884 



RADIAL AND ZYGOPTEKID SECTORS, (feC, 



bridge, from its origin basal ]y on M up to the second oblique 

 vein O^,, represents, in the venation, the original Zygopterid 

 sector Ms, with the oblique vein O, standing upon it to mark 

 the point where trachea Ms has been captured by trachea Rs. 

 Tlie vein continuing the bridge is the true Rs, of which also 

 O2 must be an essential part. 



The above explanation, as it seems to me, offers the only solu- 

 tion possible in favour of a partial homology between veins Ms 

 and Rs, as now present in the wings of Odonata. Text-fig. 6 

 would show, on this supposition, the two lines of development 

 leading to Zygoptera and Anisoptera respectively. For the 

 common ancestor, we should have to postulate a form possessing 

 both a true lis and a true Ms (Text-fig. 6, «). From this, the 

 Zygoptera would be derived by reduction, Rs being entirely 

 eliminated.* The Anisoptera, on the other hand, would be 

 derived from the ancestral form via the Petalura-Gordulegaster 

 stage, trachea Ms being first of all captured by Rs, as in these 

 forms, and then entirely eliminated; while, in the imagines, the 

 short Anisopterid bridge would remain as the tme vein Ms, and 

 therefore homo/ogoits with the long bridge of Lestidff, and also 

 with the basal part of Ms in the Calopteryyidre and AgrionidcB. 



This might be exhibited as follows : — 



The new view of Bridge-formation, embodied in the above 



^' Unless, indeed, the subnodal vein itself be the remains of Rs. 



