INSECTS OF WESTERN NORTH AMERICA 



243 



sawflies. The predominantly northern genera Dolerus, Nematus, 

 Amauronematiis, Rhadinoceraea, and many others have entire 

 complexes within which all western North American species either 

 are Holarctic or have a sister species in Eurasia. It is obvious in 

 these cases that probably during the immediate past either an 

 existing species or its immediate ancestor became widespread 



o 



0) 



U5 



PLEISTOCENE. 



PLIOCENE 



MIOCENE 



0LI60CENE 



EOCENE 



PALEOCENE 



CRETACEOUS 



EUROPE \ ASIA 



NORTH AMERICA 

 WEST ] ^AST 



Fig. 9. Phylogenetic dispersal chart of several species groups from one 

 of the distinctive branches of Rhyacophila, comprising caddisflies which 

 inhabit cold, rapid rivers and streams. (From Ross, 1956.) 



across the Holarctic region. The same situation is shown graphically 

 by evidence from the herb-feeding leafhopper genus Macrosteles 

 (Moore and Ross, 1957). The present distribution of known species 

 of this genus can plausibly be explained only by the intercontinental 

 dispersal of at least sixteen phylctic lines (indicated by black dots 

 on Fig. 10). These sixteen movements were of course not all con- 

 temporaneous, but judged by the uniform morphological similarity 

 of many pairs of sister species, at least the upper thirteen dispersals 



