Springtail Insect On A Green Leaf. Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Though the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be more closely related to one another than they all are to insects, which have external mouthparts.