66 THE WINGS [CH. Ill 



When a bridge is formed, the original point of departure of the 

 vein so lengthened is marked by a more or less oblique cross-vein, 

 the oblique vein (0). The most important bridges in the Odonata 

 are (a) the short bridge of Anisoptera, continuing Rs back to 

 M 1 _ 2 ; (6) the long bridge of the Lestidae, continuing Ms similarly 

 backwards, but much further, so that the bridge junctions either 

 with -M 1 _ 2 (Lestinae) or M s (Synlestinae) close to their point of 

 union. A comparison of the venation of Synlestes (fig. 23 A) 

 with one of the Megapodagrioninae (fig. 23 c) shews how this 

 formation has produced a remarkable resemblance between the 

 two forms. This "hidden convergence" has hitherto led to the 

 inclusion of Synlestes in the Megapodagrioninae, a subfamily with 

 which it is in no way closely related. [176.] 



In the Agrionidae, but not so much in the Lestidae, most or 

 even all of the cross-veins have become arranged in transverse sets, 

 producing a series of complete transverse veins (Plate II, fig. G}. 

 This arrangement, when complete, as in the Agrioninae, must be 

 regarded as one of the most beautiful, as well as one of the highest, 

 specializations yet attained by the Odonate wing. 



We may conclude this chapter by exhibiting the successive 

 stages of evolution of the Dragonfly wing in consecutive order, as 

 far as the evidence allows us : 



1. Original slight inequality of wings (hind- wing broader than fore-wing; 



Protodonata). 



2. Fusion of A with Cu, and formation of A' (completed in Protodonata). 



3. Fusion of M with E (nearly completed in Protodonata). 



4. Formation of pterostigma. 



5. Formation of nodus. 



6. Bifurcation of Cu. 



7. Completion of arculus and discoidal cell. 



Anisoptera Zygoptera 



8. Presence of Rs a constant character. Absence of .Rs a constant character. 



9. Sub-division of discoidal cell into two Strengthening of discoidal cell into 



parts. a single strong quadrilateral. 



10. Development of the two parts into a Development of quadrilateral, either 

 well-formed supra-triangle + triangle, 

 eithei 

 (a) of about the same shape in both wings (a) towards a regular rectangular 



(Aeachnidae), shape (Calopterygidae), 



or (b) of widely different shapes in fore and or (b) towards further irregularity 

 hind" wings (Libellulidae). by formation of a sharply 



acute distal angle (Leatidae 

 and Agrionidae). 



