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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: