How many legs does an insect have?

Six, on three body sections, and that count is the entire membership test. Spiders have eight and are not insects; centipedes and millipedes have far more and are not either; woodlice are crustaceans, more closely related to a crab than to a beetle.

The group they define is the most successful on Earth by almost any measure. Around a million insect species have been described and estimates of the true number run into the millions; roughly one in four known animal species is a beetle. J.B.S. Haldane's line, when asked what study of creation had revealed about its creator, was that He appeared to have an inordinate fondness for beetles.

Their size ceiling is set by how they breathe. Insects have no lungs; air diffuses in through holes along the body and reaches the tissues through fine tubes, which works beautifully up to a point and then stops scaling. In the Carboniferous, when the atmosphere held perhaps 35 per cent oxygen rather than today's 21, dragonflies had wingspans of seventy centimetres. As oxygen fell, the giants went with it.