168 THE WI?fG-VENATlON OF LEPIDOPTERA, 



symmetrical dichotomous forkings of the main veins, Rs, M, 

 and Cuj; (b) the formation of large elongated polygonal (usually 

 hexagonal) cells; (c) the formation of sessile apical forks upon 

 the distal ends of the two principal cells (viz., the discoidal cell, 

 dz, and the median cell, mc); and (d) the capture by Cuia of 

 the fourth or last branch of the media (M^). 



Reduced to its essentials by the removal of numerous excess 

 cross-veins and irregularities in the courses of the main veins, 

 the Protomecopterous Venational Plan is shown in Text-fig. 1 

 (forewing). For those readers who are not familiar with the 



Text-fig. 1. 



Diagram of the Protomecopterous scheme of venation, with all excess 

 cross-veins removed. Notice the partial fusion between Cuu and M^- 

 (In Mecoptera and Lepicloptera, this fusion becomes complete). 



Comstock-Needham system of nomenclature, an explanation of 

 the notation used is given at the end of this paper. The costal 

 vein C is present in the Protomecoptera, but becomes entirely 

 fused with the anterior border of the wing in the four descendant 

 Orders (as, indeed, in all recent Insecta). The only differences 

 between fore- and hindwings are : — 



(a) Sc is reduced in length in hind wing. 



(b) M tends to fuse basally with R in forewing, with Cu in 

 hindwing. 



(c) Cui (like Sc) is shorter in hindwing than in forewing. 



The main object of this short outline is to indicate the inter- 

 pretation of the Lepidopterous venational scheme as a direct 

 derivative from the Protomecopterous Putting aside the 

 Micropieryyid(E (which will be dealt with as an archaic side- 



