just forty-six native kinds, several of which are rare.

Because orchids are so attractive they are important in the conservation movement (and cynics call them ‘botanical pandas’). They may look fragile but many are tough. Their capital lies in the wet and cool hill-forests of the tropics, and a third of all known species are found in Papua New Guinea. Plenty more live in the Arctic or in temperate woodlands, fields and marshes. They grow on the ground or high in the branches of trees, or on rocky slopes and grasslands. A few live underground and never see the light of day. In some places the plants are short of water and, like cacti, develop thickened stems or tubers to store a reserve. Some have leaves as big as their relatives the bamboos while a few are parasites with almost no foliage at all. Others, such as the vanillas, make vines twenty metres long. Some kinds are tiny, with a flower head that would fit on the head of a pin, while the flower of a certain tree-dweller from New Guinea is fourteen metres around and weighs about a thousand kilograms (a specimen caused amazement at the Great Exhibition in 1851). Plenty of others have multiple displays several metres long. A few have opted out of the endless and expensive conflict and are pollinated by the wind while one Chinese kind has abandoned the whole business of sex and indulges in a strange internal dance in which its male element curves backwards and inserts itself into its own female orifice.

Some of the flowers are simple. They are dark and look rather like the entrance to a burrow, which attracts a bee to come in for a snooze and pollinate as it does so. Many others use far more elaborate tactics. Some are perfect six-pointed stars while others resemble a glass-blower’s nightmare with fine tendrils that hang together in delicate and lurid bunches. Yet others look as if they are moulded from thick pink plastic. The flowers are scarlet, white, purple, orange, red or even blue. One species is pollinated by a wasp. It generates a chemical identical to that emitted by a leaf chewed by grubs - the wasp’s favourite food. The wasp as it visits gets not a meal of tasty flesh, but a load of unwanted pollen. For those over-impressed by the beauties of botany, certain orchids smell like putrid fish to attract carrion-feeding flies.

The biological war between flower and insect, like the whole of evolution, involves an endless set of tactics, but no strategy. It has produced a vast variety of blooms, each of which evolved in a manner that depends on the preferences of their pollinators and on what turns up in the form of mutations. Darwin noted what strong evidence the orchids were against the then common notion that the beauties of nature emerged from some kind of plan: their structures ‘transcend in an incomparable degree the contrivances and adaptations the most fertile imagination of the most imaginative man could suggest’. They were another weapon in the battle against the idea of design, a ‘flank movement on the enemy’.

However remarkable the details, all their flowers are based on the same fundamental plan. It resembles that of the distantly related, but simpler, lily (and Goethe himself, with his interest in botany, described orchids as ‘monstrous lilies’). The parts are arranged in threes, or multiples of that figure. The central lobe is often enlarged into a coloured lip which acts both as a flag to attract insects and as a landing strip that allows the visitors to reach the sweet reward at its base. Often, the flower rotates to turn upside down as it develops. The male organ sits at the end of a long column and the male cells, the pollen, are not powdery as in other plants but instead are held together in large masses, with up to two million minute grains in each. They are covered with a sticky secretion that can attach the whole lot to an insect. The female part lies deeper within, on the same column as the male. Once fertilised, the orchid may produce thousands of tiny seeds in every capsule - no more than a minute proportion of which have any hope of success.

When the pollinator enters, some means is found to attach male sex cells to it. Many orchids have a spring-loaded mechanism that fires a mass of pollen in the right direction. It sticks on with powerful glue. As Darwin found by stimulating the flowers with a pencil, the stalk of the transferred pollen sac quickly dries out and the mass of male cells takes on a more vertical position, just right to fertilise the female part of the next plant visited. In a few kinds, should the mass miss its target, its energy is enough to shoot it for a metre away from the plant (the pollen is ‘shot like an arrow which is not barbed’). The blow is unpleasant enough to cause an insect that has been hit to concentrate, if it can, on the female part of the flowers it visits subsequently, which is a real help to the male who scared it off. In other species, the pollen masses crack open to the sound of a buzz like that of a particular species of bee. Yet other kinds have a see-saw that tips the insect on to the crucial male cells. It pays the plant to do the job as well as it can, for many orchids are limited in their ability to reproduce by a shortage of visitors.

Once the pollinator has been enticed to arrive it expects to be repaid. The first reward of all, in the earliest flowers, was pollen itself, which is expensive as it contains lots of protein. Even so, plenty of orchids still provide a solid meal made up of the stuff, or of bits of tissue that resemble it. Others give nectar, which is simpler and can be provided in very dilute form. Honey bees, for example, must extract the sugary liquid from several million flowers, a few of which may be orchids, to make a kilo of honey.

Some botanical entanglements are intimate indeed. Certain bees are so tied to their partners that their own sex lives have come to depend upon them. They obtain their sexual scents - their pheromones - from an orchid flower and without a visit they are unable to mate. The pheromones may have more than a dozen ingredients. As in the Chanel factory, the bees practise ‘enfleurage’: they mix an odoriferous base taken from the plant with an oily substance of their own that helps the smell to persist. A special grease is smeared on to the flower and the sexual mix transmitted to pockets in the bees’ hind legs. The habit evolved from the insects’ ancient habit of marking their sexual readiness - like dogs around lampposts - with scents extracted from flowers, rotten wood and even from faeces.

The battle is not evenly matched, for the insects themselves - many of whom visit a variety of plants - are under less pressure to retain an accurate fit with their partner than are the flowers. The orchids evolved well after insect pollination began and have had to adapt to the needs of their partners, rather than the other way around. Some insects - many bees included - are quite catholic in their tastes and some orchids are indifferent as to who moves their pollen, as long as somebody does. A few species are visited by more than a hundred different insects, while only around half of all orchids are more or less faithful to a single pollinator. To become too closely connected to a particular insect is risky. Darwin himself speculated that the giant orchid of Madagascar would disappear if its specialised pollinator died out, and he may have been right.

The orchids face a higher risk of failure if they cannot find a pollinator than do animals in the same predicament, for an insect can always try another kind of flower if its prime source of food becomes too rare or too mean. Many flowers - those of orchids included - are in fact visited by several pollinators, even if particular species do tend to concentrate on similar insects; on long-tongued bee-flies and long-tongued flies, or on tiny bees, flies and beetles, each of which picks up the pollen on its legs. Even the bees that pick up their own sexual scents from an orchid are less dependent than they seem. A certain South American species has become naturalised in Florida, where its host does not grow. It finds its chemicals instead in aromatic plants such as basil and allspice when it chews their leaves and extracts the smelly substances. The bee pollinates a wide variety of local plants, which reciprocate with nectar rather than with an aphrodisiac. Once again, the insect has more freedom of action than does its partner.

As a result, the two parties are often less entangled than Darwin imagined. A shift in one is not always matched by an equivalent move by the other, with deeper flower trumped by longer tongue. Molecular trees of plants and pollinators suggest that the insects have instead often switched to species with shallower flowers from which nectar can be sucked with less effort.

The orchid’s ability to force its ally to serve its selfish interests is further limited because such gorgeous beings are often rare and scattered among other species. Make life too hard and the insect will sip elsewhere. Infidelity by the pollinator is bad news for the orchid as it may fail to export its own genes and in addition it may get pollen from the wrong species. Nevertheless, not all the pollinators have been promiscuous, for fossil water lilies from ninety million years ago have flowers quite like those of their modern ancestors as evidence that their association with beetles is ancient indeed.

Orchids bolster Darwin’s case that species arise through the action of natural selection and he soon realised that their diversity had been driven by the vagaries of insect behaviour. He was much less certain of the origin of flowers themselves, which he called ‘an abominable mystery’ and a ‘perplexing phenomenon’. The mystery has been cleared up and the orchids have helped.

Plants colonised the land more than four hundred million years before the present. Those pioneers had no flowers and neither did the huge forests of giant ferns that covered large parts of the planet a hundred million years later. The fern forests declined and the dinosaurs flourished for an age in a flowerless world. Not until the first flowers of all, perhaps a hundred and fifty million years ago, did the conflict between insect and plant begin. It led to an explosion of change in both parties. Their joint transformation was spectacular, for more than three hundred thousand species of flowering plants have evolved, together with several times that number of insects.

The oldest fossil flower comes from a famous bed close to the estuary of the Yellow River in China. It dates from around a hundred and twenty-five million years ago, at the time when the white cliffs of Dover were being formed in a shallow sea. It looked rather like a water lily and floated in fresh water with its small flowers above the surface. For tens of millions of years such structures remained modest, but sixty-five million years ago - just as the dinosaurs left the stage - the world burst into bloom.

The orchids played their part in beautifying an unpeopled world. A distinctive pollen sac attached to a stingless bee has been found in twenty-million-year-old amber from the Dominican Republic. That orchid’s modern relatives use just the same group of insects to transfer their male cells. The molecular clock suggests that orchids as a whole originated around the time of the extinction of the dinosaurs. Their massive radiation happened just after that memorable event and was accompanied by parallel change in the insects that pollinate them.

The great blooming was evidence of an early skirmish in the war between orchid and insect. Conflict between plants and pollinators has gone on ever since. It is expensive and never more so than when it escalates. The Soviet Union collapsed under the financial pressures of its attempts to match the power of the Americans and for centuries Britain and France spent a third of their wealth in mutual conflict. In war, as in love and business, lavish display is a test of merit. A military parade intimidates the enemy and a costly publicity campaign is a sign of a high-class company. The medium becomes the message, the powerful stay in charge, cheats go bankrupt and, for most of the time, truthful ostentation prevails. The best signals are too expensive to copy, which is why McDonald’s sues anyone who imitates their golden arches and why Japanese Yakuza gangsters cut off their fingers.

The interaction between plants and pollinators is a matter of economics - and economists have been quick to notice. Signalling theory tries to explain how decisions are made when the information available is less than perfect - what used car to buy, who to hire for a job, what flower to visit. One test is to look for a reliable sign of quality, whatever it might be. An applicant for a job in a bank might have a first-class degree in genetics. Useless as that certificate might be to a prospective financier, it is at least an honest (and expensive) statement of overall merit. The system works well - as long as everyone is honourable. Sometimes they are not. Straight fraud - a forged Harvard degree - can often be picked up but what of a parchment from one of the many bogus universities that nowadays advertise their wares? The University of Dublin sounds respectable but is a website. How can employers tell Redding University (an American degree mill) from the University of Reading (a respectable institution to the west of London)? Thousands of people now have such qualifications and if too many degrees turn out to be false then the whole machine breaks down. The risk is real. Nine-tenths of the ‘Tiffany’ jewellery on sale on eBay is fake, and Tiffany & Co. has spent millions in attempts to shut down the sellers, who cause huge damage to its brand. If the bogus continue to prosper at the expense of the genuine, the entire jewellery market may collapse.

A study of the economic implications of such false signals (or ‘asymmetric information’, as financial experts call it) won the Nobel Prize for Economics in 2001. Plants and animals have done such sums for years. Most of the time, they get it right and honesty more or less prevails. Sometimes, the cheats get in, for if the reward is large enough and the penalty for swindling not too stringent, natural selection can favour sharp practice. The temptation to invest in display rather than product means that the price of sex is eternal vigilance. Some orchids - like some traders - allow others to pay for the publicity while they double-cross their pollinators. Life at the top faces a constant challenge from fraudsters.

Plenty of pollinators, too, are duplicitous. Insects gnaw into a flower to gain a reward at minimal cost while humming birds can poke a hole in its side to do the same. Even legitimate pollinators like honey bees become robbers at once when someone else has broken in. For them, dishonesty pays and they turn to it whenever they get a chance.

The flowers have hit back. What they offer may be quite different from what they promise. Orchids have a wide range of lures. Some subvert their pollinators’ desires with blossoms that resemble female insects. The flowers are larger than real females, and may emit a hundred times more of their attractive sexual scent. The male bees or spiders - understandably - try to copulate with their spurious brides and in their failed attempt to pass on their own DNA do the same job for the plant. Their amatory experience is futile but intense, as many of the befuddled males produce copious amounts of sperm that costs them a lot to make and goes nowhere.

Darwin found it hard to believe that a bee could be so stupid as to frot a flower but, in the world of sex, stupidity can pay. A naive male bee faced with females in short supply, as they often are because males emerge earlier in the season than their partners, is well advised to travel hopefully because he might arrive; he should copulate with anything that looks even a little like a member of the opposite sex on the off-chance that, now and again, he will be lucky. The bees oblige and, most of the time, the orchids win. Other orchids exploit the aggressive, rather than the amatory, instincts of their pollinators. They mimic a male insect rather than a female - which annoys the local territory-holder who tries to drive out the supposed intruder and pollinates as he does so.

Other kinds exploit the greed, rather than the lust, of their visitors. They advertise not sex but a free meal but again, they provide nothing. That baffled Charles Darwin. It was ‘utterly incredible’ that ‘bees . . . should persevere in visiting flower after flower . . . in the hope of obtaining nectar which is never present’. He suggested instead that the empty flowers had hidden reserves, which the insects would reach if they made a hole and sucked the plant’s juices.

Life is less honest than he imagined and the flowers were in fact cheats. About a third of all orchids act in this underhand way - flashy signal, but no food reward. Some other plants do the same, often with a few ‘cheater flowers’ on an individual in which most are honest, but the orchids are the real confidence tricksters, for they make up nine-tenths of all flowers known to fool their visitors. DNA shows that the habit has arisen again and again within the group - but it does not always pay, for some orchids that now provide a generous recompense to their visitors have evolved from species that once led a dishonest life.

Often, such false flowers are - like the harmless flies that look like wasps - mimics, with a resemblance, more or less accurate, to other local plants that do make a reward. They flaunt a badge of quality such as bright colour to attract an assistant on the cheap. Some work hard to fool their visitors and are uncannily similar to a particular model in shape and colour. Certain Australian kinds, for example, look like mushrooms and are pollinated by fungus gnats in search of a place to lay eggs. A few even make small orange and black spots on their flowers which attract aphid-feeding flies that see the spots as potential prey. More often, their displays are no more than general statements of reward that attract a variety of insects. The parasite joins a whole guild of locals in which the various species share a resemblance and attract about the same mix of insects. Honest plants pay the price when insects avoid them after an anticlimactic experience with a cheat. Some orchids are doubly duplicitous for individuals vary in colour, one from another, which allows them to parasitise a wider range of victims.

The cheats tend to grow scattered among their hosts, for a group of fraudsters close together is soon detected by the pollinators, who move away to a more worthwhile patch. They do best at fooling insects that have just emerged into the wicked world and have not yet learned to detect double-crossers. As a result such orchids tend to flower in the spring rather than later in the year. but in many cases a shortage of pollinators foolish enough to revisit a dishonest plant force it to make a long-lasting flower and pollen that, unlike that of most of its fellows, survives for weeks or months. A certain Australian orchid uses the opposite strategy, for all the plants open on the same day of the year, giving the pollinators no time to learn about the gigantic fraud being perpetrated upon them.

Experienced insects soon become cynical for they move away faster - and fly further - from empty flowers than from those with nectar. The dishonest orchids may reap a subtle benefit from their disappointed visitors, for the still hungry insect may buzz off to a new individual, rather than shifting its attentions to a second flower on the same plant. Such behaviour cuts down the chance of self-fertilisation.

Orchids may be the real experts, but plenty of other associations between plants and pollinators have been subverted by natural selection. Wild peas and beans often make nutrient-rich rewards that attract birds to spread their seeds but some of their offerings contain nothing of value although they look like a tasty meal. Yuccas - those spectacular flower spikes of the American deserts - are pollinated by a certain moth, who carries a bundle of pollen to the female, inserts it in the right place and then lays her eggs within the flower. When they hatch, the larvae feed on the seeds and once adult fly off to pollinate another yucca. Close relatives of such moths, though, eat the seeds without bringing pollen.

Many male insects use a gift of food to persuade a female to copulate with them. Dance flies, hairy-legged predatory insects of wet places, make swarms in the mating season. In some species, each male brings a gift of a dead insect larva, and mates with his female while her attention is diverted by the meal. Once the bribe has been eaten, the male is pushed off. Other species prolong the sexual experience, for they wrap the gift in a silk purse, which the female must open before she can eat. At once, a chance to cheat presents itself - and it has been seized. Some male flies make elegant and complex purses that take a long time to open, but - like a dishonest orchid - are empty, or contain a desiccated corpse. By the time the female finds out, she has been inseminated. Fireflies are just as devious. Males bring a gift, a sticky mass of nutritious gel that goes with the sperm and is soaked up by their mates. Those who can afford more of the stuff make a longer flash and attract more females. A successful male soon runs out of energy. Some cheat, with a long flash and no reward - but they take a risk, for a certain predatory firefly uses the burst of light to find its prey. A false flasher risks death every time he exposes himself.

Darwin’s perplexity about the dishonesty of orchids opened the door to a whole universe of evolutionary discord. Many creatures are happy to lie in the race to pass on genes. The conflict extends beyond plants and pollinators, to predators and prey, pathogen and host or men and their domestic animals, all of which are locked into an endless - and often joyless - conflict. Such ancient disputes explain why the Irish had a potato famine, why some diseases are virulent and others not and why the Argentinian Lake Duck has a corkscrew-shaped penis longer than its own body.

Sexual dishonesty is widespread. Birds are at it all the time. Many species appear to live as faithful pairs, but paternity tests show that the majority are happy to cheat and that half - or even more - of the eggs of a particular female are the scions of another male, often an individual more dominant than their regular partner. Mammals are even more devious. The joys of paternity-testing reveal that a male mammal’s sexual displays are often subverted: a feeble individual can sneak in when the top stag is preoccupied with display and insert his own genes with no need for a huge investment.

Monogamy is rare, for not more than one mammal species in about twenty (some humans included) appears to indulge in it. Even some classic examples of reproductive honesty are in fact cheats. The prairie vole seems to stick to his mate through thick and thin and helps raise the young. Their happy marriage is based on a certain hormone. On his wedding night a surge of the stuff kicks in and appears to tie the male to his partner for life. A director of the US government’s family planning program saw the vole as proof that sex before marriage disrupts brain chemistry and leads to divorce. The hormone, he says, is ‘God’s superglue’. It bonds partners together and, said the politician, it does the same for society (and also proves that abstinence is the finest form of contraception). The gene that picks up the hormone in the bloodstream comes in several forms in humans, too, and - in Sweden at least - men who bear two copies of a certain variant are less likely to be married or, if they are, have a more difficult relationship than do others.

The cold eye of the paternity-tester has now fallen upon the private life of the prairies. DNA cannot tell a lie - and it shows that beneath the vole’s upright social habits lies a dark sexual universe. One in five of the young of each pair is fathered by a male other than the marital partner and around a quarter of all males and females have sex outside the household. Voles are socially faithful, but sexually fickle; happy to cheat, but quick to forgive. Foxes are even more dishonest, for more than three-quarters of their cubs are fathered by a stranger.

Even so, there has been plenty of reproductive dishonesty in our own history. Casanova, himself of uncertain paternity, posed as a soldier, a doctor, a diplomat, a nobleman and a sorcerer to gain the favours of an admitted hundred and twenty women (plus, more than likely, many more). He was a great lover, and a better liar, even if, according to a contemporary, he ‘would be a good-looking man if he were not ugly’. His wit, rather than his looks, charmed his way into the bedroom.

Now, the chance for deceit has been much improved by technology. No longer does a hopeful male need to display his talents directly; instead he can say what he chooses about his looks, his education and his wealth on an online-dating site. There he has no fear of detection, at least until his first appointment with a prospective mate. Tens of millions of people use such sexual aids, and millions of liaisons (many ending in marriage) have emerged from a digital romance. Even so, nine out of every ten users - and women more than men - are convinced that the world of electronic eroticism is filled with cheats, with dirty and decrepit Casanovas who present themselves as young lovers in the hope of reproductive success on the cheap.

In fact, such suspicions are misplaced. Surveys of on-line daters show impressive levels of accuracy in their descriptions of themselves, for almost all say something close to the truth about age, body build, wealth, education, politics, marital history and more (admittedly, men tell slightly more lies about their income and women about their weight). The daters disapprove strongly of anyone who did not live up to their claims on a first meeting and swore that they would go no further with them. Deception is not an effective sexual strategy. For men and women, honesty pays and the fraudulent are rejected as partners as soon as they are detected.