2 APHIDS OF THE ROCKY MOUNTAIN REGION 
study have not been thus prepared and the necessary characters 
cannot, at the present time, be observed in them. It would be neces- 
sary to remount and redescribe much typic material before such a 
system could be applied. It seems therefore more practical at this 
time to-use a system based on characters more easily observable. 
The bibliographies contain only the reference to the original 
description and a few later, usually fuller, descriptions aiding in 
establishing the concept of the species. 
Type locations, as far as known to the writer, are given follow- 
ing the description of each species. 
Acknowledgment 
Acknowledgments are due to nearly all the aphidologists in the 
Northern Hemisphere living in this century for aid in opinions and 
loans or contributions of determined specimens as well as in their 
published works. In the latter service, of course, all the former 
aphidologists also deserve credit. Mention of individuals deserving 
special credit will be made in connection with the species concerned. 
To avoid too numerous repetitions special mention is here made of 
George F. Knowlton who very generously cooperated in every way 
possible, especially in the loan and contribution of all his typic ma- 
terial of over 100 species; practically all the Utah and Idaho records 
were his collections. H. C. Manis also aided in submitting a list of 
aphids taken in southern Idaho. D. G. Denning kindly loaned aphid 
material from Wyoming. F. C. Hottes supplied many of the records 
from western Colorado and loaned all his typic material and many 
other specimens. 
Thanks are due to Myron Swenk for loans of specimens from 
the collection of University of Nebraska. A. A. Granovsky and, 
earlier, J. T. Monell aided by loans of specimens and valuable advice 
on the group Panaphini. A. C. Maxson lent much valuable assist- 
ance on the subfamily Eriosomatinae, in loan of specimens, and of 
data besides valuable advice. E. O. Essig kindly loaned many type 
specimens of species described in Califcrnia and freely gave his 
opinions on many of the determinations. Special mention is due to 
E. A. Chapin, C. F. W. Muesebeck, P. W. Mason and Louise M. 
Russell for making possible the examination of type specimens in 
the United States National Museum and for advice on nomencla- 
torial problems as well as for kindly encouragement. 
Thanks are also due to E. O. Essig and Miss L. M. Russell for 
reviewing the manuscript and making many valuable suggestions. 
To D. Hille Ris Lambers thanks are here extended for much 
detailed advice on classification and identity of species and for the 
opportunity of examining many European species, made possible by 
