INSECTS OF WESTERN NORTH AMERICA 



233 



invertebrates and contrast sharply with the rapid evolutionary 

 rates described as characteristic for many groups of Mammalia. 



PREHISTORIC FAUNAL ELEMENTS 



Fossil records show that many insect genera occurred previously 

 in western North America but are now absent from the area. 

 Examples from the cool-adapted biota include the fern-boring 



Fig. 2. The phylogenetic tree of the caddisfly genus Wormaldia 

 subgenus Doloclanes superimposed on the known distribution of the 

 species. This subgenus may be most closely related to more primitive 

 Baltic Amber species. 



sawfly Blasticotoma, known only from fossil remains in the Flor- 

 issant shales of Colorado and from living representatives in Europe 

 (Benson, 1942) ; and the snakefly genus Raphidia, known also from 

 the Florissant shales but not now occurring in North America 

 (Carpenter, 1953). Since representatives of the caddisfly genus 

 Phylocentropus are now known only from Baltic Amber fossils and 

 from living species restricted to eastern North America, intervening 

 areas must have been populated by this genus at some past time. 

 Because considerable evidence indicates that the intercontinental 

 connection was between northwestern North America and north- 



