The Pterygota are a subclass of insects that includes the winged insects. It also includes insect orders that are secondarily wingless (that is, insect groups whose ancestors once had wings but that have lost them as a result of subsequent evolution). The pterygotan group comprises almost all insects. The insect orders not included are the Archaeognatha (jumping bristletails) and the Thysanura (silverfishes and firebrats), two primitively wingless insect orders. Also not included are the three orders no longer considered to be insects: Protura, Collembola, and Diplura.