THE DEVELOPMENT OF WINGS 



113 



certain parts, rounding out the sharp angles of the exterior, and completely 

 occupying the narrower spaces in the wings. 



"It is also important to note that the basement membrane of the 

 hypodermis of the wing differs in no respect from that of the hypodennis of 

 the body-wall, and is continuous with it. In the thinner parts of the wing 





Fig. 114. — ^A cross-section of a fore-wing (in part omitted) of a nymph, two- 

 thirds grown, and recently molted, of Anax Junius; c, cuticula; bm, 

 basement membrane; h, hypodermis; the veins of the wing are 

 designated by the usual lettering. (After C. & N.). 



the two basement membranes meet and fuse, thus forming what has been 

 termed the middle membrane of the wing. Along certain lines, seen in 

 section in the figure, the two membranes remain separate, and thus are 

 formed the cavities of the wing-veins. 



"Figure 114 represents a partial section of a fore wing of a nymph of 

 Anax Junius two-thirds grown; the section was taken at the nodus. The 

 general features here seen are common to the wings of all nymphs — two 

 layers of very elongate, hypodermal cells, which meet in places and form 

 the middle membrane, and remain separate in other places, forming the 

 vein cavities, which usuall}^ contain tracheae" (C. and N.). 



The tracheation of the wings of nymphs. — In the course of the develop- 

 ment of the wings of nymphs, the principal tracheas pass out into the wing- 

 bud very early and extend as the wing grows. There larger tracheae give 

 off many small lateral and terminal branches so that all parts of the 

 developing wing are supplied with air. 



In preparations of wings made by the method commonly employed for 

 the study of the tracheation of the wings, the tracheoles are rarely \asible; 

 but in some preparations they are very evident. In the preparation of a 

 wing of a dragon-fly, of which Fig. i Plate VIII represents a small part, it 

 was easy to see that the entire wing was traversed by numerous tracheoles.* 



*In descriptions of the tracheation of wings, and of the other parts of the body as 

 well, it is important that the distinction between tracheae and tracheoles be kept clearly 

 in mind. 



The term trachcolc has been used frequently to designate any small trachea; but 

 this is an incorrect use of the term. The tracheoles arc certain minute tubes, which 

 are connected with the tips of tracheae or arise from their sides, but which differ from 

 tracheae in their appearance, structure, and mode of origin ; in other words, tracheoles 

 are not small tracheae, but structures that differ both histologically and morphologically 

 from tracheas. 



Tracheoles are exceedingly slender, measuring less than one micron in diameter; 

 they arc of uniform diameter, not tapering as do tracheae; they differ strikingly from 



