SOCIAL PARASITES 



species with small queens which could not found colonies 

 unaided. They can scarcely be considered degenerate, though 

 they may have led to the slave-making habit of such species 

 as the blood-red ant, Formica sanguinea. When the young 

 queen of this ant founds a new colony, she invades a small 

 nest, usually of Formica fusca, and kills the queen and any 

 of the workers which resist or attack her. Before long, all 

 the adults of the host ant will be dead and the sanguinea 

 queen tends the remaining brood. When new young workers 

 of fusca emerge they look after the brood which hatched from 

 the eggs laid by the sanguinea queen. In some regions, and 

 in some varieties of the blood-red ant, there is no further 

 development of parasitism, and as the fusca workers die out, 

 the colony becomes pure sanguinea. This behaviour is no 

 more than a form of temporary social parasitism in which the 

 relations between the host and the invader are even less 

 cordial than usual. 



More commonly, particularly in western Europe, including 

 Britain, the later history of the colony is different, since the 

 population of slave workers is maintained by a regular series 

 of raids on other nests. These are carried out by sanguinea 

 workers which leave the nest in a large straggling group 

 which may cover several yards. Sometimes several smaller 

 groups follow one another at short intervals. When this 

 happens the first to arrive surround the nest of the prospective 

 victims. When the whole group of raiders has assembled 

 they enter the nest and begin seizing grubs and cocoons. 

 Workers which resist are killed, but the others are not 

 molested. Eventually, each sanguinea worker returns, carry- 

 ing a grub or cocoon. 



This remarkable behaviour attracted the attention of 

 Darwin, and in The Origin of Species he describes a number of 

 his own observations on this ant. Darwin was puzzled how 



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