INSECTS OF WESTERN NORTH AMERICA 245 



indicated in Fig. 10 were relatively recent and without much doubt 

 within the late Pliocene-Pleistocene period. Similar circumstances 

 prevail in many genera of northern grass-feeding leafhoppers and 

 also, I am sure, in a large number of other insects abundant in the 

 North and Northwest. In Culicoides, an extremely widespread 

 genus of biting flies, Khalaf (1954) found evidence of even wider and 

 relatively recent dispersals, some indicating a total spread including 

 central and northwestern North America, Asia, Europe, and Africa. 

 Two Alaskan species, Culicoides obsoletiis (Meigen) and C. tri- 

 striatidus Hoff, belong to such widely ranging complexes. 



In addition to these intercontinental movements, dispersals 

 from the eastern to the western areas of the continent are illustrated 

 by insects. Although there is abundant evidence that members 

 from many western species flocks spread to and colonized the East 

 at various times in the Cenozoic, present information indicates 

 that dispersals to the West from eastern species flocks were much 

 rarer and occurred chiefly in the Pleistocene. Of the seven older 

 eastern species flocks in the caddisfly genus Rhyacophila, none has 

 apparently spread to the West. Some other caddisflies do indicate 

 such a dispersal. In the genus Triaenodes nineteen species form a 

 fairly old, distinctive, polyphyletic complex which appears to 

 have evolved in the eastern deciduous forest area as a series of 

 species flocks (Fig. 11). Seventeen of these nineteen species are still 

 restricted to the East and Northeast (Fig. 12), but two, baris and 

 tarda, extend westward into the Rocky Mountain region. 



Because no morphological differences have been detected between 

 eastern and western populations of the two species just mentioned, 

 the most logical interpretation is that they evolved originally in the 

 East and spread to the West along ecological corridors brought into 

 existence by Pleistocene events. An alternative logical possibility 

 is that the progenitors of tarda and baris spread from the East to 

 the West, that the western populations of each became isolated 

 and evolved into distinctive species, and that these two species in 

 turn spread eastward during Pleistocene. The restricted western 

 distribution and extensive eastern range of T. tarda, however, 

 strongly suggests that it was of eastern origin. 



Several species of the related genus Athripsodes, notably cancell- 

 atus and tarsipunctatus, exhibit parallel phenomena and may also 

 represent recent dispersals from the East to the West. It is quite 



