230 THE WINGS OF ODONATA 



determination of the course of specialization as are furnished by surrounding 

 parts." (Needham '03). 



The nodus. — "The nodus is the stout cross-vein near the middle of the 

 costal border of the wing, joining the costa, the subcosta, and the radius. 

 It is traversed by a more or less evident suture, making a flexible and elastic 

 joint which, without loss of strength in the parts which need rigidity, would 

 seem to allow more effective flexion of the distal parts of the wing" (Need- 

 ham '03), (Fig. 230, n). 



The suhnodus. — That portion of the radial sector which unites the lower 

 end of the nodus with the median vein is the suhnodus (Fig. 230, sn). 



The oblique vein. — The short oblique portion of the radial sector appear- 

 ing as a cross-vein behind vein Mo is the oblique vein (Fig. 230, o). 



It should be observed that the radial sector fuses with vein M2 for a 

 little way, carrying the oblique vein a variable distance beyond the sub- 

 nodus. 



The bridge. — The vein secondarily developed to connect the radial sector 

 proximally with a branch of media, usually vein Mi +2, is termed the bridge 

 (Fig. 230, hr). 



The formation of the bridge.- — "In the adult wmg the bridge exhibits 

 no evidence of an origin different from that of the radial sector, with 

 which it is strictly continuous. But a study of the tracheation of the 

 wings of nymphs reveals the secondary nature of the origin of the bridge 

 Plate VI, Fig. 2 is a reproduction of a photograph of a portion of a wing of 

 Anax Junius, showing the crossing of the radial sector, and the origin of the 

 trachea that precedes the bridge. The latter is a small twig which arises 

 from the distal end of that portion of the radial sector which becomes the 

 oblique vein, and extends towards the base of the wing in a direct line to the 

 media. This method of formation of the bridge is characteristic of the 

 Aeschnidas. 



"In most Libelkilidae a trachea, or a bunch of tracheoles, descends from 

 near the base of the radial sector and forks at the level of the bridge, one 

 branch going to the distal end of the oblique vein, the other going in a 

 diametrically opposite direction to the media, (Plate VI, Fig. i). 



"The illustrations just given exhibit the structure of these parts in 

 nymphs of the suborder Anisoptera. In the suborder Zygoptera (Calop- 

 terygidae and Agrionidas) there exists a striking difference. If we compare 

 adult wings of the two suborders, there can be no question as to the identity 

 of vein R^, or as to its homology in the two groups. But in the suborder 

 Zygoptera, so far as known to us, the tracheae Rg is a branch of the medial 

 trachea. The base of R^, however, forms an oblique vein, and a bridge is 

 developed secondarily, as in the Anisoptera. It is probable that there has 

 been a switching of the base of the trachea Rg from trachea R to trachea M. 

 One has only to examine a well-mounted wing of any dragon-fly nymph to 



