Life on Earth. David Attenborough
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.
By any standards, the insect body must be reckoned the most successful of all the solutions to the problems of living on the surface of the earth. Insects swarm in deserts as well as forests; they swim below water and crawl in deep caves in perpetual darkness. They fly over the high peaks of the Himalayas and exist in surprising numbers on the permanent icecaps of the Poles. One fly makes its home in pools of crude oil welling up from the ground; another lives in steaming-hot volcanic springs. Some deliberately seek high concentrations of brine and others regularly withstand being frozen solid. They excavate homes for themselves in the skins of animals and burrow long winding tunnels within the thickness of a leaf.
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