INSECTS OF WESTERN NORTH AMERICA 237 



tain regions of western North America since the time of this supposed 

 mid -Cretaceous dispersal, or if they did spread to other areas have 

 become estabUshed in no other region. This situation suggests 

 that for survival these insects must have streams which are not 

 only cold but rapid, and that in the past greater continuity of 

 mountainous terrain was a major factor in permitting their dispersal. 



Evidence is highly suggestive that during the height of the 

 Laramide orogeny, that is, the very end of Cretaceous and the 

 earliest part of Cenozoic, many members of the cool-adapted biota 

 spread between western North America and Asia. The mountain- 

 inhabiting caddisfiies offer evidence for this dispersal also. The 

 genus Wormaldia, which could very well have evolved from a 

 Cretaceous subgenus of Sortosa isolated in western North America, 

 probably spread in Paleocene into Asia and across to Europe. 

 Whichever way the dispersal occurred, it antedated the Baltic 

 Amber (in which Wormaldia occurs) and is documented by clusters 

 of Wormaldia species in many continental areas (Fig. 4). One 

 cluster of nine species occurs in western North America. It may be 

 postulated that members of two other caddisfly families dispersed 

 in similar fashion at this same time. The genus Rhyacophila, 

 belonging to the family Rhyacophilidae, is one of the largest and 

 commonest mountain caddisfly genera of the West. Of its 44 species 

 groups, 19 occur in the West but 7 are known also from other 

 areas. It is almost certain that the ancestors of the other 12 groups 

 either reached the West no later than Paleocene or evolved in the 

 mountains of western North America from older parental forms. 

 The same may be true also of certain of those seven groups that 

 now occur in two or more major areas. 



In the family Glossosomatidae (the caddisfiies whose larvae 

 construct saddle-like cases) both the archaic genus Anagapetus and 

 the subgenus Ripaeglossa of Glossosoma appear to have had similar 

 early histories. It is impossible to be sure whether the ancestral 

 forms of these groups arose in Asia or North America, but wherever 

 they did originate, they dispersed between the two continents at 

 some early date. In western North America a remnant of each line 

 appears to have evolved in, and to have been restricted ever since 

 to, the higher elevations of this area (Fig. 5). 



The next dispersals of cool-adapted insects for which we have 



