🔬 Insect classification

Scientists define an insect by a specific set of features, a body divided into three sections, head, thorax and abdomen, exactly six jointed legs, and usually one or two pairs of wings attached to the thorax. This precise definition is why spiders, centipedes and woodlice, despite often getting lumped in as bugs, do not actually qualify as insects at all. Having such a clear checklist lets scientists sort millions of different species into one enormous, organised group.

Scientists have already described more than a million distinct insect species, and many researchers believe millions more are still waiting to be discovered, especially in places like tropical rainforests. New species turn up constantly, sometimes even in cities or backyards, since insects are small enough to easily go unnoticed. Beetles alone make up such a huge fraction of known insects that one famous scientist joked the creator must have had an unusual fondness for them.

An exoskeleton is a hard outer shell made mostly of a tough material called chitin, acting as both armour and structural support since insects have no internal bones at all. Because muscles attach to the inside of this shell, insects gain surprising strength for their size, but the rigid shell also cannot stretch, which is exactly why molting becomes necessary as they grow. This same basic body plan, evolved hundreds of millions of years ago, has proven so successful that insects have barely needed to change it since.

Insects, spiders and crabs all belong to a much larger group of animals called arthropods, united by having jointed legs and a hard exoskeleton instead of an internal skeleton. Within that huge group, insects form their own branch separate from arachnids like spiders and crustaceans like crabs, each with slightly different body plans and numbers of legs. Scientists actually think crustaceans and insects share a surprisingly close ancestor, closer than many people would guess just by looking at a crab and a beetle side by side.