BY R. J. TILLYARD. 051 



Note an the Evidence concerning the Existence of vein 71/ 4 



in the Lepidoptera. 



The evidence for the existence of M 4 in tiie Lepidoptera is 

 not absolutely conclusive, as will be readily gathered from the 

 account of my researches on the Hepiahdae. The alternative 

 is that the vein here taken to be M 4 is in reality the cross-vein 

 m-cu. In the present state of our knowledge, it is impossible to 

 decide finally what is the true state of affairs. My reasons for 

 accepting the presence of M 4 , as I have done throughout Sec- 

 tions vi. and xiw, is that the new evidence brought forward in 

 this research seems to me more in favour of that view than the 

 other. The absence of a trachea M 4 in the pupal wing seemed 

 to me for a long time to be almost conclusively against the 

 existence of M- 4 in the Lepidoptera. But the small, though 

 very definite, trachea which I found in a pupal wing of Xyleutes 

 (Text-fig. 56) tells against this; and it has to be remembered 

 that comparatively very few dissections of pupal wings of ar- 

 chaic Lepidoptera have yet been carried out. If the trachea M 4 

 is present in only 1 per cent, of the wings of Hepialidae or 

 Cossidae, its existence will nevertheless be a much stronger argu- 

 ment in favour of the presence of vein M 4 than is its absence 

 from the other 99 per cent, for the cross- vein alternative. For 

 it has now to be borne in mind that the existence of vein M 5 

 could never have been proved without the discovery of Belmont ia 

 or some similar fossil; seeing that, in this ease also, trachea M 5 

 is absent from the early pupal wing, and in most cases, as with 

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



But there are other strong reasons why the presence of M 4 

 should be accepted, on the balance of evidence at present. The 

 fossil Archipanorpa (Text-fig. 57) shows us a type in which M 4 

 is reduced to the transverse -condition, so that, at first sight, it 

 would appear to be undoubtedly a cross-vein. Yet not only 

 the presence of strong maerotrichia upon it, but also the fact 

 that its fusion with C%_ is incomplete, — so that it separates 

 from this latter vein further distad, and runs freely to the wing- 

 border, — both prove that it is a main vein in this fossil. 



Again, what are we to make of the now famous individual of 

 Sthenopis, in which the same state of partial fusion is shown 

 in the hindwing (Text-fig. 55&, and Comstock, 15, fig.337) if we 

 will not accept this conclusion? If the upper arm of the Y- 



