1901 



ENTOMOLOGICAL SOCIETY. 



63 



THE TREND OF INSECT DIFFUSION IN NORTH AMERICA. 



Bv F. M. WEliSJTKR, WOOSTER, O. 



American entomologists have for many years been engaged in observing, defining and 

 recording the distribution of species of insects and their progress, if of a migratory nature. 

 In earlier days, when entomologists were few in numberand widely scattered, there was little 

 opportunity for serial observations or combined studies. But now, with entomologists in every 

 State, the distribution of species can be more thoroughly studied and their movetnents, if such 

 occur, carefully traced across State after State with surprising promptness and accuracy. 



While it is not the object of this paper to gather up all the facts bearing on this subject 

 that are scattered through our literature, it does seem proper, at this time, to sketch, tentatively 

 at least the possible if not probable paths along which so ne insects, once unknown, but now 

 ommon, have made their way to their present areas of habitation. 



Map 1. — The Trend of Insect Diffusion in North America. — Webster. 



Acquisitions to our insect fauna have come to us, almost exclusively from two sources, viz., 

 either by introduction amongst articles of commerce or through natural influences, in the latter 

 cases coming either from the north or the south, generally the latter. 



The Appalachian mountain system on the east and the Cordilleran mountain system on the 

 west, while they do not prevent the introduction of foreign insects along our sea coasts, they d« 

 present more or less impassable barriers to the direct progress of such insects inland. In the 

 c ise of the Cordilleras these appear to separate the northwar 1 flowing stream of tropical and 

 fiub tropical species in its onward course, sending one portion along the Pacific coast and the 

 other along the coast of the Gulf of Mexico, the latter sometimes spreading broadly inland, 

 wliile the former holds more closely to the coast. The influence of the high plateau in Mexic* 

 on the eastern branch of this insect streanr, wi'l, some day, offer ma*-erial for a most interesting 

 study, even if it does not solve the problem of the evolution of several of our common insects. 

 The Appalachian system does not approach the gulf coast at its southern terminus, but leaves a 

 broad avenue that enables species moving eastward to pass on along the gulf to the Atlantic 

 coast and thence northward. It does, however, to the northward form an almost impassable 



