218 WING-VENATION IN ZYGOPTEROUS DRAGONPLlES, 



This trachea may be called a "supplementary anal crossing," and 

 labelled Ac'. 



We are now in a position to deal with our problems in the 

 liirht of the facts disclosed. 



A. The Problem of the Radial Sector. 



This question is of outstanding importance, since it lies at the 

 very root of the relationships between Zygoptera and Anisoptera. 

 It will be recalled that Professor Needham, in his famous paper,* 

 showed how, in the Anisoptera, the radius R in all cases gives off 

 a sector Rs, the radial sector, which crosses over Mj.g, and so gives 

 rise to the peculiar formation of the bridge and oblique-vein. 

 Needham pointed out that in Lestes, the only Zygopterid which he 

 examined in the larval stage, R appeared to be unbranched, while 

 M had an extra branch taking the place of Rs. He explained 

 this by assuming that, during the narrowing process in the 

 Zygopterid wing, the basal piece of Rs, lying between R and M, 

 had got nipped off and had become aborted, so that Rs finally 

 became attached to M. 



Now this explanation has been tacitly accepted by all students 

 of Odonata, including Dr. Ris and myself, for a good many years. 

 Even when, on the completion of my last paper on this subject 

 (loc. cit.), I was faced with the fact that, in all the Lestidce and 

 Agrionidce examined by me, R was always unbranched, and that 

 in the Agrionida^ there was neither a bridge nor an oblique vein, 

 I still retained Needham's notation, and accepted his explanation. 

 The peculiar case of Neosticta {loc. cit., Pl.xiii., fig. 4), of course, 

 supported this view. Further examination of this highly reduced 

 form has convinced me that it is extremely doubtful whether the 

 trachea, which I believed to be Rs, really does come off from R. 

 The dense pigmentation renders examination most difficult, and 

 I have not yet been able to get a larva which has just undergone 

 ecdysis for study. In any case, the trachea in question is so 

 weakly developed, that it represents a special case of extreme 

 reduction, which can be laid aside provisionally during the more 

 general discussion on less reduced forms. 



* "AGenealogic Study of Dragonfly Wing-venation." Proc. U. S. Nat. 

 Mua., xxvi., 1903, pp. 703-764, Plates xxxi.-liv. 



