Its condition thus recalls the pupal instar of the higher (Endopterygote) Hexapoda; and the Thysanoptera, though few in number, are seen to be of great interest to the student, exhibiting at once a transition between the biting and the suctorial mouth, and the passage from " incomplete " to " complete " metamorphosis.
The new instar - or temporary form - is often very different from the old one, and this is the essential fact of metamorphosis.
Among a few of the beetles or Coleoptera, and also in the neuropterous genus Mantispa, are found life-histories in which the earliest instar is campodeiform and the succeeding larval stages eruciform.
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.
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.
Penultimate instar a flying sub-imago.
Young resembling parents, but penultimate instar passive and enclosed in a filmy pellicle.
Penultimate instar passive in some cases.
Young animals always unlike parents, the wing-rudiments developing beneath the larval cuticle and only appearing in a penultimate pupal instar, which takes no food and is usually passive.
Pupa incompletely obtect or free, and enclosed in the hardened cuticle of the last larval instar (puparium).
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.
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.
In at least one genus the unjointed pincers of the forceps are preceded, in the youngest instar by jointed cerci.
- a, Cast-off nymphal skin of Bed-bug (Cimex lectularius); b, Second instar after emergence from a; c, The same after a meal.
2d, 3 c) is always "free," the legs, wings and other appendages not being 1 Instar is a convenient term suggested by D.