246 



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Fig. 11. Phylogenetic diagram of the North America components of 

 the case-making caddisflies belonging to the genus Triaenodes. 



possible that comparable information on other insect groups will 

 add greatly to these examples. 



The sum of the Holarctic and trans-American dispersal patterns 

 points to a truly extensive faunal interchange associated with the 

 colder climates of the Pleistocene. 



The American species of the caddisfly genus Helicopsyche il- 

 lustrate another type of range extension into the cool-temperate 

 world. The larvae of Helicopsyche live in small clear streams and 

 make strong cases shaped like snail shells. All fourteen described 

 species occur in the tropical and /or subtropical regions and three of 

 them extend northward into the temperate region (Fig. 13). For 

 two species, these extensions are slight or small: H. mexicana has 

 outpost populations in central Arizona, northern New Mexico, and 

 Arkansas, and H. vergelana has an outpost in western Louisiana. 

 The species borealis, however, extends northward into the cool- 

 temperate zone, reaching Washington in the West and Quebec in the 

 East. Thus H. borealis has in some manner acquired an ecological 

 tolerance much wider than that of other members of the genus, and 



