Palaeoptera

The name Palaeoptera (from Greek ( 'old') + ( '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.

==Disputed status== The complexities of the wing-folding mechanism, as well as the mechanical operation of the wings in flight (indirect flight muscles), are such that it indicates that Neoptera is a monophyletic lineage. The plesiomorphic absence of wing-folding does not necessarily mean the Palaeoptera form a natural group – they may be an assemblage containing all insects, closely related or not, that "are not Neoptera", an example of a wastebasket taxon. If the extinct lineages are taken into account, it is likely that the concept of Palaeoptera will eventually be discarded or changed in content to more accurately reflect insect evolution.