SOCIAL PARASITES 



wasps also have a " southern " appearance. It has been 

 suggested that if a southern species works its way into more 

 northern territory already occupied by some allied species, 

 the situation would be one in which cuckoo habits might well 

 arise. The more southern species would, as is known from 

 the observation of industrious species, tend to leave hiber- 

 nation later in the spring. The conditions for founding a 

 colony might well be more unfavourable than in southern 

 regions. At the same time, the northern species, having come 

 out earlier, would already have occupied many of the suit- 

 able nesting places. 



This is in fact what seems to happen with Bombus terrestris 

 and B. lucorum. In Europe as a whole, terrestris has a more 

 southern distribution than lucorum and where, as in England, 

 they occur side by side, terrestris tends to come out of 

 hibernation later. This probably explains the occasional 

 invasion of nests of lucorum by terrestris, and it is possible 

 that over a long 'period (perhaps to be measured in millions 

 of years) such a conjunction of species might lead the 

 southern intruder to become a cuckoo species. 



This hypothesis may account for the evolution of many of 

 the cuckoo wasps and bees, both social and solitary, in 

 Europe and North America. It can hardly account for the 

 origin of the numerous solitary cuckoo species in the tropics. 

 So far the only known social cuckoo species of bee or wasp 

 outside the temperate regions are two Psithyrus found in 

 eastern Asia. Possibly in tropical climates cuckoo habits 

 might tend to develop quite apart from climate whenever one 

 species invaded the territory occupied by one of similar 

 habits. However, much more observation of the tropical 

 species is required before such speculation can have much 

 value. 



i57 



