THE WINGS OF INSECTS 397 



Wing of a muscid. — The wing used is that of the common house-fly, 

 Miisca domestica. 



Wings of dolichopodids. — The wings of two of the long-legged flies will 

 be used. The first belongs to the genus Psilopodius; the second to the 

 genus Dolichopus. 



Wing of a syrphid. — The wings of a fly of the genus Eristalis will be 

 used. Note the \-ein-like structure between the radius and the media ; this 

 is termed the spurious vein. 



Wing of Dixa. — The wing of a dixa-midge is used as an example of the 

 venation of the midge-like flies. If a wing of Dixa is not available, use one 

 of a mosquito. In the midge-like flies the number of the branches of the 

 radial sector is reduced in a way different from that seen in the families 

 previously studied. Compare with the asilid, the tombyliid, and the 

 scenopinid. 



REVIEW" OF WORK ON WINGS OF DIPTERA 



In the preceding studies of wdngs of Diptera illustrations have been seen 

 of A'arious ways in which these wings have been modified in the course of 

 their evolution. It will clarify the subject if some of the more important 

 of these methods of modification of the primitive type be studied separately. 



Reduction of the radial sector. — Note the follow ing facts : — 



In Protoplasa fitchii (Fig. 415) the radial sector is not reduced, being 

 four-branched as in the hypothetical typical form of this vein. 



In Tabanns the radial sector is only three-branched. This is the result 

 of the coalescence of two of the branches of this vein. 



In Dixa also the radial sector is reduced to a three-branched condition 

 by the coalescence of two of its branches; but in Dixa not the same 

 branches coalesce as in Tabanns. 



These two genera represent two kinds of specialization, which indicates 

 that they belong to different lines of descent. The common progenitor of 

 these two genera had a four branched radial sector; in some of the descen- 

 dants of this primitive form one method of reduction has taken place, while 

 in other descendants another method has been followed. 



That this differentation took place comparatively early in the history 

 of the order is shown by the fact that in all Nematocera that ha\-e a three- 

 branched radial sector veins R2 and R3 remain distinct; while in those 

 Brachycera that have a three-branched radial sector veins R^ and R5 are 

 separate. The Nematocera and the Brachycera are the two chief divisions 

 of the order. 



In both divisions of the order the reduction of the radial sector is carried 

 farther in many cases. Thus in Rhyp'ius the radial sector is only two- 

 branched (Fig. 410). 



Two methods of coalescence of veins. — There are two methods of 

 coalescence of wing-veins. By one method the coalescence proceeds out- 



