The Odonafa or Dragonflies of Sovlh Africa. -55 



the writer considers this introduction as a marked progress. Besides 

 being of comparatively easy application, the study of venational 

 details has the great merit of leading very often to the true lines of 

 development within the Order, of helping us to a really phylogenetic, 

 and therefore natural classitication — all due reserve being made, of 

 course, for incomplete knowledge and possible, or even probable, 

 mistakes. The greatest progress in this branch of study is due to a 

 paper of J. C. Needham, of Ithaca, New York : "A Genealogic Study of 

 Dragonfly Wing- Venation " (' Proc. U.S. Nat. Mus.,' xxvi, pp. 703-704, 

 tab. 31-54, 1903) ; from this essay most or all of the more recent 

 authors have taken their starting-point, and its terminology has 

 rapidly become almost universally adopted. This terminology, with 

 some very slight and merely formal modifications, made as the writer 

 found them practical in his various studies on the subject, is here also 

 adopted and illustrated in its main features in Plate V, figs. 1, 2. 



Prof. Needham's studies of venation were made chiefly on the wings 

 of the Suborder Anisoptera, and interpretations of doubtful structures 

 and terminology were therefore immediately derived from observations 

 of this systematic unit. Needham arrived at his conclusions by 

 following the ontogenetic development of wing-venation as fore- 

 shadowed in the tracheation of the larval wing ; by his text and 

 beautiful photographic figures the following facts were emphasised as 

 being more especially characteristic for the f Anisopterous) Odonate 

 wing : (1) A branch of the radius (H), the single radial sector (Bs) 

 present in Odouata, branches off from the main trunk in the nodal 

 region, crosses in an oljlique transverse direction over two branches 

 of the media {Ml and M2) before taking up again the longitudinal 

 direction ; the point where Rs becomes longitudinal again is linked to 

 a considerably more proximal level of branch Ml-2 by an accessory 

 structure of veins named " the bridge " by Needham. In the matuie 

 wing Bs would appear to the observer unacquainted with the larval 

 conditions as a branch of M ; but there is au absolutely constant 

 oblique cross-vein behind il/2 and a little distally from the nodus to 

 indicate still what has taken place during the development. (2) The 

 discoidal triangles of the basal part of both wings^structures already 

 observed and named by early observers — are demonstrated to be built 

 up as follows : The proximal side of the triangle is part of the main 

 branch of the cubitus (Cm) ; the costal and distal sides of the 

 triangle are medio-cubital cross-veins modified in length, structure 

 and final position in various ways, according to systematic group 

 and to differences between front and hind wing ; but the modifications 

 are evidently determined by the mechanical needs of the wing stiaicture. 



