Vol. 1.] 



Woodwortli. — Wing Veins of Insects. 



45 



FIG. 18. Clasp on front wing of 

 Psorid, showing relation to tracheoid 

 thickenings. 



ing modification of the tracheoid ridges. Here is a vein that is 

 conspicuously tracheoid on the lower side of the wing, but free 

 from these structures on the upper side. The tracheoid struc- 

 ture is entirely normal along most of the wing, but toward its 

 tip the ridges begin to project as spine-like processes from the 

 outer margin of the wing. 

 These processes are successively 

 longer and longer, till finally 

 those nearest the end curve 

 over and form the clasp that 

 serves to hold the two wings 

 together. At the region of the 

 greatest development of these 

 spines the vein itself almost 

 disappears, all the cuticular 

 substance appearing to be ap- 

 propriated for the production 

 of the modified spines. 



Tracheoid structures can not, 

 in view of these facts, be con- 

 nected with the trachea, nor are 

 they criteria for the recognition or classification of veins, but 

 simply methods of specialization, and might be produced at 

 any point when the appropriate conditions occur. What 

 these conditions are can at present only be guessed at, but 

 we may have a clue to them in the common presence of 

 tracheoid growths in bulhe. These appear to be simply col-, 

 lapsed portions of veins whose cells have failed to produce 

 sufficient chitin. It is conceivable that such cells would have 

 a lower osmotic pressure and therefore a greater tendency to 

 cling together, so that relatively less chitin would be deposited 

 in the depths of the transverse wrinkles of the developing wing 

 in this region. The facts that weak veins exhibit a greater 

 tendency to develop tracheoid characteristics, and that in a 

 single vein regions which are suddenly narrowed often show 

 the same tendency, agree well with this hypothesis. 



To return to the question of the true tracheation of veins: 

 we have to deal with a matter that goes back to the early 

 stages of the development of a wing. The chitinization, which 

 is the most evident thing about a vein, is produced only just 

 previous to the last molt; but the vein cavitv is evident long 



