ENTOMOLOGY 



all directions and does so spread until its further progress is 

 prevented, in one way or another. The paths along which a 

 species spreads are determined, then, by the absence of barri- 

 ers. The diffusion of insects in our own country has received 

 much attention from entomologists, especially in the case of 

 such insects as are important from an economic standpoint. 

 The accessions to our insect fauna have arrived chiefly .from 

 Asia, Central and South America, and Europe. 



Webster, our foremost student of this subject, to whom the 

 author is indebted for most of his facts, names four paths along 

 which insects have made their way into the United States : 

 (i) Northwest Northern Asia into Alaska and thence south 

 and east; (2) Southwest Central America through Mexico; 

 (3) Southeast West Indies into Florida; (4) Eastern from 

 Europe, commercially. 



Northwest. - - The northern parts of Europe, Asia and 

 North America have in common very many identical or closely 

 allied species, whose distribution is accounted for if, as geol- 

 ogists assure us, Asia and North America were once con- 

 nected, at a time when a subtropical climate prevailed within 

 the Arctic Circle; in fact, the distribution is scarcely explic- 

 able upon any other theory. Curiously enough, the trend of 

 diffusion seems to have been from Asia into North America 

 and rarely the reverse, so far as can be inferred. 



Coccinella quinquenotata, occurring in Siberia and Alaska, 

 has spread to Hudson Bay, Greenland, Kansas, Utah, Califor- 

 nia and Mexico; while C. sanguined, well known in Europe 

 and Asia, ranges from Alaska to Patagonia ; and Megilla mac- 

 ulata from Vancouver and Canada to Chile. About six hun- 

 dred species of beetles are holarctic in distribution, as was 

 mentioned. Some of them inhabit different climatal regions 

 in different parts of their range; thus Melasoma (Lina) lap- 

 ponica in the Old World " occurs only in the high north and 

 on high mountain ranges, whereas in North America it ex- 

 tends to the extreme southern portion of the country," being 

 widely diffused over the lowlands (Schwarz). Similarly, 



