620 THE PANORPOID COMPLEX, iii., 



basally. As this veiu is fused with Ci 2 in soine Tipulidae, but 

 with the main stem of Cu in Stratiomyiidae, I have left it just 

 free in the Archetype, so as to allow the fusion to take place 

 independently in either position, according to the line of evolu- 

 tion followed. 



(6) Vein 3 A is, at the most, only present as a very short, 

 weakly chitinised vein bounding the jugal lobe distally. 



(7) The cross-vein system is much reduced, and consists of 

 the following only: — the subhumeral (sh) ; the subeosto-radial 

 (sc-r) ; the inter-radial between Ri and R 2 (ir') present 

 in many Tipulidae; the inter-radial {ir) closing the radial 

 cell; the radio-median (r-m) ; the intermedian (im) closing 

 the median cell; the submedian (sm) from M 3 to M 4 (found 

 only in Protoplasa) ; the medio-cubital (cu-m) ; and the first 

 inter-anal (*%) 



A comparison of this Archetype with that of the Paratrichop- 

 tera shows us at once that the Diptera can be directly derived 

 from this fossil Order by reduction. A narrowing of the wing- 

 base, with consequent reduction in the anal veins, and approach 

 of 1A to Cu ; reduction of Sc ' to the humeral veinlet ; loss of all 

 the other costal veinlets and the distal forkings of Sc and Rj; 

 some reduction, probably, in the condition of the cubito-median 

 Y-vein; and,, finally, a considerable reduction in the cross-vein 

 system; these changes would produce the archetypic Dipterous 

 forewing from that of the Paratrichoptera. As far as the 

 hindwings are concerned, we have to assume for the Archetype 

 of the Diptera that they were already evolved into well-formed 

 halteres. All the known wings of the Paratrichoptera being al- 

 most certainly forewings, we cannot say for certain what the 

 state of the hindwings was in that Order. But it is clear, in 

 any case, that the Diptera are descended from some originally 

 four-winged type, although the intermediate stages of the process 

 are not known to us. 



In connection with the Order Diptera, and probably also in 

 the case of the two other Orders, Meooptera and Trichoptera, 

 in which the pupal wing's are merotracheate, I desire here to 

 emphasise the value of a careful study of the imaginal wing, 

 during and just after expansion, when the fly emerges from the 

 puparium. My attention was first called to this when I hap- 

 pened to examine the expanding wings of a number of unde- 

 termined species of Tachinidae, whose puparia I had placed in a 



