We are therefore entitled to assume that the suppressed wings of Exopterygota tend to reappear; and, speaking of the past, we may say that if after a period of suppression the wings began to reappear as hypodermal buds while a more rigid pressure was exerted by the cuticle, the growth of the buds would necessarily be inwards, and we should have incipient endopterygotism.
If it should be objected that the wings so developed would be rudimentary, and that there would be nothing to encourage their development into perfect functional organs, we may remind the reader that we have already pointed out that imperfect wings of Exopterygota do, even at the present time under certain conditions, become perfect organs; and we may also add that there are, even among existing Endopterygota, species in which the wings are usually vestiges and yet sometimes become perfectly developed.
Sub-class Exopterygota Order Plecoptera.
Now there are many forms of Exopterygota in which the creatures are almost or quite destitute of wings.
All such families - falling into the group Exopterygota as defined in the classification of the Hexapodawere separated from the Neuroptera by W.
The change that is required to transform Exopterygota into Endopterygota is merely that a cell of hypodeimis should proliferate inwards instead of outwards, or that a minute hypodermal evaginated bud should be forced to the interior of the body by the pressure of a contracted cuticle.
These ancient Exopterygota were synthetic in type, and included insects that may, with probability, be regarded as ancestral to most of the existing orders.
It is hard to arrange the Exopterygota in a linear series, for some of the orders that are remarkably primitive in some respects are rather highly specialized in others.
Hence the grouping of the orders of winged Hexapoda into the divisions Exopterygota and Endopterygota, as suggested by D.
Altogether, eight orders are recognized in the classification adopted here, the first five of these belonging to the sub-class Exopterygota and the last three to the Endopterygota (see Hexapoda).