INTRODUCTION 
The purpose and scope of this work is to set forth as clearly as 
possible the taxonomic characters of the species of aphids recorded 
up to 1950, on both native and introduced plants in the state of 
Colorado and adjacent territory belonging to the same biotic areas 
as contained in Colorado. The region herein treated comprises the 
whole series of life zones beginning with the Arctic-alpine which is 
inhabited by practically no aphids, to, but not including, the Lower 
Sonoran or desert zone. Geographically, the region comprises the 
Southern Rocky Mountains, the Wasatch and Uintah Mountains, the 
Jemez and Sangre de Cristo Ranges and adjacent plains. Politically, 
the region comprises the entire states of Colorado and Utah, southern 
Wyoming, southeastern and southern central Idaho and northern 
New Mexico. 
This work is not a revision and responsibility is not assumed as 
to complete synonymy. However, considerable time has been spent 
on comparisons with similar species as far as they have been located 
by the regional keys in existence and by host plant records. The 
host plant lists are the only published guides to a large number of 
species described in recent years. A comprehensive revision of the 
aphids of not only North America but of the world is very much 
needed. 
A conservative attitude has been taken as to the status of 
species doubtfully distinct, especially in cases where only a small 
number of individuals and only one life cycle form have been taken. 
It has seemed best to hold such species distinct and await collections 
of larger numbers which may show intergrading variations uniting 
some forms into one species or the discovery of other life cycle forms 
which may exhibit more distinct differences, thus establishing a 
separate status in other cases. 
Careful descriptions, life history habits and drawings, the latter 
by the author and of equal authority with the text, are given for 
each species so that it will be possible for other workers to know 
definitely what the species are and to give a sound basis for writing 
of revisions by later workers. 
The plan of classification followed is mainly that of A. C. Baker 
(1920a). Some workers will disagree with this, and doubtless im- 
proved systems will appear or, in the opinion of some, have already 
appeared. However, the characters on which these latter classifi- 
cations are based are less obvious than those used by Baker and re- 
quire clearing of the specimens. Most of the aphids used in this 
