iQog] Homologies of Wing Veins iii 



interest concerning it is centered in the cephalic branch, or the 

 trachea preceding vein Rj. 



In the case of two widely separated Hemiptera, a Cicada and 

 a Coreid, this trachea was found to be but weakly developed in the 

 n^^mphal wing pads,* being apparently crowded out by the strong 

 subcostal trachea, and it was not succeeded by a vein, — the vein 

 Rj being completely absent in these two insects. 



With the aphids no such crowding of the radial trachea occurs, 

 for, as has been shown, the costal and subcostal are both absent. 



It is, however, of great phylogenetic interest to find that in the 

 tracheation of the aphids, the weak character of this same tra- 

 chea (that is the one preceding vein Ri) is evident. In Lachnus, 

 probably the most generalized genus in the family, this branch is 

 scarcely to be distinguished from the secondary branches of the 

 radial trachea and is emphasized chiefly from the fact that it is 

 succeeded by a vein R^ (fig. i). And in all of the generalized 

 genera it is characterized by a wavering and uncertain course. 

 It is consistent with this general tendency to find that this is the 

 tracheal branch which if it appears at all in the specialized Cher- 

 mesinas is so weakly indicated as to become indistinguishable 

 from the secondary branches and loses its significance as it is not 

 succeeded by a vein, vein R^ being absent in the Chermesinae. 



The wings of the Chermesinae are at a glance conspicuously of 

 a difterent type from those of the more generalized aphids, and 

 this difference in venation has heretofore been interpreted as due 

 to the absence of media (the "third discoidal"). Especial atten- 

 tion is called, therefore, to figures 28 and 29. These are two draw- 

 ings made of the same identical wing of Chermes pinifolicE, — 

 figure 28 drawn immediately after mounting when most of the 

 tracheae showed, and figure 29 drawn from the same mount 24 

 hours later when the trachea had cleared and become invisible 

 and the veins are apparent instead. These drawings indicate 

 the character of the difterence between the Chermes venation and 

 that of the more generalized genera. Ri has disappeared and 

 Rs, losing its characteristic curve, lies along the caudal margin of 

 the stigma. Media, apparently to replace the position and func- 

 tion thus deserted by the radial sector, migrates forward to ap- 

 proximately the position which the "stigmal vein" or radial sec- 

 tor occupies in the generalized genera. This is an interpretation 

 which could in no wise be settled except by appeal to the trachea- 



* Comstock and Needham. Wings of Insects, pp. 245 — 251. 



