A. G. Mayer has examined the late development of the wings in Pieris rapæ. Fig. 149 represents a frontal section through the left wing of a mature larva and shows the rudiment of the wing, lying in its hypodermal pocket or peripodal cavity. How the trachea passes into the rudimentary wing, and eventually becomes divided into the branches, around which the main veins afterwards form, is seen in Figs. 144, 147, 159.
The histological condition of the wing at this time is represented by Fig. 151, the spindle-like hypodermal cells forming the two walls being separated by the ground-membrane of Semper.
“While in the pupa state,” says Mayer, “the wing-membrane is thrown into a very regular series of closely compressed folds, a single scale being inserted upon the crest of each fold. When the butterfly issues from the chrysalis, these folds in the pupal wings flatten out, and it is this flattening which causes the expansion of the wings.... It is evident that the wings after emergence undergo a great stretching and flattening. The mechanics of the operation appears to be as follows. The hæmolymph, or blood, within the wings is under considerable pressure, and this pressure would naturally tend to enlarge the freshly emerged wing into a balloon-shaped bag; but the hypodermal fibres (h) hold the upper and lower walls of the wing-membrane closely together, and so, instead of becoming a swollen bag, the wing becomes a thin flat one. And thus it is that the little thick corrugated sac-like wings of the freshly emerged insect become the large, thin, flat wings of the imago.... The area of the wing of the imago of Danais plexippus is 8.6 times that of the pupa. Now, as the wing of the young pupa has about 60 times the area of the wing in the mature larva, it is evident that in passing from the larval state to maturity the area of the wings increases more than 500 times.”
Fig. 150.—Diagrammatic reproduction of Fig. 149 showing the wing-germ in its peripodal cavity (p): h’drm, hypodermis; tr, trachea; cta, cuticula; a, anterior end.—After Mayer.
Fig. 151.—Section of the wing-germ, the upper and lower sides connected by spindle-like hypodermic cells (h), forming the rods of the adult wing; mbr, ground-membrane of Semper.—After Mayer.
f. The primitive origin of the wings
Farther observations are needed to connect the mode of formation of the wings in the holometabolous insects with the more primitive mode of origin seen in the hemimetabolous orders, but the former mode is evidently inherited from the latter. Pancritius remarks that the development of the rudiments of the wing in a hypodermal cavity is in the holometabolic insects to be regarded as a later inherited character, the external conditions causing it being unknown.
Fritz Müller was the first to investigate the mode of development of the wings of the hemimetabolic insects, examining the young nymphs of Termites. He regards the wings as evaginations of the hypodermis, which externally appear as thoracic scale-like projections, into which enter rather late in nymphal life tracheæ which correspond to the veins which afterward arise.