BY K, J. TILLY AKD. 555 



base of the wings in that Order, R and M being fused basally, 

 and then diverging further distad, so that M approaches Cu. 

 The curve thus formed is completed by a strong cross-vein des- 

 cending from M to Cu. The arculus consists of the curved por- 

 tion of M, from the point at which it leaves R, to the top of 

 this cross-vein, plus the cross- vein itself. The portion formed 

 from M has been called the anterior arculus, that from the cross- 

 vein the posterior arculus (Text-fig. 41a). 



Comstock has extended the use of the term to other Orders 

 also, in which somewhat similar formations are to be met with 

 (15, p. 78) . In the Triehoptera and Lepidoptera, in particular, 

 he has applied the name posterior arculus to the supposed cross- 

 vein that descends upon Ciix not far distad from the cubital 

 fork . 



The discovery of the fossil Order Parameeoptera (29) has 

 shown us very clearly that this supposed cross-vein is not a 

 cross-vein at all, but a true branch of M, whose existence might 

 otherwise have passed unsuspected. As this discovery is of the 

 utmost importance, for the right understanding of the Phylogeny of 

 the Complex, it is necessary here to undertake a careful analysis 

 of the region of the arculus, and to give the full proof of the 

 true nature of this basal posterior branch of M, which has hither- 

 to been regarded as a cross-vein. 



In order to follow this argument clearly, it will be necessary 

 to go back, first of all, to the Odonata, in which the arculus still 

 exists in a more primitive condition than it does in any Order 

 of the Panorpoid Complex. In the Zygopterous Odonata, which 

 preserve the most archaic condition of the venation as regards 

 the media and its branches, there are actually five branches of 

 M represented. But, owing to the fact that the venational 

 scheme was worked out from the more specialised Anisoptera, in 

 which Rs has crossed over Mi and M 2 , and has captured the 

 third branch (leaving its basal portion as the so-called bridge- 

 rein), the notation originally applied to these five branches, 

 beginning with the most anterior, was M v M 2 , Rs, M 3 and M 4 

 respectively. "When I pointed out that, in the Zygoptera, the 

 supposed Rs was really a branch of M, I adopted the name 

 Zygopterid Sector for it, with the notation Ms (19) . Hence 

 the notation for the five branches now stands at M 1; M 2 , Ms, 

 M 3 and M 4 respectively, the last three being actually the third, 

 fourth and fifth of these branches, in order. The correct nota- 



