Class of arthropods that have three pairs of legs and, usually, two pairs of wings borne on the thorax. Typically, there is a single pair of antennae and one pair of compound eyes. Gas exchange takes place through a tracheal system and the gonoducts open at the posterior end of the body. The oldest fossil insects occur in Devonian rocks, and the first winged representatives are known from Carboniferous rocks. Dragonflies and beetles were established before the end of the Palaeozoic; social varieties such as ants and wasps are present in Cretaceous sediments. The evolution of the flowering plants had a marked influence on insect development, so that many new forms appeared in the Cretaceous. More than 750 000 extant species of insects have been described. Insects are now believed to be descended from a crustacean ancestor, and the Hexapoda and Crustacea are grouped in the clade Pancrustacea.