ORIGINS OF parasitism: evolution of parasites 51 



they have retained the more primitive predacious habits and are gain- 

 fully employed, as far as the host is concerned, killing and eating their 

 flea parasites. In the intensely social environment of a termitary or 

 ant hill the behaviour of the staphylinid beetles has radiated out along 

 several different lines. Firstly, the predatory habit has developed into 

 straightforward parasitism and the larvae of some species are parasitic 

 upon the nidicolous pupae of certain flies. Secondly, there are species 

 which are commensals of the ants. They live as tolerated guests and, 

 for example, accompany the foraging expeditions of the legionary 

 ants, picking up some of the food captured on the way. They have 

 come to resemble their hosts to a remarkable degree. Thirdly, there 

 are scavengers and corpse eaters. Towards this category the ants show 

 a certain degree of hostility — with good reason, because in the case of 

 diseased and disabled workers, the beetles sometimes reduce them 

 prematurely to the status of a corpse. Fourthly, there are the symbiotics 

 which, like those from birds' nests, prey on certain enemies of the ants, 

 such as parasitic mites and the larvae of certain flies. Finally, we have 

 the true guests (symphiles) which are housed, fed and even reared by 

 the ants and, in return for their hospitality and solicitude, eat their 

 eggs and young. To encourage their ministrations the beetles exude a 

 glandular liquid which the ants find madly attractive and lick up with 

 intense eagerness. It is not known if the liquid exuded by the beetles 

 confers any benefit on the ants. It appears more probable that they are 

 merely sacrificing their brood in order to indulge an irresistible craving 

 for the exudation — a situation which the beetles have learned to exploit. 

 Such curious behaviour is also found in human communities where men 

 will sacrifice their own health and the welfare of their families in order to 

 indulge in drugs and drink. 



Thus it will be seen that, once established in a communal environ- 

 ment, the rove beetles have exploited the situation in a variety of ways. 

 They have launched out into different kinds and different degrees of 

 commensalism, symbiosis and parasitism. 



There are certain features (see p. 38), both morphological and 

 biological, which appear to be characteristic of parasites. These 

 attributes are useful and adapt them to the parasitic mode of life, in the 

 same way that the streamlined shape of whales and sharks adapts them 

 to a wandering life in the sea. How these modifications have come about 

 is the subject of much discussion. Parasites themselves are very varied 

 organisms, pertaining to widely separated groups of animals, and it is 



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