242 THE WONDERS OF INSTINCT 



whereas, in the Horned Osmia and the Three-horned 

 Osmia, the male is only half or a third the size of the 

 female, as we have seen from the respective bulk of their 

 cocoons. In the Mason-bee of the Walls there is also a 

 difference in size, though less pronounced. 



The Three-pronged Osmia has not therefore to trouble 

 about adjusting the dimensions of the dwelling and the 

 quantity of the food to the sex of the egg which she is 

 about to lay; the measure is the same from one end of 

 the series to the other. It does not matter if the sexes 

 alternate without order: one and all will find what they 

 need, whatever their position in the row. The two other 

 Osmiae, with their great disparity in size between the two 

 sexes, have to be careful about the twofold consideration 

 of board and lodging. 



The more I thought about this curious question, the 

 more probable it appeared to me that the irregular series 

 of the Three-pronged Osmia and the regular series of 

 the other Osmiae and of the Bees in general were all 

 traceable to a common law. It seemed to me that the 

 arrangement in a succession first of females and then of 

 males did not account for everything. There must be 

 something more. And I was right : that arrangement in 

 series is only a tiny fraction of the reality, which is re- 

 markable in a very different way. This is what I am 

 going to prove by experiment. 



The succession first of females and then of males is 

 not, in fact, invariable. Thus, the Chalicodoma, whose 

 nests serve for two or three generations, always lays male 

 eggs in the old male cells, which can be recognized by 



