BEES WHICH VISIT ONLY ONE KIND OF FLOWER 



dance of pollen and nectar, and the commonness and wide 

 distribution of many species are the factors which attract these 

 bees. No other family of plants blooming at the season offers 

 equal advantages. The different genera of the ComposiiGP 

 vary greatly in the length of the floral tubes, while in the genera 

 of bees the length of the tongue also varies greatly. Thus it is 

 the tube length of the flower which is the factor limiting the 

 visits of many species of bees to certain composite genera. 

 Small bees with short tongues do not resort to the same flowers 

 as larger bees with longer tongues. (Fig. 58.) 



Practically all of the species of Perdita are oligotropic. 

 They are small bees with a short flight. A part of the species 

 are vernal; but the majority fly in late summer and autumn 

 and many visit the Composite. The flowers visited by them 

 occur in immense profusion and include the best honey -plants 

 of this country, as 8a/i.r, SoUdago, Cleome, Prosopis, Helianthus, 

 Verbesina, Bidens aristosa, and Monarda punctata. It is note- 

 worthy that we find these flowers also visited by oligotropic 

 bees belonging to other genera. This behavior on the part of 

 so many species of bees is very similar to that of higher forms of 

 life when they gather at some feeding-ground where there is 

 a superabundance of food. 



Since the nest-building bees are compelled to collect pollen 

 for brood-rearing they are naturally more constant in their 

 visits to flowers than the parasitic bees, which do not gather 

 pollen and require nectar only for themselves. Nevertheless a 

 number of the parasitic bees with a short term of flight visit 

 wholly or largely the Compositce and may be regarded as oligo- 

 tropic species. This is of great interest since it is not claimed 

 that they have acquired this habit as the result of competition. 



We may sum up the matter as follows. All bees including 

 the honey-bee show a strong tendency in collecting both nec- 

 tar and pollen to be constant to one species of flower. This is 



121 



