Life on Earth. David Attenborough
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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