INSECTS OF WESTERN NORTH AMERICA 239 



plausible evidence occurred in mid-Cenozoic and involved chiefly 

 species which may be considered ecologically as forming the warmer 

 fringe of the cool-temperate biota. The best evidence involves tree- 

 feeding leafhoppers of the genus Erythroneiira and caddisflies of the 

 genera Pycnopsyche, Agapetus, and the more warm-adapted 

 species of Rhyacophila, which presumably became widespread 

 across North America and Eurasia along with the temperate 

 deciduous forests of that era. Most of these insect examples had a 

 greater eventual effect on the fauna of eastern North America than 

 on that of the West, and established nuclei for groups that became 

 important elements of the eastern temperate deciduous forest. 



Certain of these mid-Cenozoic dispersals did apparently result 

 in colonizations that evolved into species flocks in the West compara- 

 ble with those that evolved in the East. The best examples with 

 which I am acquainted are the sawfly genus Neodiprion and the 

 caddisfly genus Agapetus. We do not know where the ancestral 

 form of Neodiprion (Fig. 6) arose, but it seems obvious that an 

 eastern and a western population became established, that each 

 evolved into a distinctive species flock, that each now constitutes 

 an important element in the conifer-inhabiting insect fauna, and 

 that the eastern and western branches of the genus have remained 

 separate geographically until almost the present time (Ross, 1955). 



A comparable situation may be postulated for the American 

 species of the caddisfly genus Agapetus. The larvae of these species 

 construct saddle-like cases and inhabit clear, cool, spring-fed streams 

 in hilly and mountainous country throughout the Allegheny system 

 in the East and much of the western montane region south of 

 Canada. Available evidence, based on characters of venation and 

 abdomen, indicates that the North American species arose from an 

 Asiatic ancestor which spread into North America at the time of the 

 Holarctic temperate deciduous forest. The present day North 

 American species of Agapetus appear to constitute two primary 

 phyletic branches, one in the East and one in the West. Each branch 

 has evolved into a moderate cluster of species, many of which 

 exhibit striking changes compared with more primitive forms. 

 This combination of phylogeny and distribution suggests that when 

 the American transcontinental temperate deciduous forest broke up 

 in mid-Cenozoic into widely separated segments, one segregate of 

 Agapetus persisted in the East and one in the West. There is no 



