THE OSMIiE 259 



And, in fact, she does very well with a short tube of 

 the same diameter. Such are the cells in the old nests 

 of the Mason-bee of the Shrubs and the empty shells of 

 the Garden Snail. With the short tube the two disad- 

 vantages of the long tube are avoided. She has very lit- 

 tle of that crawling backwards to do when she has a 

 Snail-shell for the home of her eggs and scarcely any 

 when the home is the cell of the Mason-bee. Moreover, 

 as the stack of cocoons numbers two or three at most, 

 the deliverance will be exempt from the difficulties at- 

 tached to a long series. To persuade the Osmia to nidify 

 in a single tube long enough to receive the whole of her 

 laying and at the same time narrow enough to leave her 

 only just the possibility of admittance appears to me a 

 project without the slightest chance of success: the Bee 

 would stubbornly refuse such a dwelling or would con- 

 tent herself with entrusting only a very small portion of 

 her eggs to it. On the other hand, with narrow but 

 short cavities, success, without being easy, seems to me 

 at least quite possible. Guided by these considerations, 

 I embarked upon the most arduous part of my problem: 

 to obtain the complete or almost complete permutation 

 of one sex with the other; to produce a laying consisting 

 only of males by offering the mother a series of lodgings 

 suited only to males. 



Let us in the first place consult the old nests of tlie 

 Mason-bee of the Shrubs. I have said that these mortar 

 spheroids, pierced all over wuth little cylindrical cavities, 

 are adopted pretty eagerly by the Three-horned Osmia, 

 who colonizes them before my eyes with females in the 



