Insects

If you could count every animal on Earth, the overwhelming majority would be insects. Around a million species have been named, more than all the other animals combined, and entomologists think several million more await discovery. There are perhaps ten quintillion individual insects alive at this moment. They were the first animals to fly, they pollinate most of the plants we eat, and they are the base of food webs on every continent except Antarctica. They are small, which is why we overlook them, and small is exactly why they succeeded.

The design

An insect's body comes in three sections: a head carrying the eyes, antennae and mouthparts; a thorax carrying six legs and, usually, two pairs of wings; and an abdomen holding the gut and reproductive organs. Instead of bones inside, it wears its skeleton outside, a hard shell of a material called chitin. The exoskeleton is light, strong and waterproof, but it cannot grow, so a young insect must periodically split it and crawl out, soft and vulnerable, to swell into a new, larger one.

Insects breathe without lungs. Air enters through small holes along the sides of the body and travels through tubes directly to the tissues. This works beautifully at small sizes and badly at large ones, which is the main reason there are no cat-sized insects, except in films. The largest living insects, such as the giant wetas of New Zealand and some beetles, weigh under a hundred grams. Three hundred million years ago, when the air held more oxygen, dragonflies had wings 70 centimetres across.

Why so many?

Several things explain the insects' extraordinary diversity. Being small, they can divide a single tree into hundreds of separate homes: leaf, bark, root, fruit, gall. Flight, which they invented about 400 million years ago, lets them find food and mates over wide areas and escape predators. Short lives and large families mean fast evolution. And metamorphosis, which most insects undergo, lets one species exploit two completely different ways of life. A caterpillar is an eating machine; a butterfly is a flying, mating machine; they do not compete with each other for food. More than 80 percent of insects, including beetles, flies, bees, wasps, butterflies and moths, rebuild their bodies this way inside a pupa.

Beetles are the champions: about 400,000 named species, one in every four known animals. When asked what his studies had taught him about the Creator, the biologist J. B. S. Haldane is said to have replied that He had "an inordinate fondness for beetles".

Societies

Some insects live in colonies that behave almost like single organisms. An ant or honeybee colony has one or a few egg-laying queens and thousands or millions of sterile workers, who forage, build, nurse the young and defend the nest, communicating by chemical signals and, in bees, by a dance that encodes the direction and distance of flowers. Leafcutter ants farm fungus on chewed leaves; weaver ants sew nests from living leaves using silk squeezed from their own larvae; termites build towers metres high with ventilation systems that keep the interior at a steady temperature. These societies arose independently several times, and a single Argentine ant supercolony stretches 6,000 kilometres along the Mediterranean coast.

What insects do for us

Insects pollinate roughly three quarters of the world's crops, including most fruits, vegetables and nuts; without them diets would shrink to grains and little else. They recycle dead plants, animals and dung at a scale no other group matches. They are food for most birds, freshwater fish, amphibians and many mammals, and for two billion people. Silk, honey, beeswax, dyes and medicines come from them.

They also cost us. Locusts have destroyed harvests since the dawn of farming, and mosquitoes, by carrying malaria, dengue and yellow fever, have killed more people than any other animal, including other humans.

What we still don't know

The sheer scale of insect life means we have barely catalogued it; a single hectare of tropical forest canopy can yield species never seen before by the dozen. More worryingly, surveys in Europe and North America suggest flying insect numbers have fallen by three quarters in some places over thirty years, through habitat loss, pesticides and climate change. Why, how widespread it is, and what happens to the birds, fish and crops that depend on them, are questions biologists are now racing to answer.