No. 373-] THE WINGS OF INSECTS. 47 
the wings of aquatic nymphs. The irregular blotches of dark 
color in the figures are due to this cause. The dark lines 
traversing the disk of the wing represent the trachez, and the 
pale bands the cuticular thickenings destined to form the wing- 
veins. 
It will be observed that the principal veins are formed along 
the courses of trachez, while in most cases the cross-veins 
have fio tracheze within them. It will also be observed that 
the tracheze extend in straight lines or in gentle curves, while 
in some cases the corresponding veins are much more angular. 
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 
Fic. 3. — Hind wing of a nymph of Nemoura. 
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 trachez 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 tracheze 
of the immature wing, serving the purpose of respiration, and 
lying more or less free within the wing-sac, have not been 
forced to follow closely the changes in the cuticular thicken- 
ings of that sac. 
The operation of this principle is shown only to a slight 
extent in the wings figured here. But when we study more 
highly specialized forms, it is seen that the divergence of these 
