THE WINGS OF DIPT ERA 



349 



The truth of the conclusion that in Rhy pints veins Mi and Mo are dis- 

 tinct is made evident by a comparison of the wing of this insect with the 

 fore wing of a caddice fly (Fig. 3 59), in which media is not reduced. In this 

 comparison, the position of the medial cross-vein should be noted. It will 

 be seen that this cross-\-ein divides cell M2; and consequently the two 

 branches of media in front of the cell divided by this cross-vein are veins 

 Ml and Mo. 



Owing to the fact that in the Diptera a great reduction of wing tracheae 

 has taken place, it is not possible to determine by a study of the tracheation 

 of the wings of pupae whether vein M4 has coalesced with vein M3 or with 



S-, 



2d A Cu, Qiv^' 

 Fig. 360. — Wing of Proloplasa fitchii. 



vein Cui; our conclusions, therefore, must be based on a study of wings 

 of adults. 



Among the dipterous wings that are ccmparati\-ely little modified are 

 those of Rhyphus. An examination of a wing of this insect (Fig. 357) 

 would lead one to believe that the three-branched condition of the media is 

 due to a coalescence of veins M3 and Mi and that consequently the vein 

 labeled M3 is really M3H-4. 



That the reduction of media may have been attained in another way is 

 indicated by a study of the wings of Proloplasa fitchii (Fig. 360). This 

 species is one of the most generalized members of the order Diptera; this 

 is shown by the fact that the subcosta is two-branched and that all of the 

 branches of the radius are retained distinct. The suggestive feature 

 indicating that vein M4 may have coalesced with vein Cui is the presence 

 of a vein extending from near the middle of the length of vein M3 to vein Cui. 



This vein has the appearance of a cross-vein; but it is quite possible 

 that it is a section of vein M4, the more distal part of this vein having 

 coalesced with vein Cui. Such a coalescence wovild be quite in accord 

 with what commonly takes place in this order. There are many instances 

 where the distal part of a vein has coalesced with an adjacent vein, the 

 coalescence beginning at the margin of the wing and proceding towards the 

 base of the wnng; with the result that the free part of the vein is pulled out 

 of its earlier position and comes to occupy one transverse to the length of 

 the wing; illustrations of this are given later. If this is what has taken 



