SOCIAL PARASITES 



rather later in the spring, instead of beginning a nest of their 

 own they search for a nest of a humble bee of the correct 

 species, since each kind of parasite is attached to one or a few 

 host species. The entry into a nest is a dangerous process, 

 and is often unsuccessful if the host colony has reached a 

 moderate size. The smaller the colony, the more easily it is 

 invaded. The resistance is due entirely to the workers, and 

 if there are enough of them the Psithyrus queen may be 

 killed. One or more Psithyrus may be found dead in the 

 tunnel leading to a humble-bee nest. If an entry is success- 

 fully accomplished, probably after a few of the workers have 

 been killed, the cuckoo queen becomes established in the 

 nest and leaves it no more. For some time she frequently 

 mauls workers when they meet inside the nest, usually with- 

 out killing them. Eventually an uneasy truce is established. 

 There has been disagreement about the relations between 

 the cuckoo humble bees and the host queens. Sladen, who 

 published in 19 12 a well-known work on British humble bees, 

 maintained the host queen was always killed. Hoffer in 

 Austria and Plath in the United States, however, have 

 numerous records of the two queens living side by side and 

 taking little notice of one another. Cumber also found a 

 nest in which both the Bombus and its parasite had been 

 laying eggs. Some of these discrepancies may arise from the 

 different behaviour of different species, but it seems almost 

 certain that Sladen exaggerated the antagonism between the 

 queens. Nevertheless, once the colony has been invaded the 

 Bombus never produces any young queens, very rarely any 

 males, and probably no more workers. This would be 

 quite understandable if the Bombus queen were killed, but 

 it is not clear how it happens when she is not. Possibly the 

 cuckoo queen eats the eggs; or it may be that, as described 

 below in some of the ants, the behaviour of the workers 



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