It appears probable that the partition has been destroyed, because the space between the two teguments is strewn with numerous bits, many of which adhere to the chitinous integument, while others are scattered along the edges of the wings, in their folds, or between the wings and the wall of the thorax.
Another series of sections showed that the exit of the fore wings had been accomplished, while the hinder pair was undergoing the process of eversion. In this case the partition showed signs of degeneration: deformation of the nuclei, indistinct cellular limits, pigmentation, granular leucocytes, and fatty globules.
After the destruction of the partition, what remains of the layer of the envelope is destined to make a part of the thoracic wall and undergoes for this purpose a superficial desquamation. The layer of flattened cells is removed and replaced by a firmer epithelium like that covering the other regions. It is this renewed hypodermis which conceals the wing within, serves to separate it from the cavity of the body, and gives the illusion of a complete change in its situation. Other changes occur, all forming a complete regeneration, but which does not accord with the description of Van Rees for the Muscidæ. Finally, Gonin concludes that the débris scattered about the wing comes from the two layers of the partition of the sac, from the flattened hypodermis of the renewed envelope, from the chitinous cuticle of the wing, and from the inner surface of the chitinous integument.
He thinks that the metamorphosis of Pieris is intermediate between the two types of Corethra and of Musca, established by Weismann, as follows:
Corethra.—The wing is formed in a simple depression of the hypodermic wall. No destruction.
Pieris.—The rudiment is concealed in a sac attached to the hypodermis by a short pedicel. Destruction of the partition and its replacement by a part of the thoracic wall by means of the imaginal epithelium.
Musca.—The pedicel is represented by a cord of variable length, whose cavity may be obliterated (Van Rees). The imaginal hypodermis is substituted for the larval hypodermis, which has completely disappeared, either by desquamation (Viallanes), or by histolytic resorption (Van Rees).
Extension of the wing; drawing out of the tracheoles.—When it is disengaged from the cavity, the wing greatly elongates and the creases on its surface are smoothed out; the blood penetrates between the two walls, and the cellular fibres, before relaxed and sinuous, are now firmly extended.
Of the two tracheal systems, the large branches are sinuous, and they are rendered more distinct by the presence of a spiral membrane; but the two tunics are not separated as in the other tracheæ of the thorax; moreover, the mouth choked up with débris does not yet communicate with that of the principal trunk. The bundles of tracheoles on their part form straight lines, as if the folds of the organ had had no influence on them. As they have remained bound together, apart from the chitinous membrane of the tracheal trunk, they become drawn out with this membrane, at the time of exuviation, i.e. of pupation, and are drawn out of the neighboring spiracle.
