580 



THE PANORPOID COMPLEX, 111., 



a. 3A 



Text-Fig. 53. 

 Evolution of the anal Y-vein in the fore wings of Lepidoptera Homo- 

 neura, showing progressive stages of reduction, a, original complete 

 double Y-vein formation with inter-anal cross-vein (iai) present, in 

 Sabatinca (fam. Microplerygidae) ; b, loss of zai, in Mnemonica 

 (fam. Eriocraniidae) ; c, loss of lower loop in Eriocrania (fam. 

 Eriocraniidae) , (but note presence of sub-anal cross- vein sa); d, re- 

 duction of upper loop in Mnesarchaea (fam. Mnesarchaeidae) ; 

 e, partial loss of upper loop, with retention of ia\, in Trictena 

 (fam. Hepialidae) ; J, last stage of reduction in upper loop, in Leto 

 (fam. Hepialidae). Lettering as on p. 535. 



formed by 2 A and 1A. When we turn to the family Hepialidae, 

 we find a further stage in the reduction of the loop indicated 

 in the genus Trictena (Text-figs. 53e, 79) . Vein 3 A has ceased 

 to be chitinised, though its trachea still persists. Further, the 

 distal end of vein 2A has become aborted, and a new connection 

 with 1A is being made via the cross-vein ioj. In other genera 

 of this family, the anal veins become still further reduced (Text- 

 fig. 53, /) until, in Charagia (Text-fig. 78) there is no sign of 

 the original formation left. 



In the Heteroneura, the loop formed between 1A and 2A 

 persists in the great majority of forms, including the Butterflies; 

 but 3 A is absent from the forewing. The most archaic con- 

 dition in this Suborder is probably that in which trachea 2A 

 reaches 1A, and the chitinisation of the loop in the imaginal 

 venation is formed around the two tracheae in situ. But we may 

 note that, in cases in which 2A is reduced, and does not succeed 

 in quite reaching 1A, a recurrent trachea may grow out down- 

 wards and backwards from this vein towards the end of 2A, as 



