Vol. XXvl ENTOMOLOGICAL NEWS. 317 



a social bee to maintain its organization throughout the sea- 

 son ; but for a solitary insect it is desirable that it mate and de- 

 posit its eggs as soon as possible. The longer the female flies 

 before this happens the greater the probability is that she will 

 be destroyed by some one of many causes. There are numer- 

 ous other insects besides bees which have a short term of 

 flight. A solitary bee, which like Halictus flies throughout the 

 season, and of which the adult females pass the winter dor- 

 mant is evidently exposed to more dangers than a species which 

 passes the larger part of the year in an immature state in its 

 burrows. vSince many polytropic bees have either a short term 

 of flight, or one which does not exceed 100 days, it is clear that 

 a shorter term of flight is not necessarily correlated with 

 oligotropism. 



If severe competition did exist among the solitary bees for 

 flower food, the oligotropic habit would not be desirable. It 

 is not an advantage for a bee to restrict its visits to one kind 

 of flower unless it is always certain to obtain the food supply 

 it requires ; otherwise it is clearly at a disadvantage as com- 

 pared with the polytropic species. If severe competition is in- 

 duced by artificial means, as by overstocking a locality, then 

 the oligotropic bees will either tend to disappear or become 

 polytropic. The small number of oligotropic bees reported 

 from central Europe is noteworthy. If, however, a very com- 

 mon flower yields a surplus of food then a bee with a periofl 

 of flight nearly coinciding with the period of inflorescence 

 would save time and labor by restricting its visits to this spe- 

 cies ; and since bees instinctively learn from observation it 

 would naturally be expected that the oligotropic habit would 

 be formed. To some extent, at least, Robertson recognizes 

 the correctness of this inference in the case of a part of the 

 oligotropic visitors of the Compositae. He says, "It is not so 

 surprising that some of these are exclusive when we consider 

 that at their maximum the Compositae form 34 per cent, of 

 the indigenous flowers." (Can. Ent., 42:327.) Why, in- 

 deed, should the nest-building bees, which restrict their visits 

 to the great tribes Astereae and Heliantheae do otherwise? 



