The document summarizes the evolution of insects over 400 million years based on the fossil record, from the Ordovician period to the Triassic period. Early insect fossils from the Ordovician period were small in size. By the Carboniferous period, insects had already diversified greatly but their fossils remain incomplete. Some insect groups from this time, such as Meganisoptera, were very large while others were similar in size to modern insects. The Permian period saw the emergence of the first holometabolous insects. Many modern insect orders appeared by the Triassic period, though the fossil record remains imperfect.