Insects

Insects are by far the most successful group of animals on Earth. There are around 1 million known species of insect, and probably several million more that have not yet been described. Roughly 4 out of every 5 animal species on the planet is an insect. At any moment, the Earth's surface holds an estimated 10 quintillion individual insects: more than a billion insects for every human on the planet. Most are tiny and harmless; many are crucial pollinators; a few are pests; and some are starting to disappear in worrying numbers.

• Known speciesapprox. 1 millionapprox. 80% of all animal species
• Estimated total5 to 10 millionMost undiscovered
• Total individualsapprox. 10 quintillionapprox. 1.4 billion insects per human
• Six legsAlwaysSix-leggedness is the defining trait
• First appearedapprox. 400 million years agoBefore dinosaurs, before forests
• Most successful orderBeetles (Coleoptera)Over 400,000 species

What makes an insect?

• Six legs (three pairs): the most reliable defining feature.
• Three body parts: head, thorax (middle), abdomen (rear).
• One pair of antennae: for smelling, touching and tasting.
• An exoskeleton: a tough outer body case made of chitin.
• Compound eyes: each made of hundreds or thousands of tiny lenses.
• Often wings: most adult insects have one or two pairs of wings (the only invertebrates that fly).

The main insect groups

• Beetles (Coleoptera): the biggest order of all, over 400,000 species. Includes ladybirds, dung beetles, weevils, fireflies, stag beetles.
• Flies (Diptera): around 160,000 species. Just two wings instead of four. Includes houseflies, mosquitoes, hoverflies, fruit flies.
• Butterflies and moths (Lepidoptera): around 180,000 species. Beautiful scaled wings.
• Bees, wasps and ants (Hymenoptera): around 150,000 species. Mostly social, with complex colonies.
• True bugs (Hemiptera): around 80,000 species, including aphids, cicadas, water boatmen and bedbugs.
• Dragonflies and damselflies (Odonata): around 6,000 species. Ancient and impressive predators.
• Grasshoppers and crickets (Orthoptera): around 25,000 species.
• Stick and leaf insects (Phasmida): around 3,000 species. Champions of camouflage.

Why insects are so successful

Several features explain why insects rule the planet.

• Small size: lets them live in tiny niches with very little food.
• Flight: lets them disperse easily and escape predators.
• Fast life cycle: many species go from egg to adult in just weeks, so they can adapt to new environments quickly.
• Huge variety of diets: there are insects that eat almost every other living thing on Earth.
• Exoskeleton: protects them from drying out and from many predators.
• Specialised mouthparts: have evolved into thousands of different forms (sucking, chewing, piercing, lapping) for every possible food.

Insect lifestyles

Insects have evolved into almost every possible role.

• Pollinators: bees, butterflies, moths and many flies pollinate around 75% of food crops.
• Decomposers: dung beetles, carrion beetles, flies and others recycle dead matter.
• Predators: dragonflies, mantises, beetles, wasps hunt other insects.
• Herbivores: grasshoppers, caterpillars, aphids eat plants.
• Parasites: lice, fleas, parasitic wasps live on or in other animals.
• Social insects: bees, ants, wasps and termites live in colonies of thousands or millions, with specialised castes.
• Builders: termites build mounds up to 9 metres tall; honey bees build combs of perfect hexagons.

Fact The total biomass of all the ants on Earth is roughly the same as the total biomass of all humans. There are an estimated 20 quadrillion (20,000,000,000,000,000) ants on the planet, weighing about 12 megatonnes of carbon. They have been on Earth for about 100 million years and are now found on every continent except Antarctica. In many ecosystems, ants move more soil than earthworms do.

Metamorphosis: the great transformation

Many insects go through a dramatic body transformation called metamorphosis. There are two main types.

• Complete metamorphosis (butterflies, beetles, flies, bees): egg → larva (caterpillar / grub / maggot) → pupa (chrysalis / cocoon) → adult. Inside the pupa, the larva's body breaks down almost completely and rebuilds itself as an adult.
• Incomplete metamorphosis (grasshoppers, crickets, true bugs, dragonflies): egg → nymph (a small wingless version of the adult) → adult. The nymph gradually grows wings over several moults.

Did you know? Insect populations are declining sharply in many parts of the world. Recent studies have found that some long-term sampling sites have lost over 75% of their flying insects in just 30 years. The causes include habitat loss, pesticides, climate change, light pollution and invasive species. This "insect apocalypse" matters far beyond insects themselves: birds, bats, frogs and many fish depend on insects for food, and roughly 75% of food crops need insect pollinators.

Deeper dive: why insects can never grow huge

One of the great trivia questions in biology is: why are there no insects the size of dogs? In the Carboniferous period 300 million years ago, dragonflies with wingspans of nearly a metre flew through swamp forests. There were giant cockroaches and centipede-relatives 2 metres long. Why nothing that big today?

Several reasons keep modern insects small.

• Breathing: insects do not have lungs. They breathe through a network of tiny tubes (tracheae) that bring oxygen directly to each cell. This works fine for small insects, but the bigger the insect gets, the longer the tracheae have to be, and the harder it is to get oxygen everywhere. The Carboniferous giants were possible because the atmosphere then was about 35% oxygen (compared to 21% today). With more oxygen in the air, the breathing tubes could be smaller relative to body size, and insects could grow bigger.
• Exoskeleton: insects wear their skeleton on the outside. As an insect grows, the exoskeleton has to get bigger and thicker. At some point the weight of the shell would crush the insect inside. This is roughly why crab- or beetle-style bodies cannot scale up to mammal size.
• Moulting: insects grow by shedding their exoskeleton and producing a new bigger one. While the new exoskeleton is hardening, the insect is helpless. The bigger the insect, the longer this dangerous period.
• Birds: when birds appeared 150 million years ago, they ate big insects faster than the insects could survive. Smaller insects were too small to be worth catching, so they survived; bigger insects became dinner. After 150 million years of selection, modern insects are all relatively small.

None of these obstacles is absolute. If oxygen levels rose again, or birds disappeared, insects might evolve to be much bigger over millions of years. For now, the limits are set, and we live in a world of small insects, not giant ones. Lucky for us, perhaps.

For other invertebrate groups, see arachnids, crustaceans and molluscs.