The Mother Decides the Sex 



bers, formerly the homes of Chalicodomae, the 

 Osmia settles females and none but females. 



Let us now consider the long vestibule. 

 Here, a partition is constructed, encroach- 

 ing slightly on the cell proper, and the resi- 

 dence is divided into two unequal storeys, a 

 large room below, housing a female, and a 

 narrow cabin above, containing a male. 



When the length of the vestibule permits, 

 allowing for the space required by the outer 

 stopper, a third storey Is built, smaller than 

 the second; and another male Is lodged In this 

 cramped corner. In this way the old nest of 

 the Mason-bee of the Pebbles Is colonized, 

 cell after cell, by a single mother. 



The Osmla, as w^c sec, is very frugal of 

 the lodging that has fallen to her share; she 

 makes the best possible use of It, giving to 

 the females the spacious chambers of the 

 Mason-bee and to the males the narrow vesti- 

 bules, subdivided into storeys when this Is fea- 

 sible. Economy of space is the chief consi- 

 deration, since her stay-at-home tastes do not 

 allow her to indulge In distant quests. She 

 has to employ the site which chance places at 

 her disposal just as it is, now for a male and 

 now for a female. I Icre we sec displayed, 



139 



