906 THE AMERICAN NATURALIST. [VoL. XXXII. 
its usual position. It will be convenient to designate that part 
of the radial sector which appears as a cross-vein behind vein 
Mz as the oblique vein (Fig. 62, o); and the winters | longi- 
tudinal trunk as the dridge (Fig. 62, 67). 
In the adult wing 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 
Fic. 63. Th d Anax junius showing the crossing of the par: sector and 
the origin of the trachea which precedes the bridge. o, oblique vein; 47, the bridge. 
wings of nymphs reveals the secondary nature of the origin of 
the bridge. Fig. 63 is a reproduction of a photograph of a 
portion of a wing of a nymph of Anax junius, showing the 
crossing of the radial sector, and the origin of the trachea 
which 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 for- 
mation of the bridge is characteristic of the Æschnidæ. 
