202 AMERICAN HYMENOPTERA. 



and fraternus are exceptional in this respect, but they may 

 have had a mountain inhabiting ancestry. 



No comparative study of the habits of tropical, temperate 

 and arctic species of the genus Bombus has as yet been made, 

 but it will be surprising if it is found that climate does not 

 have a marked influence on their habits. Are the colonies 

 of tropical species ever perennial ? Are tropical colonies, as 

 a rule, larger than arctic ones ? What effect does the great 

 parasitism of the tropics have on the habits of the tropical 

 forms ? Do the arctic species usually have subterranean 

 nests ? It should be noted, in this connection, that there are 

 rather strong indications that the species of both the Ter- 

 restris and the Borealis groups usually have their nests under- 

 ground. In tropical and temperate regions, the lowland spe- 

 cies, at least, seem, as a rule, to nest on the surface. 



Economic Importance. 

 Taking the world as a whole, probably the species of no 

 other genus surpass those of the genus Bombus in their im- 

 portance as pollen carrying agents, unless it be the species 

 of Apis. In the cooler climates their season of activity in 

 numbers is, of course, not so long as that of the honey bee. 

 On the other hand, however, they pollenize the flowers of 

 some plants which are not much visited by other bees, as 

 they are enabled by their longer tongues to reach the nectar 

 at the bottoms of long flower cups, which is out of the reach 

 of Apis mellifera and most other bees. The red clover, Tri- 

 folium PratensCy seems, in most countries, to rely mainly on 

 bumble-bees for the transference of its pollen, while its blos- 

 soms are seldom visited by the honey bee. The white clover, 

 Trifolium repens, is, on the other hand, one of the favorite 

 plants of the honey bee. The main difference between these 

 two clover species, from the standpoint of bee visitation, ap- 

 pears to be the difference in the length of the carolla tubes. 

 If bumble-bees are the main pollenizing agents of red clover, 

 as is commonly supposed, this clover should, as a rule, yield 

 a heavier crop of seed when grown in the Atlantic or the 

 Pacific Highland of North America than when grown in the 



