THE TRACHEATION OF WINGS 



13 



the tracheas enter the fold the two layers become approximated along lines 

 midway between the trachece, resulting in an actual fusion of the internal 

 ends of the cells. Then follows a gradual lateral extension of the areas in 



Fig. 2. — A wing of a pupa of Sialis 0\ftcr C. & N.)- 



which the cells are fused to delimit definitely the channels through which 

 the tracheas pass. This process is illustrated by the figures showing two 

 stages in the development of the wings of Psocus (Figs. 4 and 5). 



"The pale color of the bands, indicating the extent of the vein cavities 

 when viewed by transmitted light, is doubtless due to the fact that the 

 haemolymph filling these cavities is more translucent than the hypodermal 

 tissue, which completely fills the wing elsewhere. 



"As an illustration of the nature of the data that can be obtained by the 

 method described above, a half-tone reproduction of one of our photographs 

 is printed here (Plate I Frontispiece). This figure represents a wing of a 

 nearly mature m^mph of a Nemoura, one of the genera of the Plecoptera. 

 In making the preparations it was impracticable to remo\'e all of the dirt 

 adhering to the wing without danger of injuring it ; this is often the case in 

 preparing mounts of the wings of aquatic nymphs. The irregular blotches 



Fig. 3. — Cross section of a fore wing (in part omitted) of a nymph, two-thirds 

 grown, and recently molted, of Anax Junius (After C. <S: N.). 



of dark color in the figure are due to this cause. The dark lines traversing 

 the disk of the wing represent the tracheas, and the pale bands, the vein- 

 cavities alread}^ described. 



"It will be observed that the principal veins are formed along the courses 

 of tracheae, while in most cases the cross- veins have no tracheas within them. 



