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are actually unable to work ; as shown by the fact that the 

 organs for collecting and carrying pollen have been lost 

 through disuse. 



The inquiline bees that infest the nests of solitary bees 

 resemble the cuckoo-flies in habits, watching their oppor- 

 tunity to steel into a partially completed nest, and, cuckoo- 

 like, laying an egg there. The larva of the guest-bee hatch- 

 ing first is able to devour the food stored there, and thus 

 starves the rightful occupant of the nest. We have in our 

 collection many nests of solitary bees in which more than 

 half of the cells were inhabited by inquilines. 



The relations existing between the solitary bees and 

 their unwelcome guests are easy to understand ; but when 

 we study the nests of bumblebees we find that they, too, 

 have guests : but in this case the guests are apparently wel- 

 come, although, so far as we have been able to learn, they 

 are of no use to the colony. 



The inquilines infesting the nests of bumblebees belong 

 to the genus PsitJiyrus (Psith'y-rus). These bees so closely 

 resemble bumblebees in appearance and structure that it is 

 extremely difficult to determine whether a male is a 

 PsitJiyrus or a Bombus ; but the females are easily distin- 

 guished, for in PsitJiyrus the pollen-basket of the hind legs 

 has been lost through disuse. In PsitJiyrus there are only 

 two forms of individuals, the males and the females, there 

 being no working caste, as with bumblebees. 



The female PsitJiyrus lays her eggs in a bumblebee's 

 nest, and when the larvae hatch they are cared for by the 

 bumblebees as if they belonged in the nest. And even after 

 the guests have reached maturity they come and go in the 

 nest without being disturbed, although they never lend a help- 

 ing mandible in carrying on the work of the colony. Some- 

 times these guests very closely resemble their hosts in size 

 and color, but in other cases the PsitJiyrus is marked very 

 differently from the Bombus that entertains it. We cannot 

 suppose, therefore, that the guests are mistaken for mem- 



