﻿158 Journal New York Ent. Soc. [Voi. in. 



ON THE PROBABLE ORIGIN, DEVELOPMENT AND 



DIFFUSION OF NORTH AMERICAN SPECIES 



OF THE GENUS DIABROTICA.* 



By F. M. Webster. 



This genus of Coleoptera is almost exclusively confined to the west- 

 ern hemisphere, with its native habitat, seemingly, in the Neotropical Re- 

 gion. In fact South America would app_^ar to have been its original 

 home, from which country the eighteen species known to inhabit North 

 America north of Mexico might almost be supposed to have originally 

 sprung, while one of these species, balteata, appears to have unchanged, 

 spanned the borderland, so to speak, being found from Texes southward 

 to Colombia, South America. The species of this genus number some- 

 thing like four hundred, which fact, when compared with the limited 

 number occurring in our own fauna, would of itself suggest that we 

 were on the extreme outskirts of its northern habitat. With this idea 

 of the northern spread from the tropical habitat in mind, we can readily 

 comprehend the evolution of the two species, 12-piinctata and soror, 

 the former ranging over the eastern region from Canada to southern 

 California and possibly into Mexico, while the latter ranges from Ore- 

 gon southward, also into Mexico, and covering Arizona. It is as if 

 from out of the Lower Sonoran of Mexico twin species had been evolved, 

 the one, soror, making its way to the west of the great mountain range, 

 while the other, 12-piinctata, spread over the country to the north and 

 northeastward. It is true these two species occupy the same area in 

 Arizona and southern California, but I believe that a careful study of 

 their respective habitats there will show that they do not occupy areas 

 of the same altitude. That 12-pitnctata breeds freely in low, alluvial 

 sections there is ample proof. I have found their larvce in destructive 

 abundance in districts along the Mississippi River where the land was 

 subject to overflow, and Prof. Forbes (i8ih Report State Entomologist 

 of Illinois, p. 157, foot-note) finds larvce, supposed to belong to this spe- 

 cies, destructively abundant in a field in southern Illinois that had been 

 under water for nearly three weeks in spring. In an earlier publication 

 (Entomologica Americana, II, p. 174) this was thought to be the 

 larvae of longicornis, but afterwards referred to 12-punctata, because 



*Read before the Association of Economic Entomologists, at Springfield, Massa- 

 chusetts, August 28, 1895, and the N. Y. Ent. Soc. November 5, 1895. 



