io6 Annals Entomological Society of America [Vol.11, 



THE COURSE OF A VEIN. 



In the following pages the "course" and the "free part" of 

 veins are frequently mentioned. By the free part of a vein is 

 meant all that portion that is not coalesced with any other 

 vein. As for instance in Fig. 36, the free part of Cu^ is all that 

 portion of Cuj between the point where it separates from Cu^ 

 and the margin of the wing. 



As to the course of the vein it has been convenient to consider 

 each of the branches of any vein as extending from the base to 

 the .margin of the wing, as is made plain by the following quo- 

 tation :* 



"If radius and its five branches be taken as an example, the 

 stem part, always designated as R, would be considered as being 

 a combination of all the branches of radius, or as Ri + 2 + 3 + 4 + 5) 

 which divides into Ri and R^. In like manner the stem of the 

 radial sector would be considered as being a combination of all 

 the branches of the radial sector, or as Ro + 3 + 4 + 5, which divided 

 into R2 + 3 and R4 + 5, and these in turn into R, and R3, and R4 

 and R5, respectively. So that in tracing out the course of any 

 of the branches of radius by drawing a pencil along them, as R^, 

 beginning at the base of the wing, we would pass first over the 

 stem of R, then over the stem of the radial sector, then over 

 R4 + 5, and finally over the free part of R^." 



COALESCENCE OF TRACHEA. 



A comparison of the tracheation of the wing pad of an aphid 

 nymph with that of the wing of the recently emerged aphid of 

 the same species reveals the fact that there are fewer basal tra- 

 cheae in the later than in the earlier stage, although the terminal 

 branches are the same in number. For instance, there are four 

 tracheae which branch from the body trachea in the nymphal 

 wing pad of Schizoneura rileyi (fig. 21) and only two tracheal 

 stems at the base of the newly emerged wing of the same species 

 (fig. 22). Similarly as there are four tracheas at the base of the 

 nymphal wing pad of the species of Aphis which I ha\'e studied 

 and of Callipterus ulmifolii while there are but two basal tracheae 

 in the newly emerged wings of these same species, this is shown to 

 be the normal condition for the family in the more generalized 

 genera. In the nymphal wing pad of Mindarus (fig. 18) three 



* MacGillivary, A. D., Wings of Tenthredinoidea. Proc. U. S. Nat. Museum 

 Vol. XXIX, page 576. 1906. 



