BEES 20 1 



strict their visits to flowers so abundantly supplied with pollen and 

 nectar, combined with their early and short time of flight, which lasts 

 only about a month, and perhaps also to their nesting near these shrubs. 

 Where bees fly only during the latter part of the season it seems very 

 natural for them to restrict their visits to the Composite. These flow- 

 ers, as in the case of the golden-rods and thistles, are very common, con- 

 tain ample food supplies and are easy to visit. They are actuated not 

 by the need or desire of avoiding competition, but by the same motives 

 which lead honey-bees to visit the white clover exclusively while it is in 

 bloom. 



The oligotropic habit is not beneficial to flowers, it concerns the bees 

 alone. The oligotropic bees are almost without exception solitary forms, 

 to which there are no flowers specially adapted. The social bees, as a 

 rule, visit a great variety of flowers, though in Europe it is stated that 

 there is a bumblebee (Bomb us gerstaecJceri) which visits a single species 

 of monkshood (Aconitvm lycoctonum). Here, of course, the adapta- 

 tions are mutual. This mode of flight, however, has not in general been 

 determined by floral adaptations. Certain species of bees have be- 

 come satellites of certain flowers because of the advantage thus gained 

 for themselves, and partly also perhaps as the result of habit. Just as 

 there are fly-flowers, butterfly-flowers and bumblebee-flowers, so, on the 

 other hand, there are willow-bees, golden-rod bees, a pickerel-weed bee, 

 a loosestrife bee, a violet bee and a strawberry bee. 



Two most important influences are the season of the year and the 

 length of time the bee is on the wing'. It is clear that bees which fly 

 only in spring or autumn for about a month have not a great choice of 

 flowers ; and, of course, we never look for autumnal bees on spring flow- 

 ers. Usually the length of time an oligotropic bee flies and the flower 

 it visits is in bloom are about the same. The honey-bee is practically 

 a monotropic bee at certain seasons of the year. While the basswood 

 and white clover are in bloom the honey-bee visits these flowers almost 

 exclusively. Again in the fall in Maine it confines its attention solely 

 to the golden-rods. In California at times it collects nectar exclusively 

 from the sages ; in Michigan from the willow-herb, and in other regions 

 from other plants. If from any one of these plants it also obtained its 

 supply of pollen and was on the wing only while it was in bloom, it 

 would be regarded as a monotropic bee in the strict sense of the word. 

 That it exhibits a strong tendency, when collecting pollen, to be con- 

 stant to one plant species is well known; and the little packets of poller: 

 it brings into the hive seldom consist of two kinds of pollen. But. 

 when a bee flies from spring till fall and requires a large amount of 

 stores, it is evident that it can never become oligotropic. 



Another important factor is the small size of many oligotropic bees, 

 This is true of Prosopis nehtmbonis, Halictus nelumbonis, and many 



VOL. LXXXI. — 14. 



