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 trachese are rarely seen 

 even in carefull}' 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 tlie.se tracheae should not be compressed by the 



K vm 



Fig. 16. — Packardia. 



bending of the wing during flight, therefore any tendency to 

 protect these tracheae by a thickening of the membrane along 

 their cour.se would be beneficial and would result in the de- 

 velopment of veins enclosing these tracheae. 



These veins at first extend in a transver.se direction, and are 

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



