IIYMENOPTERA. 
671 
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 Psithyrus (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 
Psithyrus or a Bombus\ but the females are easily distin¬ 
guished, for in Psithyrus the pollen-basket of the hind legs 
has been lost through disuse. In Psithyrus there are only 
two forms of individuals, the males and the females, there 
being no working caste, as with bumblebees. 
The female Psithyrus lays her eggs in a bumblebee’s 
nest, and when the larv$ 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 Psithyrus is marked very 
differently from the Bombus that entertains it. We cannot 
suppose, therefore, that the guests are mistaken for mem- 
