1915] Middle Membrane in Wings of Platyphylax 213 
are still present but now restricted to the perpendicular strands 
(Fig. 11). 
The wings reach their maximum thickness after the con- 
traction of the pupa and they then lie stretched over and at the 
sides of its body; this chitinous case which now encloses them 
while ample in extent for the wings in their present condition is 
not sufficient to allow of the next growth, that of an increase in 
surface area. When this occurs it is necessary for the wings to 
decrease in thickness and become very much folded which 
folding continues until the imago is ready to issue from the 
pupal skin. To the perpendicular strands that pass across the 
wing from one surface to the other Mayer (5) has assigned a 
probable contractile power and to this attributed the drawing 
together of the two surfaces of the wing. This would account 
for the decrease in thickness and the limited area of the cuticular 
sack which now encloses the wings would necessitate the folding 
which becomes so marked during the remainder of pupal life. 
The narrowing of the wing is noticeable before its folding has 
commenced. 
During the process described above certain changes take 
place in the different layers of the wing, of these the most 
noticeable is the reforming of the middle membrane which 
again occupies its old place in the median part of the wing 
(Fig. 12) and is now thinner and more membrane like than 
during the earlier stages before its disappearance. The per- 
pendicular strands again pass from the hypodermis to the mid- 
dle membrane and are irregular and branched. The degen- 
erated nuclei which earlier wandered from the hypodermal 
layers to the middle membrane and which, upon the disappear- 
ance of the latter, remained on the perpendicular strands (Fig. 
11) are again found in the middle membrane. In the hypo- 
dermal layers many nuclei are seen which have wandered away 
from the outer surfaces of the wing and come to he between the 
hypodermis and the middle membrane. These are the nuclei 
of those cells, trichogens, from which later will develop the hairs 
upon the surface of the wings. It is at once noticed that these 
trichogens are more abundant upon one side of the sections than 
upon the other; this fact, knowing that there are many more 
hairs upon the dorsal than upon the ventral surface of the wing, 
enables one to distinguish these surfaces from each other rather 
early in pupal life. 
