THE SOCIAL INSECTS 



becomes perverted; the colony therefore produces a brood 

 of Psithyrus males and queens and then dies out rather earlier 

 than usual, since the host workers are not being replaced. 



HOW CUCKOO BEES AND WASPS AROSE 



It is possible to form some idea how this type of behaviour 

 may have arisen in bees and wasps, and this applies also to 

 the solitary kinds of these insects, amongst which there are 

 quite a number of cuckoo species. In both wasps (Vespula) 

 and in humble bees, suitable nesting sites are not very 

 numerous, and this seems often to set a limit to the abundance 

 of a species in any locality. As has been stated earlier, this 

 leads to a certain amount of fighting even between the 

 ordinary industrious species. Queens which have tried to 

 force their way into an already occupied site are quite 

 commonly found dead at the entrance. Sladen showed that 

 in two very similar British humble bees, B. terrestris and 

 B. lucorum, the former sometimes succeeds in invading young 

 nests of the latter and taking them over. Thus, after a short 

 period in which there are workers of both species present, 

 the B. lucorum workers die out because they are not being 

 replenished, and the colony becomes a pure one of B. 

 terrestris. This behaviour, though exceptional for these 

 species, is very similar to the " temporary social parasitism " 

 of certain ants, mentioned in the previous chapter. Such 

 successful invasions by normally industrious species seem 

 to be very rare in wasps, but Nixon in 1935 recorded one 

 example: a nest of Vespula vulgaris taken over by a queen 

 of V. germanica. 



In 1927 I showed that cuckoo bees, and in particular the 

 species of Psithyrus, tend to have the colours and shorter 

 hairs which are often seen in southern races of industrious 

 species. Bischoff and Weyrauch have noted that cuckoo 



156 



