DISTRIBUTION 379 



Silpha lapponica is strictly arctic in Europe, but is distributed 

 over most of North America; Silpha opaca, on the contrary, 

 is common all over Europe, but is strictly* arctic in North 

 America. Silpha atrata, common throughout Europe and 

 western Siberia, was introduced into North America, but failed 

 to establish itself. 



Southwest. Very many species have come to us from Cen- 

 tral America and even from South America. South America 

 appears to be the home of the genus Halisidota, according to 

 Webster, who has traced several of our North American spe- 

 cies as offshoots of South American forms. Many of our 

 species may be traced back to Yucatan. H. cinctipes ranges 

 from South America to Texas and Florida; H. tessellaris has 

 spread northward from Central America and now occurs over 

 the middle and eastern United States, while a form closely like 

 tessellaris ranges from Argentina to Costa Rica; H. caryce 

 follows tessellaris, and appears to have branched in Central 

 America, giving off H. agassizii, which extends northward 

 into California. Similarly in the case of the Colorado potato 

 beetle (Leptinotarsa decemlineata) and its relatives. Accord- 

 ing to Tower, the parent form, L. undecemlineata, seems to 

 have arisen in the northern part of South America, to have 

 migrated northward and, in the diversified Mexican region, to 

 have split into several racial varieties. The parent form 

 grades into L. multilineata of the Mexican table lands, which 

 in turn, in the northern part of the Mexican plateau, passes 

 imperceptibly into L. decemlineata, which last species has 

 spread northward along the eastern slope of the western high- 

 lands, west of the arid region. In the lower part of the Mex- 

 ican region the parent form may be traced into L. juncta, 

 which has spread along the low humid Gulf Coast, up the Miss- 

 issippi valley to southern Illinois, and along the Gulf Coast 

 and up the Atlantic coast to Maryland, Delaware and New 

 Jersey. In general, the mountains of Central America and 

 Mexico and the plateau of Mexico have been barriers to the 

 northward spread of many species, which have reached the 



