BV R. J. TILLYARD. 



577 



The most archaic condition for these three veins is that in 

 which all three run free, and well separated from one another, 

 and 1A is also separated from Cu 2 above it. The third anal 

 can always be recognised, in archaic forms, from the fact that, 

 if it is simple, then it bounds the area of the jugal lobe in the 

 forewing externally; but, if it is a forked vein, then its pos- 

 terior branch Occupies the same position. 



Specialisations of these veins occur by the approach of one 

 towards another, not far from the base of the wing to form 

 typical Y-veins of the type described in Sections iii., vi., as being 

 formed, between M and Cu. Text-fig. 51 shows the formation 

 of (a) an incomplete Y-vein, by partial fusion of two veins near 



Text-Fig.51. 

 Diagrams to show the formation of a, incomplete cubito-anal Y-vein in 

 hindwing, and b, complete anal Y-vein, in forewing. Lettering as 

 on p. 535. 



their bases, while they again diverge distally, and reach the 

 wing-border far apart, and (b) a complete Y-vein, by complete 

 fusion of the two veins from near their bases onwards as far 

 as the wing-border. Of these formations there are two in 

 which the anal veins play a part, viz. the cubito-anal Y-vein, 

 formed by fusion of 1A with Cu 2 7 and the anal Y-vein, formed 

 by fusion of two or more of the anal veins amongst themselves. 



The Cubito-Anal Y-vein. 



If the vein 1A, which is well separated from Cu 2 basally, 

 approaches it a little further from the base, and fuses with it 

 for a greater or less distance, then a cubito-anal Y-vein is 

 formed, the upper arm being the basal piece of Cu 2 , the lower 

 arm the basal piece of 1A, and the main stem the fused portions 

 of these two veins (Text-fig's. 36, 51a, Plate xxxiii., fig. 20), 

 which may be indicated by the notation Cug-j-lA. 



