578 THE PANOUPOID COMPLEX, iii., 



This formation is an ordinal character for the Meeoptera, 

 Trichoptera and Lepidoptera, in the hindwing only. It also 

 occurs in the hindwings of a few Megaloptera, and in both 

 fore and hindwings of some of the higher Planipennia. It also 

 occurs in the forewings of some Diptera (see Text-fig. 50) . 



The condition of the cubito-anal Y-vein is generally that of 

 an incomplete Y-vein, since the veins C112 and 1A, after fusing 

 for a greater or less distance, diverge again, and run separately 

 to the wing-border. This condition may be studied in Text- 

 figs. 36, 62,^ 76, 77, 80, 90, and Plate xxxiii., fig. 20. In the 

 Lepidoptera, the formation is as clearly shown in most of the 

 Homoneura as it is in" the Trichoptera and Meeoptera. But, in 

 the Heteroneura, it is necessary to study the pupal tracheation, 

 in order to show that a cubito-anal Y-vein does exist. In the 

 pupal hindwing of Wingia (Text-fig. 90), I found it absolutely 

 complete; in Xyleutes (Text-fig. 87), the fusion was not quite 

 accomplished; and, in many of the higher groups, a secondary 

 specialisation has resulted in the gradual reduction and final 

 extinction of trachea 1A, so that the Y-vein is no longer present. 

 The hindwing of Carpocapsa (Text-fig. 89) shows the stage in 

 which the fusion is still present, but 1A is much reduced; while 

 in the Butterflies Euschemon and Euploea (Text-figs. 99-102) 

 no sign of the Y-vein can be seen. 



The best example of this type of specialisation in the Mega- 

 loptera is to be seen in Raphidia (Text-fig. 107) . 



In the Planipennia, the approach of 1A towards Cu 2 is first 

 indicated in the Osmylidae. In the Nympltidae these two veins 

 just touch at a point. In many of the Myrmeleontidae the 

 fusion is completed, and the same is true for the Chrysopidae, 

 Apochrysidae and Ascalaphidae. Text-fig. 52 shows the stages 

 in this line of evolution. 



The Anal Y-vein. 



An anal Y-vein may be formed in different ways, as follows: 

 — (1) . In the forewings of all Trichoptera, and of the most 

 archaic Lepidoptera, 2 A loops up with 1A, and 3 A with 2 A, to 

 form a double anal Y-vein. or double anal loop, as shown in 

 Text-fig. 53a. This formation is confined to these two Orders, 

 for which it is an archetypic character. In the Lepidoptera, 

 the double anal Y-vein may be seen complete in the genera 

 Sabatinca and Mnemonica, the two most archaic genera of the 



