BY R. J. TILLYARD "J.39 



choptera and in the Micropterygidce. This character is also to 

 be found in the Uepialidce. In studying the pupal wings of 

 Lepidoptera, the same condition can be seen to exist throughout 

 the Order, the bend of Cuj being distinctly shown, with the 

 posterior arculus, usually devoid of a trachea, above it. In the 

 imagines of the more advanced types of both Trichoptera and 

 Lepidoptera, the posterior arculus cannot usually be recognised, 

 as it becomes obliterated by further specialisation in this region 

 of the wing. 



In naming this formation the posterior arculus, Comstock 

 assumes that it is the homologue of the posterior portion of the 

 complete arculus found in the Odonata, i.e., that it is formed 

 from a cross- vein descending from M to Cu, which later on 

 becomes specialised as a strong oblique connection carrying Cuj 

 back basally to join M. Another possibility is that the posterior 

 arculus is really the remnant of the posterior branch of the first 

 dichotomy of M; in other words, that it is the homologue of M 4 

 in the Odonata. If this be so, then this branch of M must very 

 early have united with Cuj not far from its origin, in the same 

 manner that M 4 united with Cu la later on in the Lepidoptera, 

 and M ;}+ 4 with Cuj in the Planipennia. If this latter supposi- 

 tion be true, then the correspondence between the media in 

 Zygopterous Odonata and in the Trichoptera and Lepidoptera is 

 exact, both having five branches similarly placed, and the primi- 

 tive dichotomic branching being still preserved in the Synlestince. 

 Only the notation is different, the branches in the Zygoptera 

 being called M u M 2 , Ms, M 3 , and M 4 respectively; whereas, in 

 the other Orders, they are M 1} M„, M 3 , M 4 , and M 5 , the latter 

 being the posterior arculus. 



The evidence as to whether the posterior arculus is really a 

 branch of M, or only a specialised cross-vein, has hitherto been 

 inconclusive. On the one hand, we find that, in the freshly 

 turned pupse of Hepialidce, Cossidce, and other archaic Lepido- 

 ptera, the posterior arculus is marked by a very distinct pale 

 band, not traversed by any trachea; on the other, a distinct 

 trachea can generally be seen to have grown out into it from 

 near the base of M, in pupa 3 of more advanced age. This trachea 



