56 THE WINGS [CH. 



state of affairs in the genus Telephlebia. It will be seen that, in 

 this case, Sc does not pass through the nodus, but that a slight 

 beginning of the formation of a double instead of a single row of 

 cells beyond the nodus is the real cause of the apparent prolonga- 

 tion. This change is associated with the production of wing- 

 pigment, and is known to occur in many other genera and in other 

 parts of the wing. In the case of the fossil Aeschnidium (fig. 163) 

 the apparent prolongation is much more marked, but its nature 

 is entirely clear from the presence of a similar extra longitudinal 

 vein formed actually between C and Sc, proximal to the nodus. 

 This latter could not, from its position, possibly be mistaken for 

 a main vein, and its formation is clearly due to the extreme density 

 of cell-formation in this wing. The same explanation appears to 

 be the obvious one in the case of the apparent prolongation of Sc 

 through the nodus. 



The Arculus and Discoidal Cell (fig. 20). 



Had there been no basal fusion of R with M, we may safely 

 say that there would have been no arculus. The fusion of R with 

 M was probably brought about, like the reduction in Sc, by the 

 adoption of the aquatic habit by the larva, and the consequent 

 shifting of the course of the oxygen supply of the developing wing 

 from the costal to the anal end of the alar trunk. Originally R 

 was the strongest trachea in the wing. When M became placed 

 closer to the oxygen supply than R was, competition set in. 

 M tended to become the strongest trachea of the wing, and finally 

 moved up close to R, so that these two large tracheae between them 

 monopolized the "lion's share" of the available oxygen. The 

 formation of the single imaginal vein R + M, along the contiguous 

 portion of the two tracheae, was the logical result of this. But, 

 as M originally supplied a large area of the wing more distally, 

 below R, the concomitant of a basal approach of M to R was 

 evidently a more distal departure of M from R, to its natural 

 lower level. This bend or departure of M was the rudiment of 

 the future arculus. The strengthening of the bend of M, by 

 thickening of a cross- vein just below it, completed this interesting 

 formation. This cross- vein would originally of course be attached 



