590 THE PANORPOID COMPLEX, iii., 



that vein M 4 does exist in the Lepidoptera, and forms the upper 

 arm of the distal Y-vein. I think, however, that the notation 

 M 4 -f-Cui a need only be applied to the fused vein when it is 

 clear that both M 4 and Cu la have taken part in the fusion, 

 as in the Hepialidae. In the Heteroneura, and also in Erio- 

 crania, vein M 4 is reduced to the semblance of a cross-vein. 

 "While still labelling this vein M 4) it does not appear to me 

 that it takes any part in the fusion; and I should prefer to label 

 the distal part of the Y-vein simply Cn la , 



The Trigamma. 



Text-tig. 59, a-d shows the stages in the alteration of the 

 direction of M 4 which result in that vein becoming aligned more 

 or less with the basal portion of Cu ljo so that the original form 

 of the distal Y-vein is quite lost. The changes are correlated 

 with the development of the basal cell, which requires a strong 

 support below it, as well as above. To the strong three-pronged 

 fork formation found in many higher types of Lepidoptera I 

 have already given the name trigamma (13) . The forks, named 

 from below upwards, are Cu^, Cu la and M 3) the connecting 

 piece between the two last being the highly specialised M 4) as 

 shown in Text-figs. 59, c, d. 



The Basal Cell and Areole. 



Originally, as in the Triehoptera, there were only two closed 

 cells lying away from the base of the wing in the Lepidoptera. 

 These were the radial cell (re), formed between R 2+3 and 

 R 4+5 , and closed distally by the cross-vein ir; and the median 

 cell (me), formed between M 1+2 and M 3+4 and closed distally 

 by the cross-vein im. The primitive condition is shown in Text- 

 fig. 59, a. The cross-vein ir is retained in the Tortricina and 

 Tineina, as well as in a few other genera, e.g. the Cossid Macro- 

 cyttara (32) . But in most cases it has disappeared, and the 

 closure of the radial cell distally is brought about by other means, 

 such as a partial fusion between R 3 and R 4+s (Text-fig. 59, g) . 

 The cross-vein im, on the other hand, is present in most Lepi- 

 doptera, and may persist long after the main stem of M has 

 become aborted 



The radial cell, when it persists as a separate entity in the 

 imaginal venation of the Lepidoptera, is known as the areole. 

 The basal part of the stem of R 4+S) which separates the areole, 



