6y ft. J. TILLVARI). 



881 



obliquity of the cross-vein usually in this position. The cross- 

 vein has, in fact, become an oblique vein, and the proximal part 

 of M3 has become a bridge* 



Now, with this example of how a bridge may be formed for- 

 tuitously before us, we must ask ourselves whether Needham was 

 right in determining the bridge-vein as a secondary formation. 



Sc 



M ■.:== 



:u- 



Cu, 



Text-fig.4. — x^berrant tracheation of larval wing of Neosticta canescens 

 Tillyard. M3 has been captured per sal turn by Ms. Original 

 course of basal pai-t of M3 persists in the imaginal venation as a 

 bridge (Br). 



Is it not more rational, after all, to see in the bridge-vein the 

 07-iginal basal part of a main vein, while the oblique vein is the 

 sign that the tracheation has become specialised, l^et us examine 

 the two well-known cases of Bridge-formation in Odonata in 

 this new light : — 



(a) The long bridge of Lestidcp,. If this new view be correct, 

 the Lestidce must be derived from ancestors in which Mj, Ms, 

 M.;, and M4 all came off separately from M. Owing, however, 

 to progressive narrowing of the wing, Ms must have come to lie 

 close under Mj at a point near its base. Finally, trachea Mo 



* It should be carefully noted that this aberration has nothing to do 

 witli the long bridge of Lestid<e, which is caused by Ms becoming attached 

 to M„. 



