Insects are the most diverse group of animals in existence. With over a million described species, and estimates of total species count ranging up to 30 million, insects make up between half and 90% of all animalian biodiversity. This is all the more impressive because insects aren't even an animal phylum, they're just a class within phylum Arthropoda. The oldest known insect fossil is Rhyniognatha hirsti from the Devonian, about 396 million years old. Since then, insects have been diversifying, particularly since the emergence of flowering plants about 125 million years ago.
The most primitive insect group is order Archaeognatha (jumping bristletails), which only has about 300 species. Along with order Thysanura (silverfish), these two orders are the only two within class Insecta that are wingless. Other wingless animals formerly classified as insects, such as springtails, proturans, and diplurans are now considered basal lineages apart from Insecta, and are instead placed in their own class, Entognatha. Formerly, all these wingless animals were called Apterygota, meaning "without wings."
All other insects have wings, being part of subclass Pterygota. Formerly, Pterygota was divided into infraclasses Palaeoptera, or "old wing" and Neoptera, or "new wing," but now it is suspected that these categories are paraphyletic. That means that members of the infraclasses may not be exclusive descendants of a common ancestor, and hence not a true clade. Still, the categorizations may be useful as a reference to body type. Palaeoptera consists of insects that lack the ability to fold their wings back over their abdomen, while Neopterans can. The fossil record for Palaeoptera is extensive, but today the group only includes two surviving orders: mayflies and dragonflies. Most other insects are Neopterans.
Neoptera is a huge group, containing 32 orders, five of which are extinct. These orders are tentatively placed in seven superorders based on physiological characteristics and suspected common ancestry. The members of Neoptera include webspinners, angel insects, earwigs, grasshoppers, stick insects, ice-crawlers, gladiators, cockroaches, termites, mantids, booklice, barklice, thrips, lice, true bugs, ants, bees, wasps, beetles, twisted-winged parasites, snakeflies, alderflies, net-veined insects, scorpionflies, fleas, snow fleas, flies, caddisflies, butterflies, and moths.