Ill] THE WINGS 63 



In the hind- wings of the older Anisoptera, A l , after reaching 

 Cu, fuses with Cu 2 for a considerable length (fig. 17 B), and finally 

 bends inwards towards the wing-border. The second branch, A z , 

 leaves A a little distad from Ac, and runs downwards nearly 

 parallel to Cu 2 + A 19 to the wing-border. Thus a distal portion 

 is separated off from the anal field, between A 2 basally and 

 Cu 2 + A!, or A I} distally. This area remains thus in the Peta- 

 lurinae and most Gomphinae. In the Aeschninae, however, it 

 becomes definitely closed off below, about half-way between Ac 

 and the wing-border, by development of a cross-vein from the 

 in- turned portion of A^. Thus is formed the interesting closed 

 area known as the anal loop (al). In Aeschna and a few other 

 genera, this may be supplemented by an additional closed area 

 placed distally to it (al'). 



The anal loop remains compact and more or less rounded 

 throughout the Aeschninae, in the Macromiini, and in the Syn- 

 themini. With the beginnings of a further broadening of the anal 

 field in Libellulidae, a new tendency is set up. A 2 shifts close 

 under Ac, and nearly monopolizes the oxygen-supply received 

 from it. A dwindles, and fails to reach Cu. Consequently A 2 

 grows out strongly into the widened area below it, and tends to 

 assume a slanting course parallel to Cu 2 . The latter gives off an 

 inwardly projecting branch Cu^, tending to run to the wing- 

 border very close to the ending of A 2 . Thus, between A 2 and 

 Cu 2 , a long loop is formed quite different in shape from the original 

 compact form seen in the earlier Libellulidae. This I have recently 

 termed the cubito-anal loop [176], We may, however, retain the 

 term "anal loop," since the formation is clearly homologous with 

 the older type, except in the lack of fusion of A 1 with Cu 2 . 



For most of its width, this long loop is only two cells wide. In 

 the higher forms, there is developed, along the inner boundaries of 

 these two cell-rows, a strong straight cubital supplement (Cuspl), 

 whose formation is exactly analogous to that of Rspl and Mspl. 

 This forms the mid-rib of the long loop. Owing to its assuming 

 the form of a complete vein, it was originally mistaken for A 2 , 

 while A 2 was regarded as A 3 . A glance at the tracheation of this 

 area, however, quickly settles the point. 



In the highest forms, the long loop becomes further specialized 



