INSECTS OF WESTERN NORTH AMERICA 



251 



these two populations. Only in comparatively recent times have 

 shifts in climates or topography produced conditions under which 

 northeastern species of Ripaeglossa spread westward and north- 

 western species eastward. This and other similar examples of 

 caddisfly distribution indicate that in these mountain areas Pleisto- 



Eostern Ranges 



White, Wosotch, Rockies, etc. 



^LARAMIE BREAK 



SO-CEN 

 ARIZ NM COLO WYO UT WYO MOM IDAl 



Western Ranges 



Coscodes, Sierra Nevado, etc. 



CALIF 



^CALIFICA 

 WENATCHEE 



PTERNA 



ALASKA 



Fig. 15. Phylogenetic dispersal chart of Glossosoma subgenus Ripae- 

 glossa, a group of caddisflies inhabiting cold, rapid rivers and confined 

 to the mountainous area of western North America. (From Ross, 1956.) 



cene events effected a mixing of phyletic lines previously separated 

 from each other for long periods. 



In exploring the origin of the cool-adapted western insect fauna 

 we have surveyed data from only a small fraction of the insects 

 that occur in the cooler parts of the West. It is noticeable, however, 

 how most of the data fits readily into the concept of an almost 

 cyclic alternation of brief dispersals and long separations. The many 

 small but different types of evidence pointing in the same direction 

 give us reason to visualize an extensive dispersal and intermingling 



