Apterygota in the context of "Dicondylia"

In biology, the imago (Latin for "image") is the last stage an insect attains during its metamorphosis, its process of growth and development; it is also called the imaginal stage ("imaginal" being "imago" in adjective form), the stage in which the insect attains maturity. It follows the final ecdysis of the immature instars.

In a member of the Ametabola or Hemimetabola, species in which metamorphosis is "incomplete", the final ecdysis follows the last immature or nymphal stage. In members of the Holometabola, in which there is a pupal stage, the final ecdysis follows emergence from the pupa, after which the metamorphosis is complete, although there is a prolonged period of maturation in some species.

The name Palaeoptera (from Greek παλαιός (palaiós 'old') + πτερόν (pterón 'wing')) has been traditionally applied to those ancestral groups of winged insects (most of them extinct) that lacked the ability to fold the wings back over the abdomen as characterizes the Neoptera. The Diaphanopterodea, which are palaeopteran insects, had independently and uniquely evolved a different wing-folding mechanism. Both mayflies and dragonflies lack any of the smell centers in their brain found in Neoptera. Their midgut epithelium has a dual origin: the anterior and posterior regions develop through cellular differentiation, while the central region originates from yolk cells. In all other winged insects, the midgut epithelium is formed solely by cellular differentiation, whereas in the remaining hexapods, including Apterygota, it develops entirely from yolk cells. Which suggests that Palaeoptera represent a transitional stage between these two developmental strategies.

The Entognatha, from Ancient Greek ἐντός (entós), meaning "inside", and γνάθος (gnáthos), meaning "jaw", are a class of wingless and ametabolous arthropods, which, together with the insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning that they are retracted within the head, unlike the insects. Entognatha are apterous, meaning that they lack wings. The class contains three orders: Collembola (springtails, 9000 species), Diplura ("two-tail", 1000 species) and Protura ("first-tail", 800 species). These three groups were historically united with the now-obsolete order Thysanura to form the subclass Apterygota, but it has since been recognized that these orders might not be closely related, and Entognatha and Apterygota are now both considered to be paraphyletic groups.

Thysanura is the now deprecated name of what was, for over a century, recognised as an order in the class Insecta. The two constituent groups within the former order, the Archaeognatha (jumping bristletails) and the Zygentoma (silverfish and firebrats), share several characteristics, such as of having three long caudal filaments, the lateral ones being the cerci, while the one between (telson) is a medial cerciform appendage, specifically an epiproct. They are also both wingless, and have bodies covered with fine scales, rather like the scales of the practically unrelated Lepidoptera. In the late 20th century, it was recognized that the two suborders were not sister taxa, therefore Thysanura was paraphyletic, and the two suborders were each raised to the status of an independent monophyletic order, with Archaeognatha sister taxon to the Dicondylia, including the Zygentoma.

Although the group Thysanura is no longer recognized, the name still appears in some published material. Another name used to separate the two groups from winged insects is Apterygota.