64 THE WINGS [CH. 



by the formation of a sharply pointed "toe" (figs. 132-136), ending 

 almost on the wing-border, and a long flat "sole," formed by 

 Cuy,. This form has received the name of the "foot-shaped," 

 "stocking-shaped" or "Italian" loop, owing to its shape. 



The third branch of A (A 3 ) plays an important part in forming 

 the distal side of the anal triangle of the male, in all those Anisop- 

 tera in which the anal angle is angulated, and in marking off a weak 

 corresponding basal area next to the membranule in rounded forms. 

 This triangle may be wide or narrow, short or long, and is usually 

 crossed by one or more veins. The shape of the membranule, as 

 well as its size, seems in most cases to be correlated with the shape 

 and size of the area marked off by A 3 . 



Coming finally to A , we find this small branch proceeding 

 basad from Ac. Along its course, and beyond it to the base, the 

 secondary or recurrent anal vein A' is laid down. 



(ii) Zygoptera. In this suborder we find trachea A much 

 reduced, so that it rarely reaches Cu. The original more distal 

 portion has become quite cut ofE and fused with Cu, at a point 

 where we can still recognize Ac, and in a few forms (fig. 17 G) the 

 four branches of A coming off from it. A l still runs distad towards 

 Cu, and in Calopteryx (fig. 17 E) almost reaches it. In most cases, 

 A is very short. The line of A l is continued in all imaginal 

 Zygoptera by the formation of a special vein, the anal bridge (Ab), 

 running from Ac to Cu 2 , and forming the lower side of the sub- 

 quadrangle. A' is formed backwards as in Anisoptera. The rest 

 of the anal field is reduced either to (a) a triangular area below 

 A' and Ab (Calopteryx and other non-petiolate forms) ; (b) a single 

 row of cells in the same position (most Calopterygidae) ; (c) a narrow 

 uncrossed space below Ab (Lestidae and most Agrionidae) ; or 

 (d) it is completely obliterated (many Protoneurinae). 



By studying the corresponding tracheation and venation in Cali- 

 agrion (Plate II, fig. C and fig. 17 G) we realize what a large original 

 area of the Odonate wing has been sacrificed in the formation of 

 the petiole or stalk. The sub-division of the Agrioninae into tribes 

 has been based on the extent of petiolation, with reference to the 

 position of Ac. 



