14 



THE TRACHEATION OF WINGS 



It v.'ill also be observed that the trachese extend in straight lines or in gentle 

 curves, while in some cases the corresponding veins are much more angular. 



Fig. 4. — Fore wing of a partly grown nymph of Psociis (After C. & N.)- 



"It is evident from this that in the perfecting of a wing as an organ of 

 flight the position of a vein in the adult may become quite different from 

 that of the corresponding trachea of the immature form. In other words, 

 although there is no doubt that the courses of the principal wing- veins of 

 primitive insects were determined by the position of the principal tracheae 

 of the wings, the wing-veins have been more or less modified to meet the 

 needs of adult life; while at the same time the tracheae of the immature 

 wings serving the purpose of respiration, and lying more or less free within 

 the wing-sac, have not been forced to foUow closely the changes in the 

 cuticular thickenings of that sac. 



"The operation of this principle is shown only to a slight extent in the 

 wing figured here. But when we study more highly specialized forms, it is 



Fig. 5. — Fore wing of a full grown nymph of Psocus (After C. & N.). 



seen that the divergence of these two sets of structures is sometimes very 

 wide, and must be taken into account in an interpretation of the characters 

 presented by a wing. 



