2 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



THE DIFFUSION OF INSECTS IN NORTH AMERICA. 



[Author's Abstract.] 

 BY F. M. WEBSTER. 



It does not appear necessary to at this time go into minute 

 details with reference to the possible general trend of insect 

 diffusion in North America, as this matter has been previously 

 covered in a paper published by myself in Psyche tor April, 

 1903. 



Possibly it might, however, be well to call attention to some 

 facts, notably the occurrence of European species on the 

 northwest Pacific Coast, and the greater similarity between 

 specimens found there and those taken in Europe than speci- 

 mens of the same species taken in the Eastern United States. 

 Also, to the well-known fact of a temperate or even tropical 

 climate having once existed in what are now the cold and 

 barren sections of the North. And, in the light of Lieutenant 

 Shackleton's recent discoveries in the south polar regions, we 

 may assume an almost parallel condition to have once obtained 

 within the Antartic Circle, thus tending to prove what has 

 previously been suspected, namely, a land connection between 

 Australia, South America, and South Africa. What followed 

 this period of warm temperature in the southern Antartic re- 

 gions we do not know, but in the north we know that the 

 ice sheets of the Glacial period crowded their way southward 

 in many cases far into what is now the United States. Just 

 what the effect of these immense ice sheets was beyond the 

 southern extremities of the glaciers themselves we can only 

 suspect, but judging from the fossil remains of insects found 

 in the Tertiar}' rocks of Colorado, Wyoming, and British 

 Columbia, we have every reason to suppose that northern 

 species were driven far to the southward, and that, with the 

 disappearance of the ice sheet and the recovering of the gla- 

 ciated area with vegetation, these species, perhaps more or 

 less modified in habits and appearance, would gradually drift 

 back and reinhabit the glaciated territory. 



This appears to be sufficient basis for assuming a post- 

 Glacial trend of insect diffusion from the tropical regions 

 northward. And it would seem that the species working their 

 way northward from Central America through Mexico might in 

 some cases become greatly changed both in appearance and 

 habits, There seems, however, to be a factor in insect diffusion 

 from the south northward that has heretofore escaped notice. 

 Ordinarily, insects accidentally imported into the United States 

 or Canada from foreign countries become established along the 



