112 Annals Entomological Society of America [Vol.11, 



tion, and it is perhaps as pretty an illustration as can be found of 

 the conclusive evidence which tracheation can sometimes bear in a 

 ■doubtful case of venation. 



The same condition is shown in Chermes abietis (figs. 2 4 and 25) . 



Although the radial trachea has no main branches except Ri 

 and the sector, this trachea very frequently as is the case of the 

 ■other tracheae, bears a great number of secondary ramifications. 

 These secondary branches are omitted in many of the drawings 

 accompanying this study as they are the first to clear and become 

 invisible,* and were frequently neglected when first attention was 

 concentrated on other points. However, a glance at Chermes 

 abietis (fig. 24) and Lachnus sir obi (fig. i) representing the genera 

 at the extreme ends of the Aphididas show the general condition 

 of these radial ramifications which are further treated under the 

 discussion of the subcosta (see page 108). 



With an understanding of the tracheation, little explanation 

 is required for the vein radius. Except for the Chermesince, 

 the free part of R^ forms the caudal boundary of the stigma and 

 extends to the margin of the wing. The radial sector originates 

 in the vicinity of the stigma and curves strongly to near the tip 

 of the wing. The main stem of radius extends from the junction 

 of these branches to the base of the wing in a line about parallel 

 to the subcosta, — forming indeed a part of that strong main 

 composite vein channel of the wing. 



In Chermesinse, radius is unbranched, as has been explained, 

 .and takes a straight course from the base of the wing to the ex- 

 tremity of the stigma. A double adjustment takes place here. 

 Correlated with the loss of R^ which in the more generalized 

 genera bounds the stigma along its distal edge, is a straightening 

 of the caudal margin of the stigma and a straightening of the 

 course of the radial sector so that the radial sector runs along the 

 edge of the broadened stigma in this type of wing (figs. 3 1 and 32). 



MEDIA OF APHIDIDiE. 



For a correct interpretation of media an understanding of the 

 tracheation is necessary. The freshly emerged wing of Myz^t,s 

 cerasi (fig. 3) gives a fairly typical tracheation for the more gen- 

 eralized wings. The medial trachea will here be seen, as is true 

 for all the genera of Aphididas, to be the second of the two main 

 tracheae to enter the wing from the bod^^ cavity. This trachea 



* See discussion of preparation, page 103. 



