Pterygota group comprises 99.9% of all insects.[4] The orders not included are theArchaeognatha (jumping bristletails) and theZygentoma (silverfishes andfirebrats), two primitively wingless insect orders. Unlike Archaeognatha and Zygentoma, the pterygotes do not have styli or vesicles on their abdomen (also absent in some zygentomans), and with the exception of the majority of mayflies, are also missing the median terminal filament which is present in the ancestrally wingless insects.[5][6][7]
The oldest known representatives of the group appeared during the mid-Carboniferous, around 328–324 million years ago, and the group subsequently underwent rapid diversification. Claims that they originated substantially earlier during theSilurian orDevonian based onmolecular clock estimates are unlikely based on the fossil record, and are likely analytical artefacts.[8]
Traditionally, this group was divided into theinfraclassesPaleoptera andNeoptera.[9] The former are nowadays strongly suspected of beingparaphyletic, and better treatments (such as dividing or dissolving the group) are presently being discussed[citation needed]. In addition, it is not clear how exactly the neopterans are related among each other. TheExopterygota might be a similar assemblage of rather ancienthemimetabolous insects among the Neoptera like the Palaeoptera are among insects as a whole. TheholometabolousEndopterygota seem to be very close relatives, indeed, but nonetheless appear to contain severalclades of related orders, the status of which is not agreed upon.
The following scheme uses finer divisions than the one above, which is not well-suited to correctly accommodating the fossil groups.
