BY II. J. TILLYARD. 



557 



Text-Fig-41. 



Diagrams to show the structure of the arculus and cubito-median Y-vein. 

 a, arculus in Odonata, with the discoidal cell or quadrilateral (dc) 

 formed between two cross- veins ; b, cubito-median Y-vein in the 

 Panorpoid Complex, formed by fusion of Ms with Cm. Lettering 

 as on p. 535. 



resulting formation may be called the cubito-median Y-vein. 

 Its relationship with the Odonate arculus will now be quite clear. 

 Part (i. ) of that arculus, denned above as the part of M from 

 its divergence from fi to the origin of M 5) takes no part in it 

 at all. Part (ii.j corresponds with the upper arm of the Y, 

 in so far as it is the basal portion of M 5 ; it is not, however, 

 terminated below by a cross-vein, but by the point of junction 

 of Cuj with M 5 . Part (iii. ), the cross-vein, is absent; if it 

 were ever present, as is very probable, then it has been entirely 

 eliminated by the fusion of aT 5 with Cu]_. The lower arm of 

 the Y is the basal portion of Cu 1( and corresponds with the 

 same portion of the same vein in the Odonata, which, as we can 

 see from Text-fig. -Ha, lies distad from the discoidal cell. 



The cubito-median Y-vein, then, in the Orders of the Panor- 

 poid Complex, is a specialisation akin to the arculus in the 

 Odonata, and serving the same purpose of strengthening the 

 venation at a point of stress near the base. It is, however, 

 only partially homologous with it; and the condition necessary 

 to its formation is the opposite of the condition for the forma- 

 tion of the Odonate quadrilateral, viz. that the cubital fork 

 should lie at a level nearer to the base than does the distal part 

 of the arculus itself. 



The proof that the upper arm of the Y-vein in the Parame- 

 coptera is actually a main vein. i.e. M 5> and not a cross-vein, 

 may be given briefly as follows (see Plate xxxi., and Text-fig. 

 63 ) :— 



(a) As regards its structure, an examination of the fossil 



