318 ENTOMOLOGICAL NWS. [July, '14 



From the very beginning an oligotropic bee saves time and 

 labor, and later may enhance this advantage by learning "to 

 manipulate the pollen better than other bees." 



In a previous paper I have reviewed the oligotropic species 

 of Wisconsin Andrcna, and have showed that there is no evi- 

 dence that they have acquired this habit in competition for 

 flower food. (Graenicher, S., "The Relations of the And- 

 renine Bees to the Entomophilous Flora of Milwaukee Coun- 

 ty," Trans. Wis. Ac. Nat. Sci., 15:89-97.) Six vernal spe- 

 cies, including the two earliest on the wing, are oligotropic to 

 Sali.v, a very widely distributed genus producing a great 

 abundance of pollen and nectar. The willow aments are also 

 visited by all the vernal polytropic species, but their average 

 time of flight is 33 per cent, longer than that of the oligotropic 

 species. Of the eleven autumnal Andrenid species ten, or in 

 view of the limited distribution of A. parnassiae, practically 

 all are oligotropic to the Compositae. The single exception, 

 A. parnassiae, occurs only at Whitefish Bay, Lake Michigan, 

 where it is oligotropic to Parnassia caroliniana, a plant very 

 common in that locality. Since the flower is widely distrib- 

 uted, while the bee is local, evidently the former has not de- 

 termined the habitat of the latter. These figures certainly in- 

 dicate that the adaptation of the vernal oligotropic species to 

 the Salicaceae and of the autumnal species to the Compositae 

 is determined by the character of the flowers. In a similar 

 manner honey bees restrict their visits almost exclusively dur- 

 ing the period of inflorescence to Solidago, which yields im- 

 mense quantities of nectar and pollen ; otherwise they would 

 suffer great loss. If their flight were synchronous with the 

 blooming period they would become oligotropic to the golden 

 rods. 



An examination of the characters and habits of the genus 

 Perdita should throw some light on the origin of oligotropism. 

 This large genus of bees is confined to North America and in- 

 cludes not far from 150 described species and varieties, most 

 common in the arid regions of New Mexico. "Practically all 

 are oligotropic," says Cockerell (Psyche, 18:154.) Graeni- 



