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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.”