THE SOCIAL INSECTS 



the habit of raiding could be transmitted from generation to 

 generation if, as he supposed, it was manifested only by the 

 sterile workers. Much later, however, it was discovered that 

 the behaviour of the queen in founding new colonies is not 

 so very different from that of the workers. Moreover, as 

 Darwin remarks, the queens whose worker offspring be- 

 haved in a way which made the colony as a whole most 

 successful would be the ones which leave most offspring. 

 That is, the process of natural selection involves evolutionary 

 change in the whole colony, rather than in single individuals. 

 The sterility of the workers from this point of view is 

 immaterial. 



Darwin also suggested that the raids are partly for obtain- 

 ing food, like the raids of driver ants, and that the slaves 

 hatch from grubs or cocoons which fail to get eaten. There 

 is little doubt that this theory is partly true. Sanguinea does 

 eat a varying proportion of the grubs and cocoons which it 

 brings home, and does sometimes raid nests of species which 

 it never uses as slaves. Indeed, some other species, such as the 

 wood ant, whose queens found colonies as temporary social 

 parasites, do also raid other ants' nests for food, at least 

 occasionally. 



It may be that the slave-making habits of sanguinea have 

 developed from two directions; partly from the way in which 

 the queen founds new colonies and partly from food raids 

 undertaken by the workers. One remarkable feature of the 

 raids is that they may be made on a nest as distant as ioo 

 yards from the sanguinea nest, and that on the way to the 

 chosen nest the ants may pass and ignore other nests of the 

 slave-providing species. This suggests that scouts have 

 previously reconnoitred the way. The ants are not, however, 

 led by the scouts, since the position at the head of the raiders 

 is occupied by new individuals at frequent intervals. 



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