Silent Earth

Averting the Insect Apocalypse

Dave Goulson

• Publisher: Harper
• Year: 2021
• ISBN:
  9780063088221
• Topic: Scienze biologiche
• Subtopic: Entomologia

"A terrific book…A thoughtful explanation of how the dramatic decline of insect species and numbers poses a dire threat to all life on earth." (Booklist, Starred Review)

In the tradition of Rachel Carson's groundbreaking environmental classic Silent Spring, an award-winning entomologist and conservationist explains the importance of insects to our survival, and offers a clarion call to avoid a looming ecological disaster of our own making.

Drawing on thirty years of research, Goulson has written an accessible, fascinating, and important book that examines the evidence of an alarming drop in insect numbers around the world. "If we lose the insects, then everything is going to collapse, " he warned in a recent interview in the New York Times —beginning with humans' food supply. The main cause of this decrease in insect populations is the indiscriminate use of chemical pesticides. Hence, Silent Earth's nod to Rachel Carson's classic Silent Spring which, when published in 1962, led to the global banning of DDT. This was a huge victory for science and ecological health at the time.

Yet before long, new pesticides just as lethal as DDT were introduced, and today, humanity finds itself on the brink of a new crisis. What will happen when the bugs are all gone? Goulson explores the intrinsic connection between climate change, nature, wildlife, and the shrinking biodiversity and analyzes the harmful impact for the earth and its inhabitants.

Meanwhile we have all read stories about hive collapse syndrome affecting honeybee colonies and the tragic decline of monarch butterflies in North America, and more. But it is not too late to arrest this decline, and Silent Earth should be the clarion call. Smart, eye-opening, and essential, Silent Earth is a forceful call to action to save our world, and ultimately, ourselves.

Silent Earth includes approximately 20 black-and-white illustrations and charts and graphs.

Part I

Why Insects Matter

I fear the majority of people don’t much like insects. In fact, I would go further: I think many people loathe insects, or are terrified of them, or both. They are often referred to as ‘creepy-crawlies’, or ‘bugs’, the latter a term we also use for disease-causing organisms. For many of us, these terms are associated with unpleasant, scuttling, dirty creatures, living in filth and spreading disease. Increasingly, most of us live in cities, and grow up seeing few insects other than house flies, mosquitoes and cockroaches, so perhaps we should not be surprised that insects often inspire fear. Most of us are frightened of the unknown, of the unfamiliar. Few therefore appreciate how vitally important insects are to our own survival, and fewer still how beautiful, clever, fascinating, mysterious and wonderful insects are. My mission in life is to persuade people to love insects, or at least to respect them for all that they do. Here I want to explain why we should be teaching everyone from a young age to cherish these tiny creatures; why they matter.

1

A Brief History of Insects

Let’s start at the beginning. Insects have been around for a very, very long time. Their ancestors evolved in the primordial ooze of the ocean floors, half a billion years ago, strange, armoured creatures with an external skeleton and jointed legs, known today to scientists as arthropods (meaning jointed feet). We have few fossils from that time, but those that exist, such as from the famous Burgess Shale deposits of the Canadian Rockies, give us tantalising glimpses of that early world. They are enormously diverse, with numerous types having body plans and numbers and shapes of limbs, eyes and other mysterious appendages unlike anything found today. It was as if Mother Nature had hit upon a successful concept, and was tinkering away like a child with a Meccano set, trying out different ways to bolt a creature together. For example, the aptly named Hallucigenia was a worm-like creature that was originally thought to walk on long, spine-like legs, with a crazy hairdo of waving tentacles on its back, but in more recent illustrations it has since been flipped over so that it walks on the tentacles and perhaps would have used the spines in defence. Meanwhile, Opabinia had five eyes on stalks, and a single lobster-like claw extending from its head, whereas Leanchoilia was a woodlouse-like creature equipped with two long arms at the front, each divided into three tentacles. Then there was Anomalocaris, an animal originally described as being three separate creatures – one shrimp-like, another a jellyfish, and the third something similar to a sea-cucumber – but it is now thought that these were all part of a single creature, the sea-cucumber being the body, the jellyfish its mouth parts, and the shrimp-like creature actually one of a pair of legs. At about 50cm long, Anomalocaris was the largest of the Burgess Shale fossils to have been described so far. We can only guess as to the behaviours and life cycles of these diminutive sea monsters of 500 million years ago. The early seas became crowded with these weird and wonderful creatures, but all are now extinct, although some must have founded lineages that are still present in the seas today.

What we do know is that a few of these early arthropods eventually experimented with moving to the land, perhaps to escape competitors or predators, or in search of their prey.

Having an external skeleton proved handy on land: most small sea creatures such as jellyfish and sea slugs rely on the water for support, and simply flop about in a helpless mess if they are stranded by a retreating tide, but with a rigid skeleton the early arthropods could walk, and this they did, exploring ever further from the water. They went on to found the most successful dynasty of creatures ever to tread the Earth. To this day, they are easily the most successful group on land, if you measure them by number of species or number of individuals (and not by their ability to mess up the planet). They, of course, are the insects.

Starting perhaps 450 million years ago, various different lineages of arthropod had a crack at life on land. Early arachnids dragged themselves from the sea and went on to become spiders, scorpions, ticks and mites – perhaps not the most glamorous of creatures to our human eyes, but very successful in their way. Millipedes ambled slowly onto land and occupied shady, damp habitats, quietly nibbling on decaying organic matter in the soil and under logs and stones, where they remain peacefully ensconced to this day. There, the millipedes were pursued by their fiercely predatory and rather faster relatives, the centipedes, also denizens of the soil and other dark, damp places. A few crustaceans (crabs, lobsters, shrimps and so on) had a go at terrestrial life, but most never really got the hang of it. This group remains enormously diverse and abundant in the oceans to this day, but its most successful terrestrial representative is the humble woodlouse, an endearing and important creature in its way but with no serious claim to global domination.

Creatures of the Burgess Shale, animals that lived in the sea 500 million years ago: These weird creatures include many early arthropods, ancestors of the insects: sponges Vanuxia (1), Choia (2), Pirania (3); brachiopods Nisusia (4); polychaetes Burgessochaeta (5); priapulid worms Ottia (6), Louisella (7); trilobites Olenoides (8); other arthropods Sidneyia (9), Leanchoilia (10), Marella (11), Canadaspis (12), Molaria (13), Burgessia (14), Yohoia (15), Waptia (16), Aysheaia (17); molluscs Scenella (18); echinoderms Echmatocrinus (19); chordates Pikaia (20); along with Haplophrentis (21), Opabina (22), lophophorate Dinomischus (23), proto-annelid Wiwaxia (24), and anomalocarid Laggania cambria (25). From Wikicommons https://commons.wikimedia.org/wiki/File:Burgess_community.gif

The early arthropod adventurers on land, like woodlice and millipedes today, were presumably confined to damp places, along the water’s edge, in mud, under stones or in clumps of moss. Aquatic creatures tend to die of dehydration very quickly on land, especially small ones like most arthropods. To really explore the land, waterproofing is vital. Spiders got the hang of this, evolving a waxy cuticle that now enables them to live even in the most arid places; I have seen them sitting patiently in their delicate webs, constructed on a scraggy leafless bush in the middle of the Sahara Desert. However, it was the insects that truly mastered terrestrial life. Their precise origin remains mysterious: insects are thought to have evolved on land about 400 million years ago,^* perhaps from an early crustacean, perhaps from a millipede, but more likely to have come from some other ancient arthropod group that did not survive to the present and has not yet been found in fossils.

How, though, do we define or identify an insect? The answer is that all insects share certain common characteristics that distinguish them from other arthropods. Their body is divided into three sections: a head, thorax and abdomen. Unlike any other arthropod group, insects have six legs, which are attached to the thorax. Like the spiders, insects developed a waterproofed cuticle, sealed with waxes and oils.

Equipped with this basic design, insects set out to conquer the land, but they would probably not have got far were it not for one further huge evolutionary leap that was to be the key to their global success. One early insect took to the skies. Some primitive flightless insects still survive to this day – silverfish are perhaps the best known (which is to say, not very well known at all). Those able to fly, on the other hand, became enormously successful.

Powered flight has only evolved four times, so far as we know, in the three and a half billion years since life began, and insects were the pioneers of life in the air, about 380 million years ago (followed by the pterosaurs 228 million years ago, birds about 150 million years ago, and bats about 60 million years ago). For 150 million years insects had the skies to themselves. It isn’t clear how flight first evolved, but a popular theory is that wings were originally flap-like gills, as seen in mayfly nymphs today. To start with they may have simply facilitated gliding, but eventually they became motile, and the first powered flight began.

Being able to fly bestows significant advantages. Escaping from land-bound predators becomes easy, and finding food or a mate is greatly facilitated, for flying is much quicker than walking. Migration becomes possible, with some insects such as the monarch and painted lady butterflies eventually evolving to fly thousands of miles each year to avoid the cold of winter. Migration is not a viable option if you are a woodlouse or a millipede.

With their new-found superpower the flying insects proliferated in the Carboniferous period (359 to 299 million years ago), with many new insect groups appearing, including the weak-flying mantises, cockroaches and grasshoppers, and also more accomplished flyers such as mayflies and dragonflies.

While the insects were busy learning to fly, the plants were not resting on their laurels. They too had developed better waterproofing of their leaves, and in competition with one another for light had grown ever taller, creating forests of giant tree ferns (some of which were of course to become fossilised as coal when they sank into the boggy forest floor). Although by this time there were amphibians and the first lizards, life on land must have been very largely dominated by insects. The air at the time was richer in oxygen than today, and that may be one reason why some insects were able to grow larger than any present-day species. If one could travel back to those ancient forests, one might glimpse Meganeura soaring between the trees – huge, dragonfly-like insects with wingspans of over 70cm.

Although flying may have been the insects’ most important innovation, they had a couple more tricks up their six sleeves. Firstly, just after the end of the Carboniferous, about 280 million years ago, an insect species somehow achieved metamorphosis, the remarkable ability to change from a grub-like immature stage (the larva) into an adult insect with an entirely different appearance; from a caterpillar into a butterfly, or from a maggot into a fly.

Metamorphosis is as magical as any frog-to-prince transformation in a fairy tale, except that it is real, and happens all the time all around us. Imagine you are a full-grown caterpillar. You digest your final meal of leaves, then spin yourself a silken pad to hold you tight to a stem. You then split out of your old skin, revealing a new, smooth brown skin beneath. You no longer have eyes, or limbs, or any external openings except tiny holes called spiracles to allow you to breathe. You are entirely helpless, and will remain so for weeks, perhaps months in some species. Inside your shiny pupal skin your body dissolves, the cells of your tissues and organs pre-programmed to die and disintegrate, until you are little more than a soup. A few clusters of embryonic cells remain, and these proliferate, growing entirely new organs and structures, building you a brand-new body. Once it is ready, and the time is right, you split open your pupal skin and underneath have grown another one, this time complete with large eyes, a long, coiled proboscis for drinking, and beautiful wings covered in iridescent scales that you must inflate by pumping blood into their veins before they harden.

There is much debate as to how this astonishing phenomenon came about, including one recent and somewhat bizarre theory which suggested that metamorphosis evolved via a freak successful mating between a flying, butterfly-like insect and a velvet worm (a caterpillar-like relative of the arthropods). A more plausible suggestion is that caterpillars came about via the premature emergence of an embryonic insect from its egg. However they did it, metamorphosis is a remarkable phenomenon, and the insects that have this ability have become the most successful of all: flies, beetles, butterflies and moths, and wasps, ants and bees.

On the face of it, it may not be obvious why being able to transform oneself from a maggot into a fly is such a useful skill, impressive though it is. It seems like an awful lot of effort, and anyone who has ever reared butterflies can attest that emergence from the pupa is a delicate and precarious manoeuvre which often goes wrong, not least when wings fail to expand correctly, leaving the poor insect crippled and doomed. One theory as to why metamorphosis is such a successful strategy is that it enables the immature stages and the adults to each specialise in different tasks, and to have a body designed for the purpose.^* The larva is an eating machine, little more than a mouth and anus connected by a gut, which is pretty much all that a maggot is. It does not need to be able to move quickly or travel far, as its mother will have ensured that she laid her eggs somewhere where food was plentiful. Larvae tend to have only rudimentary senses, with poor eyesight and no antennae. The adults, on the other hand, are often short-lived and feed rather little, other than perhaps sipping on some nectar to fuel their activity.^* Their main task is to find a mate, copulate and, in the case of females, lay eggs. In some species they may also migrate. The adults need to be mobile and have acute senses, able to travel to seek out a partner by sight or smell or sound, so they often have large eyes and large antennae. They may also be equipped with bright colours to impress a potential mate.

For comparison, consider the lot of insects that do not go through metamorphosis. The grasshoppers or cockroaches, for instance. An immature grasshopper or cockroach is essentially a miniature version of the adult, with small wing ‘buds’ in place of functioning wings. Unlike insects that undergo metamorphosis, these young grasshoppers may have to compete for food with adult grasshoppers, something that is not a concern for a maggot or caterpillar. The body of a grasshopper is essentially a compromised design which has to be able to do everything: feeding, growing, dispersing, finding a mate, finding somewhere good to lay eggs. To be fair to grasshoppers, they succeed pretty well, as any farmer in Africa who has faced a swarm of hungry locusts can attest, but in terms of numbers of species they are outclassed by their metamorphosing cousins. There are about 20,000 known species of Orthoptera (grasshoppers and their kin), and 7,400 species of Blattodea (cockroaches). In contrast, insects undergoing metamorphosis include 125,000 species of Diptera (flies), 150,000 species of Hymenoptera (bees, ants and wasps), 180,000 species of Lepidoptera (butterflies and moths) and an astonishing 400,000 species of Coleoptera (beetles). Together, these four groups of insects comprise about 65 per cent of all known species on our planet.

Aside from flight and the magic of metamorphosis, the final trick insects pulled off during their evolution was the development of complex societies, in which teams of individuals work effectively almost as if they were a single ‘superorganism’. Termites, wasps, bees and ants all adopt this strategy, living in a nest with one or a small number of queens laying more or less all the eggs, and daughter workers performing various specialist jobs, such as caring for the queen, looking after the young, defending the nest and so on. By specialising, each individual insect can become an expert in its particular task, and in some cases even has a specially adapted body, as in for example the huge-jawed soldier castes found in some ant nests, principally engaged in defending the nest against attack by large predators such as anteaters or aardvarks. The famous American biologist E. O. Wilson, a specialist in ants, once estimated that there are in the region of between one and ten quadrillion individual ants in the world (1,000,000,000,000,000 to 10,000,000,000,000,000). In some terrestrial ecosystems they may make up 25 per cent of the total animal biomass, and overall the weight of ants on our planet is similar to the total weight of humans, to a very rough approximation. Ants alone outnumber us by about one million to one. Until perhaps the last 200 years, an alien looking down on Earth at any time in the last 400 million years would have concluded that this was the kingdom of the insects.

‘Femme Fatale’ Fireflies

Fireflies, also known in some countries as glow-worms, are surely among the most magical of insects. They are not flies at all, but a group of beetles possessing luminous bottoms. Their lights are used to attract a mate; different species glow green, yellow, red or blue, some producing a steady glow while others flash in a pattern particular to the species. In the European glow-worm, for instance, the female emits a gentle, steady green glow, which attracts the males. In many other species the glow is emitted in short flashes while in flight, which in the darkness creates the effect of a streak of light to the human eye, leading to another common name for these insects: lightning bugs. Some fireflies of the USA and tropical Asia glow in synchrony, creating a spectacular light display as thousands of insects flash their bottoms in unison.

Fireflies are predatory, variously feeding on other insects, worms or snails, depending on the species...