BY R. J. TILLYARD. 



585 



The pupal tracbeation and imaginal venation of various ar- 

 chaic Lepidoptera have been very fully studied, and the results 

 are given in detail in Section xiv. In this Section, I propose 

 to examine the formation of the distal Y-vein, its evolution into 

 the Trigamma (13), and the stages of the evolution of the 

 basal cell and areole within the Order. 



The Distal Y-vein. 



I suggest this name for the distal fusion between M 4 and 

 Cui a which is shown in the Hepialidae, and which is a special- 

 isation closely parallel with that called the eubito-median Y-vein 

 between M 5 and Cuj, nearer to the base of the wing.* 



The original condition of M4 and Cu la must obviously have 

 been that in which they both ran separately to the wing-border; 



Text-Fig. 55. 

 Evolution of the distal Y-vein in Lepidoptera. a , the original condition 

 found in the Trichoptera ; b , the stage of partial fusion, as in hind- 

 wing of Sthenopis and in the fossil Archipanorpa (see Text-fig. 57) ; 



c, formation of the Y-vein completed, as in Hepialidae generally; 



d, reduction of M4 to the transverse position, as in many Lepidop- 

 tera. The final stages are shown in Text-fig. 59, b, c, d, where M4 

 becomes aligned with basal piece of Cui a . Lettering as on p. 535. 



as they still do, for instance, in the forewings of all archaic 

 Trichoptera (Text-fig. 55«). This is followed by an incomplete 

 fusion, in which the two veins are united for a space, and then 



*In this connection, the " Note on the Evidence concerning the Exist- 

 ence of Vein M4 in the Lepidoptera," on p. 651, should be read carefully. 



