No. 395-] THE WINGS OF INSECTS. 859 
ultimately form the strong costal vein. Its trachea is often 
atrophied, probably owing to the disadvantageous position of 
its base in relation to air supply, as we have hitherto indi- 
= cated. The anal channel becomes obliterated, and the dense 
hypodermis of its walls dispersed by subsequent expansion of 
this part of the wing. In some wings in which this space 
persists, as in Psocus, it is occupied by the third anal trachea. 
In wings developing within small hypodermal pockets, while 
the cells are densely crowded and while the fusion of the cells 
internally joining the two layers is more tardy and incomplete, 
the same principal channels are formed. In the Lepidoptera, 
though the development of their trachez is retarded, the tem- 
porary tracheoles pass out in tangled skeins through the original 
channels. 
But the process of reduction of trachez, already begun in 
the lower orders, finds favorable conditions for progress in 
the shorter and more open wing sacs developed internally ; 
and we find in all but the more generalized members of 
certain orders that the close correspondence between trachez 
and channels due to simultaneous development is again lost. 
Illustrations have been abundantly offered in preceding chap- 
ters; we are here offering only a suggestion as to the reason, 
first for this correspondence, and then for the loss of it. While 
the tracheze seem at first to have been the determining factor 
in the venation, and while we have been able to show a gratify- 
ingly large number of cases in which the tracheze show the 
unmistakable signs of homology, and cases in which the course 
of the veins is still determined: by them, it appears that in 
certain insects the tendency of the hypodermis to segregate 
itself and to form chitine along certain lines has become so 
well established as to be more or less completely independent 
of the tracheze. The veins have to do in these cases with loco- 
motion in adult life; the trachez, with growth and metamor- 
phosis. The adult wing, whatever it may have been originally, 
has become a dry resilient plate of chitine traversed by finely 
adjusted supports, It would be manifestly disadvantageous for 
the trachez to follow the course of these supports, sharp angled, 
and often recurrent; but in wings with slight fusion between 
