Insects are the most successful members of the animal kingdom: roughly one million described species, an estimated five to ten million in total, and a substantial fraction of Earth's terrestrial biomass. The roots of this success reach back to the Devonian, when plants were beginning to spread from coastal fringes into continental interiors.

The oldest fossil record of hexapods — the six-legged class to which insects belong — comes from the Rhynie Chert of Scotland, dating to around 407 million years ago. Rhyniognatha hirsti, this early hexapod, already possessed jaw anatomy characteristic of modern insects. The fossil record grows markedly richer at the Devonian-Carboniferous transition: the swampy coastal forests of that period offered ideal breeding and shelter environments.

Flight is the single most transformative innovation in insect evolution. Wings appeared in certain insect lineages around 325 million years ago — a full 150 million years before any vertebrate took to the air. Atmospheric oxygen in the Carboniferous was far higher than today (roughly 35% versus 21%), boosting respiratory capacity and permitting gigantic body sizes. Meganeura monyi, a dragonfly-like predator of the Carboniferous, had a wingspan of up to 65 centimetres and ranks among the largest insects ever to have lived.

The ecological importance of insects extends far beyond their own survival. Pollination of flowering plants, decomposition of organic matter, and the role of intermediate consumer in food webs — all depend critically on insects. When angiosperms diversified explosively in the Cretaceous, insects co-evolved with them: increasingly specialised flower-pollinator relationships drove diversity in both groups simultaneously. Today, insect biodiversity in tropical forests is directly proportional to plant diversity.

The reason for insect success almost certainly lies in small body size, short generation times, and the dispersal capacity that flight provides. Together, these traits allow rapid evolutionary responses to changing environments; even during mass extinctions in the fossil record, insects are among the least affected groups.