The Development of the Wings of a Caddis-fly Platyphylax designatus Walk. 579 



of the surrounding hypodermis do not, in general. show anything at 

 all similar to this increase in number or radial arrangement but one 

 sees in them only an increase in size. Certain areas over which hairs, 

 setae, etc. are beginning to develop cannot be included. No C5i;ological 

 change could be seen in any of the nuclei. It is also noted that in 

 larvae of this length, 5 mm, the area of the wing rudiment is clearly 

 marked off from the surromiding hypodermis by the absence within 

 its cells of the pigment granules (Fig. 4), which occur in the surround- 

 ing hypodermal cells; pigment is more numerous in those dorsal than 

 in those ventral to the area. The pigment granules are brown and 

 are more numerous and larger near the out er than the inner surface 

 of the hypodermal cells. 



The next noticeable change of the hypodermis at the area of the 

 wing rudiment is an increase in its thickness, the thicker part forming 

 a disk-like body both surfaces of which are convex; the disk shows 

 no distinct marginal boundary but passes gradually into the surrounding 

 hypodermis (Fig. 5). The nuclei within the disk are, with very few 

 exceptions, radial to its surface; they have increased somewhat in 

 size but this has not been proportionally as great as has the increase 

 in the mass of cytoplasm within which they lie imbedded. As a result 

 the nuclei are not so crowded together as thev were in the last stages 

 (Figs. 3 and 4). There has also been an increase in the thickness of 

 the cuticula, especially in that part lying directly over the disk. A 

 great part of this cuticula is dark, the proportional amoimt of dark 

 and light varying greatly in different specimens from larvae of the 

 same length. The outer dark layer is smooth externally, it bears no 

 setae and the dark color ceases where they begin (Fig. 4). 



The disk increases gradually in size; the nuclei change but little 

 in this respect but, as the disk becomes thicker, they begin to occupy a 

 Position near its inner surface (Fig. 7). Cell boundaries are now for the 

 first time noticeable but not distinct enough to trace them throughout 

 their entire length. The cells of the disk, especially in the outer part, 

 show a marked striation due to their secretory activity in the formation 

 of the cuticular layer. In the last stage it was noted that the disk 

 was convex on both surfaces; this now changes, the outer surface 

 shows an indentation which is the beginning of the invagination that 

 later results in the formation of the complete imaginal disk. At its 

 margin the disk passes gradually over into the surrounding hypodermis 

 there being as yet no definite boundary between the two. After the 

 invagination of the disk begins there ensues a more rapid increase in 



