Tlie Development of tlie Wings of a Caddis-fly Platj'phylax designatus Walk. 591 



and in figure 23 we have another and somewhat older specimen in 

 which the folding is a little more complicated. In the latter figure 

 the wing veins are miich more distinct than in the former, this is due 

 to the fact that different methods of preparation and varying degrees 

 of staining make the venation more distinct in some speeimens than 

 in others. That the folding of the wings becomes very complicated at 

 this period of development is seen from figure 24 which is a section 

 of one of the wings shown in figure 23. In the section the veins are 

 shown cut in different directions. 



The pupa when it has cast the last larval skin is still nearly equal 

 in length to the last larval stage; this soon changes, the pupa shortens 

 and the wings elongate. Each wing is surrounded by its own cuticular 

 layer but the pressure of the last larval skin being removed they loose 

 the folds we have mentioned and become straightened out along the 

 body and at the sides of the pupa. As the wings straighten they also 

 lengthen until they extend back to the middle or end of the fourth 

 abdominal segment, seldom as far as the middle of the fifth segment. 

 In this extended position the anterior wing lies in part over the posterior 

 one (Fig. 25) and they both bend ventrally under the anterior part 

 of the abdomen (Fig. 26). It has just been mentioned that each grow- 

 ing wing is in this stage surrounded by a cuticular layer which soon 

 encloses it as in a sac, as the wing continues to grow this cuticular sac 

 becomes so small for the wing that a third period of bending becomes 

 necessary; the wings occupy this cramped position until the emergence 

 of the pupa when they unfold to assume the normal imaginal form 

 and size. 



Attention has already been called to the secretion of chitin by 

 the cells of the rudiment. In some speeimens the activity of the cells 

 just under the peripodial pore was such that the cuticular layer directly 

 over the pore was thicker than the surrounding part thus forming, as 

 it were, a plug which fitted into the pore. Dewitz (3) noticed the 

 same structure in TricJiosagia varia in which species the pore had 

 narrowed, in Platyphylax the plug was formed at an earlier stage 

 before this narrowing occured. In Platyphylax the plug is rounded 

 on its inner surface and from it clear viscid threads, representing the 

 new secretion, passed to the cells of the rudiment lying directly under- 

 neath. This plug either occurs for only a short time or eise is present 

 in but few speeimens, the large majority of those examined did not 

 show it but the thickness of the cuticula over the pore was more gra- 

 dual. Others have found that this chitinous prop is a thin, dark, 



