They undergo a radical metamorphosis, with the larval and adult stages differing considerably in their structure and behaviour.

[3] The Holometabola are sometimes divided into three assemblages: Neuropterida (Neuroptera, Megaloptera, Raphidioptera, Strepsiptera and Coleoptera), Hymenopteroida (Hymenoptera), and Panorpida (Siphonaptera, Diptera, Trichoptera, Lepidoptera and Mecoptera).

Molecular analysis has clarified the group's phylogeny, as shown in the cladogram.

[4] Hymenoptera (sawflies, wasps, ants, bees) Raphidioptera (snakeflies) Megaloptera (alderflies and allies) Neuroptera (Lacewings and allies) Coleoptera (beetles) Strepsiptera (twisted-wing parasites) Trichoptera (caddisflies) Lepidoptera (butterflies, moths) Diptera (true flies) Mecoptera (scorpionflies) Siphonaptera (fleas) The Endopterygota are distinguished from the Exopterygota by the way in which their wings develop.

The latter trait is plesiomorphic, however, as it is found also in groups such as Odonata (dragonflies and damselflies), which are not Neoptera, but more basal among insects.