250 H. H. ROSS 



and coniferous forests are sufficiently extensive to have an air of 

 permanence. If, however, they were stable features of the landscape, 

 geologically speaking, one would expect some distinctive taxonomic 

 units to have evolved, and remained, therein. Among the insect 

 groups of my acquaintance, the species of the flat north country are 

 either the same as those in the western or eastern mountains, or 

 were obviously derived relatively recently from montane species. 

 It appeared at one stage in our studies that case-making cacldisflies 

 belonging to the Limnephilus suhlunatus complex might be a species 

 flock which had evolved in the flat northland. Most of the earlier 

 records for many species in this group were from northern Sas- 

 katchewan, the Mackenzie River Delta, and Great Slave Lake. 

 More recent collections from Yukon and Alaska emphasize the 

 possibility that the Limnephilus sublimatus complex may be associ- 

 ated historically with the northern ranges of the Rocky Mountains 

 rather than with the flatter country. 



2. The two main areas of caddisfly distribution in the mountains 

 south of Canada, the Cascade-Sierra Nevada area to the west and 

 the main Rocky Mountain area to the east, are separated by an 

 irregular strip of arid, less mountainous country, including the 

 Great Basin. This pattern is illustrated by the distribution of those 

 caddisflies of the genus Glossosoma that comprise the subgenus 

 Ripaeglossa, which abounds in large, fast, clear streams in both 

 areas. The sixteen species of Ripaeglossa form two major phyletic 

 branches. The branch comprising the alascense and traviatum 

 species groups apparently evolved primarily in the western ranges 

 and the one comprising the parvulum species group evolved in the 

 eastern ranges. At the present time the northern six of the sixteen 

 species of Ripaeglossa extend around the northern end of the arid 

 zone separating the two mountain areas, but no phyletic line appears 

 to have divided into sister species in the area to which it spread 

 (Fig. 15). From these data it is possible to reconstruct a plausible 

 series of events. When the progenitor of all existing forms of Ripae- 

 glossa lived, clear mountain rivers of the West formed a sufficiently 

 well-connected system to allow this caddisfly to spread throughout 

 the West. Increased aridity of the Interior Basin area broke this 

 river network and split the ancestral Ripaeglossa species into well- 

 separated eastern and western populations. This same condition 

 presumably prevailed during the entire subsequent evolution of 



