190 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



DendrocBschna conspersa Tillyard, the only nymph so far studied 

 of an jEschiiine with reticulated basilar space. This shows the 

 interesting fact that the anal-crossing Ac is not the first post- 

 cubital cross-vein, but the second, since there is one cross-vein 

 developed between it and the base. In the imaginal wing, at 

 first sight, the whole series of four (sometimes five) postcubitals 

 appears very similar, except that the last is placed slantwise to 

 form a weak subtriangle. But a little examination shows us that 

 the second postcubital is also slightly bent, and very slightly 

 thicker than the others. The reduction in strength from the 

 strong trachea Ac to the weak cross-vein is, of course, exactly on 

 a level with the formation of the " oblique vein " treated of in 

 the next section of this paper. But here, the obliquity of the 

 trachea was never very great, and is practically lost in the 

 imaginal cross-vein. 



We see from this how very inconvenient, and even incorrect, 

 the term " first cubito-anal cross-vein " may become. It should 

 certainly be done away with once and for all, and the term 

 " anal-crossing" (Ac) be substituted. In the closely veined 

 species of jEschnince, there should never be any difficulty in pick- 

 ing Ac out, since it is always the cross- vein lying slightly distad 

 from the distal side of the anal triangle of the male. Hence, in 

 Basiceschna and Boyei^ia, for instance, it is actually the first 

 cross-vein, in spite of their archaic reticulation. 



Section B. — The occurrence of a hridge-vein in Zygoptera. 



Needham states that a true bridge-vein is not formed in any 

 of the Zygoptera except the Lestince. In that subfamily (Plate 

 xii., figs. 3-4), it is very easily discovered in the nymphal wing, 

 and agrees entirely in its form with the bridge-vein of Anisoptera, 

 except for its greater length. In the imaginal wing (Plate xii., 

 fig. 4), it carries lis backwards in a straight line, to attach to 

 Mi.2 very slightly distad from the origin of M3. 



The occurrence of this bridge-vein in the Lestince must be of 

 very great phylogenetic importance, since it almost certainly 

 indicates a separate line of descent for that subfamily, as com- 

 pared with the non-bridgeveined Zygoptei^a. We should, there- 

 fore, search carefully for any evidence still existing as to what 



