1909] Homologies of Wing Veins 119 



pies the same position as in the front wing. Similarly there is a 

 common stem of the radial, medial and cubital tracheae. The 

 medial trachea, however, is unbranched, and the tracheal branch 

 preceding R^ is wanting. 



The loss of R, is significant. This vein is one of the least 

 stable of the Hemipterous wing veins. Its complete absence has 

 been established for the Cicada and a Coreid.* In the front wing' 

 of the aphids Ri was seen to be preceded by a wavering 

 trachea in all the subfamilies except the Chermesinas where the 

 vein Ri is altogether lacking. In the hind wing of none of the: 

 aphids does either the vein R^ occur, or the corresponding trachea. 



In the front wing of the psyllid and Aleurodicus alone for this 

 group is Ri a strong vein and here it is evidently the response to 

 the mechanical necessity not otherwise provided for. Subcosta, 

 is wanting and, as the only vein present in this portion of the 

 wing, the burden of support falls upon the radius. 



It is exceedingly interesting to find the condition of R^ as 

 predicted for the Hemipteraf on the basis of the Cicada and 

 Coreid, fulfilled in this group of highly specialized Homoptera. 



It is interesting, too, to find the same veins, vein for vein, 

 appear in the wing of the psyllid that we have in the wing of the 

 aphid — the most striking difterence is a very slight one, — the 

 cubitus branched in the psyllid and unbranched in the aphid. 

 There is experienced no difficulty, as has been seen, in homologiz- 

 ing either the aphid or the psyllid veins independently of each 

 other but the fact that in doing so the conclusions arrived at with 

 each support and bring additional evidence for the other should 

 in no wise be disregarded. 



The fact that the costal margin of the psyllid wing is strength- 

 ened in one of two distinct ways (i) by a stigma and (2) by R^, is 

 very w^ll brought out in the five psyllids which have been selected 

 to represent the venation of this family. Of these Eiiphalems 

 nidifex Schwarz| (fig. 38) would seem to be the most generalized 

 type so far as the radial region is concerned, Ri in this wing extend- 

 ing to near the tip of the wing as the trachea preceding R^ does 

 in the nymphal wing pad of Psylla floccosa (fig. 33). In Pachy- 



* Comstock and Needham: Wings of Insects. Page 245. 



t Comstock and Needham: Wings of Insects. Page 245. 



t For an ojjportunity to study and figure the wings of E. nidifex and A. mori, 

 the writer is indebted to Dr. L. O. Howard, who kindly loaned specimens of 

 these species for this purpose. 



