A Buzz in the Meadow (4 page)

Since the discovery of the Burgess Shale, other similar fossil beds from about the same period have been found elsewhere in the world, although arguably none quite so fine. These have added to our knowledge of life in the Cambrian seas half a billion years ago – 499 million years before something approximating to modern humans was to appear. In addition to Walcott's strange creatures there were arachnids, ancestors of modern spiders and ticks, including the fearsome eurypterids, scorpion-like creatures up to two and a half metres long, which used powerful pincers to hunt their prey on the beds of oceans and rivers. There were trilobites, segmented, shield-shaped animals known to us now only from fossils, but for 250 million years they were amongst the most abundant creatures on Earth, with many thousands of species, from tiny free-swimming versions thought to have lived in open water to vast, armoured bulldozers that trundled along the ocean floor, presumably trying to keep out of the way of the eurypterids.

One arthropod group that is conspicuously absent from the fossils of the Cambrian is the insects, but that would have been no surprise to Walcott, for the insects did not evolve in the seas; they evolved later, on land. At the time the Burgess Shale was laid down, life on land was pretty unexciting – there were a few primitive clubmosses and liverworts growing in wet areas near water, but little else. There would also have been a strand-line of washed-up plants and animals, edible detritus that was tantalisingly out of reach of the water-bound animals of the day. Inevitably, before long some arthropods began to drag themselves ashore to take advantage of these untapped resources. Their external skeletons, which perhaps originally evolved as a defence against predators, helped to support their bodies on land, giving them an edge in colonisation there over soft-bodied sea animals such as jellyfish and worms. To start with, these animals would presumably have been poorly waterproofed and had to return to water very regularly to prevent themselves drying out; or they would have had to stay in the dampest places, amongst rotting weed or damp moss.

We will probably never know what the first land animal looked like; it may have been something like a millipede, which grazed on the carpets of moss on the sea shores. It may have been a crustacean, perhaps similar to a woodlouse or sandhopper, feeding on rotting detritus along the high-tide line. Whatever it was, it was soon followed by predatory arachnids, scorpion-like creatures that trundled or scurried after their prey across the greenery. As the plants slowly adapted to life on land and spread away from the seas, they also grew taller as the competition for light intensified. To follow them, the animals had to improve their waterproofing. Some animal groups never really got the hang of this; for example, the few crustaceans that successfully invaded land, such as woodlice, are restricted to damp places to this day. Others, such as arachnids, evolved more-or-less waterproof cuticles and eggs, so that they were able to leave water far behind and occupy even the most inhospitable, arid environments on Earth.

The insects were the last of the major arthropod groups to arrive, about 400 million years ago, but they have more than made up for lost time since then. We do not know what they evolved from – perhaps it was the crustaceans, perhaps some early millipede-like creature. It is most likely that they evolved on land rather than in the sea like the others, and they have become the masters of terrestrial life.

Along with the spiders, insects mastered waterproofing early on; their cuticle is coated in waxes and oils that cut water loss to a minimum. Insects differ from other arthropods in having fewer legs – just six. Their body is divided into three distinct sections: the head, which carries the sensory organs: eyes, antennae, palps, and so on; the thorax, to which all the limbs are attached; and the abdomen, which contains the reproductive parts. The earliest insects were not particularly impressive. They were probably similar to the silverfish that survive to this day: small, scurrying creatures that live in damp places, notably under carpets in poorly maintained houses, presumably not their original habitat. Somewhere along the line these early insects acquired better waterproofing and, with that, the terrestrial world was their oyster. They proliferated, exploiting the abundant food provided by the spreading forests and specialising into a myriad of forms. By the Carboniferous period about 360 million years ago, there were numerous types of cockroach, mantis, grasshopper and probably many others; insects rarely fossilise, so we have only a very fragmented picture. These insects provided abundant food for the predatory and parasitic arachnids, and they too thrived and became better adapted to life on land, so that the Carboniferous was blessed with the first spiders, and also with bloodsucking ticks, scavenging harvestmen and mites, and various other horrendously unattractive but eerily fascinating creatures, such as whip scorpions and vinegaroons, which have survived to the present.

The insects were to prove to have a couple of other tricks up their sleeve – two more evolutionary innovations that would leave the rest of the animal kingdom far behind. Perhaps most importantly, they were the first creatures on Earth to take to the air, to evolve powered flight, perhaps 350 million years ago. The earliest flying insects included grasshoppers and cockroaches, but these are not accomplished fliers. Even today, most species can only fly a few metres at most before they crash to the ground. The first true masters of the air were the dragonflies, swift and agile in flight. This must have given them huge advantages over earth-bound animals. They could swoop down on their prey from above and easily escape from predators such as amphibians. They could swiftly travel long distances to find food, or flee from approaching winter by migrating southwards. Colonising new habitats as they appeared would have been easy, and so it was flying insects that would always have been among the first to arrive as new islands arose from the oceans, and the first to colonise new ponds and lakes as they formed.

The final major innovation of the insects was the evolution of metamorphosis. Primitive insects do not change much as they grow, other than in getting bigger. The eggs hatch into miniature copies of the adults called nymphs, and these grow gradually larger by moulting. This is much the same as all the other arthropods, such as shrimps and spiders. Even today, many insects develop in this way: grasshoppers and crickets, earwigs, aphids and cockroaches, to name a few. Just a small number of insect groups undergo metamorphosis. In these, the eggs hatch into a grub or caterpillar, properly known as a larva, which looks nothing whatsoever like its parent. It is an eating machine, a mouth and digestive system contained within a flabby sac, designed for growth. It is usually not very mobile, has poor eyesight and generally weak senses. Most larvae rely on having been placed on or in a supply of food by their mother, and their job is to convert that food into insect tissue as quickly as possible. Once this job is done, the larva pupates, shedding its skin and turning into an immobile, helpless chrysalis or pupa. Inside the pupa, the tissues dissolve and are rebuilt from scratch. Wings form, and legs, eyes, antennae, a brain – all the body parts of the adult – are assembled. Once this is done, the adult bursts forth from the pupa, pumps up its wings to full size and is ready for action.

The job of the adult is to reproduce. Adult insects are sex machines: the males are ‘designed' by evolution to find and seduce as many partners as possible, the females are designed to mate and then find a good source of larval food on which to lay their eggs. To do these things adult insects need to be highly mobile, and need to have well-developed senses with which to locate an appropriate mate and a place to lay eggs. Many adult insects, such as butterflies and mayflies, may live for just a few days; some for only a few hours. Some do not feed at all, or even have mouths to speak of. The advantage of metamorphosis is that it enables a division of labour between the different stages of the life cycle. The larva has a body that has been honed by evolution to allow rapid growth, while the adult has a completely different body design that is equipped for mating. More primitive insects, and other arthropods such as the arachnids and crustaceans, have to make do with the same body plan throughout their life, a kind of morphological compromise.

The advantage that metamorphosis gives to those insects that undergo it is illustrated by the enormous success of these groups, in terms of numbers of species. The four most speciose groups of animals on Earth are the beetles, flies, moths and wasps. The famous evolutionary biologist J.B.S. Haldane was once asked what his studies of evolution had taught him about the nature of God. He was an atheist, and replied, ‘He must have an inordinate fondness for beetles.'

Today, of the roughly 1.5 million known species of plant and animal on Earth, 1.2 million are arthropods, of whom about one million are insects and, of these, about 800,000 are beetles, flies, moths or wasps. Of course there are millions more insects awaiting naming, should we ever get round to it. Take a swipe with a butterfly net in any tropical forest and you will almost certainly catch a handful of species that are new to science. The tricky bit is working out which ones have already been named and which ones haven't; for most groups of insects there may be only one or two specialists in the world who can work out which are which.
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For all of the last 500 million years almost every habitat on Earth has been dominated by arthropods, in terms of numbers of species and numbers of individuals. On land and in fresh water, most of these are insects. The dinosaurs came and went, followed by the great age of mammals, but through all of this there have been insects, swarming in mind-boggling diversity in the lakes, rivers, forests, grasslands and deserts, from the seashore to the top of the highest mountains.

Insects fill almost every conceivable ecological niche: they can be predators, parasites, herbivores or detritivores.
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There is almost nothing of organic origin, alive or dead, that is not avidly consumed by insects of one sort or another. Some, such as clothes moths, can survive through their entire life cycle on foodstuffs that contain no moisture whatsoever. Others variously specialise in eating blood, wood, seeds, the tongues of frogs, dung, owl pellets, bacteria, leaves, algae, lichens, spiders, fungi and, of course, other insects. They vary in size from Bornean stick insects, which can grow to over thirty centimetres, to speck-like parasitoid wasps that weigh in at just twenty-five millionths of a gram. Their life cycles are staggeringly varied and unfamiliar. Leaf-mining flies may live almost their entire life burrowing inside a single leaf, while monarch butterflies regularly migrate 8,000 kilometres every year, from Canada to their hibernation grounds in Mexico and back again. Some, such as ants and termites, live in vast social colonies, with workers specialising as soldiers, gardeners or nursemaids, while others, such as the death-watch beetle, may spend ten years alone and in darkness, slowly munching through the timber of a dead tree. Nymphs of the periodic cicada in North America spend seventeen years living underground, sucking on tree roots, before all emerging together to mate and die, while a fruit fly dashes through its entire life cycle in a fortnight.

Just as their life cycles are infinitely diverse, so their mating habitats are extraordinarily varied and often bizarre. While butterfly males use their beautiful wings to woo a mate, male scorpion flies offer piles of dried saliva as an enticement to females. Crickets, grasshoppers and cicadas sing to impress, while other insects such as moths release pheromones that waft for kilometres on the wind. Male stag beetles use their jaws to fight over mates, while male stalk-eyed flies (so named because their eyes are suspended on long, slender stalks on either side of their head) face off against one another – the one whose eyes are furthest apart winning the competition, and the female. Picture-wing flies perform elaborate dances for their mates, while fireflies and glow-worms use light-emitting bacteria in their bottoms to attract a partner. In dance flies, a group of small black flies that swarm in clouds above streams and ponds on a summer's eve, the usual pattern of sexual selection is sometimes reversed, with males providing expensive nuptial gifts to their mates – small dead insects carefully wrapped in silk – so that they are fussy about whom they choose, preferring large females who will have many eggs. To earn their gift, females have evolved swollen legs that make them look larger and thus fool the males. Some insects, such as earwigs and shield bugs, care for their offspring, even sacrificing their life to save that of their young. Corpse-eating carrion beetles look after their offspring, but if they have too many they casually consume the surplus, ensuring an adequate supply of dead meat for the remainder. No doubt there are many more marvellous behaviours that remain to be discovered, if we ever find the time to look.

In terms of numbers of individuals, insects rule supreme. A single leaf-cutter ant nest in the forests of South America can house four million ants. At any one point in time there are currently thought to be very roughly ten million trillion individual insects alive on Earth (Ethel got a bit carried away in ‘The Insects' World'). Whatever way you look at it, we are seriously outnumbered. Some pest insects, such as aphids and house flies, are perhaps more common than ever because of the food we unwittingly supply for them. But most insects are declining, and many thousands of species have already gone extinct. As the most famous entomologist alive today, E.O. Wilson, once said, ‘If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed ten thousand years ago. If insects were to vanish, the environment would collapse into chaos.' Insects
are
vanishing. By every indicator we have, the bulk of insects are declining. Butterflies, bees, dragonflies, grasshoppers and the countless species that live only in the dwindling rainforests are disappearing one by one.