THE OSMLE 247 



study some old nests of the Mason-bee of the Shrubs, 

 which are clay spheroids with cylindrical cavities in them. 

 These cavities are formed, as in the old nests of the 

 Mason-bee of the Pebbles, of the cell properly so-called 

 and of the exit-way which the perfect insect cut through 

 the outer coating at the time of its deliverance. The 

 diameter is about 7 millimeters ; 1 their depth at the center 

 of the heap is 23 millimeters 2 and at the edge averages 

 14 millimeters. 3 



The deep central cells receive only the females of the 

 Osmia; sometimes even the two sexes together, with a 

 partition in the middle, the female occupying the lower 

 and the male the upper story. Lastly, the deeper cavi- 

 ties on the circumference are allotted to females and the 

 shallower to males. 



We know that the Three-horned Osmia prefers to 

 haunt the habitations of the Bees who nidify in populous 

 colonies, such as the Mason-bee of the Sheds and the 

 Hairy- footed Anthophora, in whose nests I have noted 

 similar facts. 



Thus the sex of the egg is optional. The choice rests 

 with the mother, who is guided by considerations of 

 space and, according to the accommodation at her dis- 

 posal, which is frequently fortuitous and incapable of 

 modification, places a female in this cell and a male in 

 that, so that both may have a dwelling of a size suited 

 to their unequal development. This is the unimpeach- 

 able evidence of the numerous and varied facts which 



1 .273 inch. — Translator's Note. 

 2 .897 inch. — Translator's Note. 

 8 .546 inch.— Translator's Note, 



