Vol. Xxiv] ENTOMOLOGICAL NEWS. [(X) 



competition with the polytropic bees severe enough to force 

 the oligotropic species to go elsewhere. 



The maximum of species of Andrena on the wing at one 

 time in Milwaukee County, Wis., says Dr. Graenicher. is 

 reached in the latter part of May and lasts throughout the 

 first week of June. Various common shrubs, which produce 

 their flowers in great profusion are then in bloom, as Vibur- 

 num, Crataegns, Conius, Ribes and Rub us, besides a great 

 abundance of umbelliferous flowers. This maximum, there- 

 fore, "corresponds with the blooming period of a great va- 

 riety of flowers, representing different families." The ma- 

 jority of the species of Andrena, or 17, are polytropic, as the 

 character of the flora would make probable. Seven species 

 are oligotropic, two getting their pollen from the late bloom- 

 ing willows, one from the strawberry, a very common and 

 widely distributed plant; one from Geranium maculatum, and 

 two from the very abundant flowers of Thaspium trifoliatwn 

 aureum and Taenidia integerrima, two species of Umbelli- 

 ferae. As in the case of the willows the flowers of the two 

 last named plants are visited by many polytropic bees. The 

 oligotropic species, then, visit very common flowers, which 

 are in bloom during the entire time of their flight, and which 

 produce pollen and nectar in quantities sufficiently large to 

 prevent injurious competition between them and the many 

 polytropic bees which visit the same flowers. 



The seventh species (A. geranii} in Milwaukee County 

 gets its pollen from Hydrophyllum, but at Carlinville, 111., 

 according to Robertson, it also gathers pollen from BlephiKa 

 ciliata. Observations on the visits of the various species of 

 Andrena are far too few in number. Robertson says that he 

 has observed 595 visits of 51 species. This is only n.6 visits 

 for each species. A large number of observations from many 

 localities is very desirable, and might modify our ideas on 

 the constancy of oligotropic bees. 



The evidence against oligotropism being an effort on the 

 part of bees to avoid competition for pollen becomes much 

 stronger when we consider the summer and autumnal species 



