Penultimate instar passive in some cases.
In the metabolic Hexapoda the resting pupal instar shows externally the wings and other characteristic imaginal organs which have been gradually elaborated beneath the larval cuticle.
The change from an exopterygote to an endopterygote development could, therefore, be brought about by the gradual postponement to a later and later instar of the appearance of the wing-rudiments outside the body, and their correlated growth inwards as imaginal disks.
Young resembling parents, but penultimate instar passive and enclosed in a filmy pellicle.
In at least one genus the unjointed pincers of the forceps are preceded, in the youngest instar by jointed cerci.
Further, although the wing-rudiments appear externally in an early instar of an exopterygotous insect, the earliest instars are wingless and wing-rudiments have been previously developing beneath the cuticle, growing however outwards, not inwards as in the larva of an endopterygote.
As the life-conditions and feedinghabits of the larva and imago become constantly more divergent, the appearance of the wing-rudiments would be postponed to the pre-imaginal instar, and that instar would become predominantly passive.
Penultimate instar a flying sub-imago.
Some pupae are thus more active than some nymphs; the essential character of a pupa is not therefore its passivity, but that it is the instar in which the wings first become evident externally.
The new instar - or temporary form - is often very different from the old one, and this is the essential fact of metamorphosis.