Why are there so many insects?

Biologists point to a few things that came together: a light suit of armour suited to life on land, wings, a very long history, and a partnership with flowering plants. The result is staggering. More than a million insect species have been named, which is more than half of all animal species, and perhaps 5.5 million exist in total. Five groups alone (beetles, flies, butterflies and moths, wasps with ants and bees, and true bugs) each have more than 100,000 named species.

Start with the body. An insect wears its skeleton on the outside, as a tough shell. As an adaptation to life on land, insects harden that shell with an enzyme that uses oxygen from the air, rather than the heavy calcium that crustaceans use, so the armour stays light. Insects are found on every continent and in almost every land habitat. Only perhaps a hundred species live in the sea.

Then add flight. Insects are the only invertebrates that can fly under their own power for long stretches. The oldest known flying insects are about 328 to 324 million years old, and after that the group went through a rapid, explosive diversification. Some of the most successful groups later grew up alongside flowering plants, each shaping the other.

People have long been struck by the body plan. Our word comes from the Latin for "cut up", because an insect looks as if it had been cut into three parts: head, thorax and abdomen, with three pairs of legs on the thorax. That is also the quick test when you meet a creepy-crawly: spiders, centipedes, mites and scorpions wear jointed armour too, which is why they get confused with insects, but only an adult insect has that three-part body and three pairs of legs.