588 Wm S. Marshall, 



Platyphylax push out from the peripodial cavity and become external 

 is not known; very many larvae were examined but in no case could 

 there be seen, from an external view, any indication that the wing 

 rudiments were partially protruded. It is very doubtful that if a 

 specimen with the rudiment partially protruded was cleared and mounted 

 in baisam any difference could be seen between such a wing rudiment 

 and one entirely within the cavity, as the folding and the transparency 

 would render it difficult to distinguish the one from the other. One 

 half of the thorax of one specimen which we possess was mounted 

 and, as the metathoracic rudiment showed very plainly the broad 

 shape characteristic before elongation and also, dorsally, a lobe which 

 had appeared (Fig. 16) the other half of the same specimen was cut 

 transversely. When studying these sections it was seen that while 

 the rudiment of the metathorax was as yet entirely within the peri- 

 podial cavity (Fig. 16^4) that of the other wing bearing segment was 

 partially protruded (Fig. \%B). A comparison of these two sections 

 shows that the lip forming the margin of the peripodial pore has re- 

 ceeded making an opening nearly as large as the surface of the rudiment 

 itself. The distal margin of the rudiment is seen in the metathorax 

 (Fig. 16.4) to be curved towards the body; in the mesothoracic rudi- 

 ment (Fig. 165) this part has straightened and become external. What 

 causes the great enlargement of the peripodial pore we are unable to 

 say but, this once started, a pressure from within would easily force 

 the entire rudiment from the cavity and place it external to the body. 

 A further study of these sections shows that in the space between the 

 two layers of the rudiment there are very many leucocytes which 

 would tend to confirm the view that the body fluid plays the principal 

 part in the expulsion of the wings. The opening of the peripodial pore 

 might be due to an increase in the diameter of the thorax and this, 

 followed by a sudden forcing of the body fluid into the rudiment would 

 result in its change from internal to external. There is a thin cuticular 

 layer covering the wing at this stage. 



Mercer (8) gives an account of several different theories regard- 

 ing the expulsion of the wings and then details his own views of the 

 same occurence. He notes the difficulty in accounting for the great 

 enlargement of the peripodial pore and then describes a fusion of the 

 two membranes forming the lip, these are the outer hypodermal 

 layer and the inner peripodial membrane where they pass into each 

 oth^r and form the lip. He holds that these layers fuse, the wall thus 

 lormed breaks down and dissapears and we have an enlarged opening 



