THE WINGS OF DIPTERA 



355 



In Rhyphiis (Fig. 357) veins M3 and Cui retain their primitive position, 

 extending nearly parallel and ending remote from each other at the margin 

 of the wing. (For reasons given in an earlier paragraph no account is 

 taken of vein M4 in this discussion). In Thereva (Fig. 366) an approxima- 

 tion of the ends of these veins has taken place, which results in a narrowing 

 of the outer end of cell M3. In Eulonchus (Fig. 367) the tips of the two 

 veins coalesce, and cell M3 is thus closed. In Pantarbes (Fig. 368) the twj 

 veins coalesce for the greater part of their length, and cell M3 is completely- 

 obliterated. 



The coalescence of veins Cuo and the second anal vein. — Quite closely- 

 correlated with the coalescence of veins M3 and Cui there exists a coales- 

 cence of vein Cuo and the second anal vein. In Rhyphus (Fig. 357) these 



Fig. 371. — Wing of Rhamphomyia. 



two veins retain their primitive position. In Leptis (Fig. 364) the tips are 

 approximate. In Thereva (Fig. 366) the tips coalesce for a short distatic3. 

 In Conops (Fig. 369) the coalescence is more striking. In Scenopiniis (Fig. 

 3 70) it is carried still farther. While in Rhamphomyia (Fig. 3 7 1) it ha? pro- 

 ceeded so far that vein Cu2 extends towards the base of the win; and 

 presents the appearance of a cross-vein. 



The reduction of the number of cells in a wing. — -A reduction of the num- 

 ber of cells in the wings has taken place in the wings of nearly all Diptera. 

 This is the result of two causes : first, the coalescence of veins; and second, 

 the atrophy of veins. The following will serve as illustrations of this 

 process. 



In the Brachycera, the coalescence of veins R2 and R3 results in the 

 obliteration of cell R2; and in the Nematocera, a similar coalescence of 

 veins R4 and R5 results in the loss of cell Ri. 



The atrophy of a vein separating two cells results in the uniting of these 

 cells and a consequent reduction in the number of distinct cells. In the 

 wing of Musca domestica (Fig. 372) cells M and ist M> are separated by the 

 free part of vein M3. In the wing of Psilopodius sipho (Fig. 373), there 

 remains only a short spur representing the free part of vein Mi; and in the 

 wing of Dolichopus coquilletti (Fig. 374), the free part of vein M3 is completely 



