THE FLOWER AND THE BEE 



quire, and there is no occasion for them to go elsewhere. There 

 are also on the wing at the same time 6 species which are poly- 

 tropic, but they all obtain a part of their food supplies from the 

 willows, so that the oligotropic species would not escape com- 

 petition with them if there was a scarcity of pollen and nectar. 

 Their average time of flight is about sixty-three days, or 43 

 per cent longer than that of the oligotropic species, which 

 renders it necessary for them to obtain a part of their pollen 

 from other flowers than those of Salix. 



In Milwaukee County, Wis., according to Graenicher, there 

 are 11 autumnal species of Andrena; and all of them, with one 

 exception, are oligotropic visitors of the Compositcp. The single 

 exception {A. parnassice) is found only near Whitefish Bay, 

 Lake Michigan, where Parnassia caroliniana produces a great 

 abundance of flowers. Evidently this bee gets its pollen from 

 these flowers because they are very common in the one locality 

 where it is known. All the other species, 10 in number, are 

 oligotropic to the CompositoB. Many genera of this family 

 are exceedingly common, as the goldenrods, asters, sunflowers, 

 and thoroughworts, and yield immense quantities of nectar 

 and pollen. There are very strong inducements for these 

 bees to visit these flowers, and comparatively little for them to 

 go elsewhere. In New England 4 species of Andrena restrict 

 their visits to the goldenrods, from which the honey-bees gather 

 annually many tons of honey and a great amount of pollen. 

 Neither the visits of the domestic bee nor of the andrenid bees 

 are the result of competition, l)ut solely of the advantages 

 gained. 



The majority of oligotropic bees flying in summer and 

 autumn, whether they be species of Colletes, Andrena, Perdita, 

 Panurginus, or Melissodes, visit exclusively the Compositce. 

 The large and crowded inflorescence consisting of many small 

 flowers which can be quickly and easily visited, the great abun- 



120 



