no Amtals Entomological Society of America [Vol.11, 



The absence of a costal trachea is by no means unusual.* The 

 aphid ^Ying is characterized by the reduction of tracheae and the 

 loss of the subcostal trachea is only one more step in the general 

 trend of reduction. What the aphid lacks in main tracheal 

 trunks, however, it makes up in secondary ramifications, and a 

 glance at the tracheation of Chermes ahietis (fig. 24) will show that 

 the absence of costal and subcostal tracheae does not indicate that 

 this portion of the wing is untracheated. Perhaps the abundance 

 of secondary tracheae explains the absence of unnecessary costal 

 and subcostal tracheae. At any rate they might in some cases be 

 mistaken for one of these main tracheae. Refer again to Chermes 

 abieiis (fig. 24) where the ends of the radial branches are so 

 interwoven as to have the appearance of a continuous subcostal 

 trachea. And where the channel of the subcostal vein hides the 

 radial connection, we have frequently the occurrence of what 

 appears to be a strong subcostal trachea with branches extending 

 toward the radius. Further, in case the radial connection of all 

 these tracheae, except a single one near the base of the wing, is 

 obscured, as sometimes happens, there is, to all superficial appear- 

 ances, a clear indication of a strong subcostal trachea. 



Altogether the absence of the subcostal trachea was one of 

 the most troublesome points to establish. Each clue was fol- 

 lowed, however, until the evidence was absolutely conclusive. 



Although nO subcostal trachea is present, there seems to be 

 no other conclusion except to consider that the vein subcosta is 

 present in the large main vein channel of the wing, and extends 

 from the base of the wing to the stigma where it approaches the 

 margin of the wing. Indeed it seems not at all unlikely that this 

 stiff vein fold at the base of the aphid wing is responsible for the 

 atrophy of the subcostal trachea. The radial trachea from its 

 position can elude the pressure and run alongside, but this ob- 

 struction alone would seem sufBcient to explain the loss of the 

 subcostal trachea in the Aphididee. 



RADIUS OF APHIDID^. 



The tracheation of Myzus cerasi (fig. 3) gives a fairly typical 

 two-branched radial trachea. This type persists for the family 

 of Aphididae except in the Chermesinae where this trachea is not 

 branched. This trachea is one of the two main tracheae which 

 enter the wing, and from the phylogenetic standpoint the chief 



* See Footnote, page 108 (Costa and Subcosta). 



