1915} Middle Membrane in Wings of Platyphylax 205 
young developing wings become external (Text-figure I, D). 
The wing veins become clearly discernable from a surface view 
and in section they appear as empty spaces alternating with 
those parts where the two layers of the wing remain connected. 
While still within the last larval skin the wings become so large 
that, confined as they are by this covering, a second folding 
becomes necessary; during this: period the wings attain their 
greatest thickness. When the last larval skin splits and the 
pupa is free the wings straighten and grow much thinner. The 
cuticular covering now formed by the wings soon becomes too 
small for their growth and another period of folding ensues, the 
wings remain thus folded until the emergence of the imago when 
they unfold and assume their definite form and shape. 
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A. B 
TEXT-FIGURE I. 
Four views of the young developing wing of Platyphylax designatus. A, thicken- 
ing of the hypodermis; B, after invagination of this same thickened portion; C, the 
wing rudiment has evaginated, but still lies within the peripodial cavity; D, 
soon after the wing rudiment has left the peripodial cavity and become external. 
In all figures the external surface of the larva is to the right; the hypodermis 
only is represented, the cuticula not drawn. Figs. A, B and C, X 220; D, X 105. 
After evagination of the imaginal disk each wing rudiment 
consists of two layers of hypodermal cells folded down into the 
peripodial cavity and each wall of the fold is a layer of cells 
similar, except in its greater thickness, to the continuous hypo- 
dermal layer forming the wall of this cavity. The two layers 
of the rudiment lie close to each other but in the early stages of 
development do not touch along any part of their opposing 
surfaces (Fig. 1, A). Each layer is seen in section to be com- 
