BY R. J. TILL YARD. 



619 



tion in the original courses of the veins involved. All stages 

 of this line of evolution can be followed out within the single 

 family Bombyliidae; but its details need not be gone into here. 



The probable condition of the eubito-median Y-vein in the 

 oldest Diptera has already been discussed in Section iii. It is 

 only here necessary to refer to Text-tig. 43 for justification of 

 the assumption that this formation did exist in the Archetype 

 of the Diptera, though probably already in a somewhat reduced 

 condition. 



The Archetype of the Order, as constructed on the evidence 

 given above, will have the forewing venation as given in Text- 

 rig. 69. The following characters should be noted :-»- 



(1) Both Sc and Hj are simple, unbranched veins; and the 

 only costal veinlet present is the humeral (hm) . 



(2) Both Ks and M 1—4 have four branches. The radial cell 



sh 



1A Cu, Cu, 



Text-Fig.69. 

 Forewing of the Archetype of the Order Diptera. Lettering as on 

 p.535. 



(re) is present, and is closed distally by ir, since this vein is 

 indicated by a short stump-vein arising from K 4 in many Taban- 

 idae (Text-fig. 68). The median cell (mc) is also present, and 

 elosed by im. 



( 3 ) The eubito-median Y-vein was present, though probably 

 already in a somewhat reduced form. 



(4) Cu 2 was weakly chitinised, and perhaps already obso- 

 lescent . 



(5) The base of the wing had already become somewhat nar- 

 rowed ; and, as a result, 1A must have been very close up to Cu 



