Evolution and Taxonomy 



71 



brane of the wing is supplied with an immense number of mi- 

 nute tracheae extending from the main trunks contained in the 

 veins. The lateral branches of the tracheae are rarely seen 

 even in carefully bleached wings, for in mounting the speci- 

 men they become filled with the mounting medium and are 

 thus rendered invisible. But occasionally air will remain in 

 them rendering them distinctly visibly. It is by means of 

 some of these lateral tracheae that the branches of media be- 

 come connected with radius and with cubitus. 



When such a communication has been established it is im- 

 portant that these tracheae should not be compressed by the 



IX vm 



P'iG. 16. — Packardia. 



bending of the wing during flight, therefore any tendency to 

 protect these tracheae by a thickening of the membrane along 

 their course would be beneficial and would result in the de- 

 velopment of veins enclosing these tracheae. 



These veins at first extend in a transverse direction, and are 

 thus obviously cross veins (Fig. 16, c. v.). But the result of 



