zoS ENTOMOLOGICAL NEWS. [March, '13 



with their time of flight and the flowers they visit for pollen. 

 No species of Andrena has ever been seen in spring before the 

 willows blossom, "although two species of entomophilous plants 

 Brigenia bulbosa and Hepatica ocuta open up their flowers 

 earlier than this willow'' (Salix discolor}. The two species 

 which first appear fly for about a month, and are both oligo- 

 tropic visitors of Salix. At the beginning of their flight they 

 are, of course, exposed to no competition whatever from other 

 species of Andrena. From March 3ist to April 28th ten spe- 

 cies of Andrena appear successively, of which four are oli- 

 gotropic visitors of Salix with an average time of flight of 

 about 44 days. Six species are polytropic, but they all obtain 

 a part of their pollen from the willows ; in Maine they are of- 

 len present in large numbers so that the oligotropic species 

 would not escape competition with them if there was a scarcity 

 of pollen. Their average time of flight is about 63 days, or 

 43 per cent, longer than that of the oligotropic species, which 

 greatly increases the probability, or may render it necessary, 

 that they obtain a part of their pollen from other flowers than 

 those of Salix. Common and widely distributed species, like 

 A. vicina, which flies for about two months, doubtless often 

 find it more convenient to gather pollen from flowers which 

 are nearer their nests. 



Certain species of Andrena visit the willows exclusively 

 because during their comparatively short time of flight they 

 can readily obtain all the pollen and nectar they require, and 

 there is no occasion for them to go elsewhere. But they do 

 not thus, as has been shown, escape the presence of the poly- 

 tropic species, which are frequently very common. Of the 

 four species of Andrena oligotropic to Salix in Maine only 

 one is common, A. illinoensis, a small bee, not likely to fly far. 

 The oligotropic ,A. mariae is comparatively rare, while the 

 polytropic A. zveedi Vier. and A. liippotes Rob. are very com- 

 mon on the willow aments ; it sounds, therefore, somewhat 

 paradoxical to say that the former escapes competition with 

 the polytropic forms because of their absence. It is because 

 of the great abundance of pollen and nectar that there is no 



