234 NEW YORK STATE MUSEUM 



early differentiating process had to deal with a long median fork, 

 with a cross vein at each elbow of it. But the median cross vein 

 standing in its midst and binding its arms together beyond the cord 

 and opposite the fork, preventing their spreading, clearly corrects in 

 some measure the obvious weakness of this arrangement. 



In our diagram [fig. 13] the cell is represented between the cross 

 veins and adjoining forks, like a ring slung in a cord. It required 

 the median cross vein to complete the ring. This is the reason why 

 that cross vein is far more persistent than any other outside the 

 cojd. There can be no doubt of this, for that cross vein disappears 

 only' when the cord is shifted to the proximal end of cell ist M"^, 

 and it is thereby put out of commission. The testimony of the 

 figures in the plates given herewith is unmistakable as to this. Very 

 rarely, as in Conosia [pi. 21, fig. 5], there is a shift of the ccrd 

 distally, which brings the median cross vein more directly^ into 

 the line of stress : in such a case it would never be lost. 



The forward shift of veins Cu^ and M^. The tendency of vein 

 Cu^ to be deflected forward at its base and strongly joined to media 

 has been noted in the preceding pages. The accompanying diagram 

 [fig. 15] illustrates successive stages in the progress of that tend- 



Fig. IS 



ency. All these are abundantly illustrated in the plates accompany- 

 ing this paper, and one figure, that of Diotrepha [pi. 29, fig. -6], illus- 

 trates a far more extreme case. By the means here shown the tip of 

 the vein Cu^ comes to be attached directly upon the base of media and 

 in direct line therewith, and it has been usually interpreted as a 

 branch of the same. Ordinarily, this union is a strong one, and the 

 deflected portion of Cu^ is one of the stoutest veins of the wing, 

 as it is in many other Diptera. But among the crane flies are found 



^ It may be noted in passing that in the Lepidoptera an outward shift of 

 stresses, somewhat like that shown in Conosia, has brought the median 

 cross vein permanently into the cord, and the other proximal part of the 

 first median fork has atrophied, leaving three cells, the so called first and 

 second basal cells and the discal cell of the dipterous wing, to constitute 

 together, when their intervening boundaries are atrophied, the " discal cell " 

 of the Lepidoptera. 



