Heterogeneous (paraphyletic) cluster group formerly summarized as Apterygota. There are 5 orders: bristletails, silverfishes (both in the past called Thysanura), Collembola, Diplura and Protura. The last 3 orders are remarkable smaller than the Thysanura (0, 1 – 15 mm). Their relationship, especially the monophyletic of Diplura, is different discussed. All 5 orders are always primary wingless.