200 AMERICAN HYMENOPTERA. 



following the description of that species. This group is apparently 

 not present in South America unless it be in small numbers in the 

 mountain ranges of the very north portion. 



20. The general paralleling of the ranges of the following groups of 

 Bonibus and Psithyrus is suggestive : 



a. Pratorum and Laboriosus . 



b. Terrestris (without affitiis) and Ashtoni. It should be noted, in 

 this connection, that P. vestalis of the Ashtotii group, is said by 

 Schmiedeknecht to live with terrestris and to have a distribution in 

 Europe similar to that of that species. 



c. Borealis and Fernaldce. 



21. Many of the species of the Pratorum group, in the northwestern 

 part of North America, and all the species of the Borealis, Kirbyellus , 

 Terrestris {affinis perhaps excepted), Ashtoni a.Ti6. Fernaldcz groups 

 show very close afl&nities with European and Asiatic species, the rela- 

 tionship being, in some cases, so close that the American forms must 

 be considered as only subspecies or varieties of the Old World species. 

 A very few species even show no noticeable variation between Old 

 World and American individuals. 



If the number of specimens found in collections is a reli- 

 able indication, there is a comparative paucity of individual 

 bumble-bees in the Great Central Plain of North America, 

 especially in the southern United States. 



Climatic Variation. 

 Species of Bombus with a comparatively level habitat of 

 wide extent are not usually very variable. Greatly variable 

 forms are never found except in greatly diversified regions. 

 No North American species which has the greater part of its 

 range east of the Pacific Highland is very variable, while 

 several of those, the habitats of which are in Alaska, western 

 Canada and the Pacific States of the United States, show 

 astonishing variation in the coloration of their pile. Of 

 these very variable forms, we may mention edwardsii, pleu- 

 ralis, rtdocinctus, californiciis and occidentalis as being, per- 

 haps, the most strking examples. Most of these variable 

 species show the same variation to a greater or less extent 

 in all parts of their habitat. It sometimes happens that 

 a marked degree of variation in coloration is found even 

 among the individuals of the same sex taken from the same 

 nest. 



