^74 



Evolution and animal life 



struggles to loosen it from its rock or from whatever it may 

 be growing on, and does not rest until it has torn it loose and 

 placed it on its shell; 



There are numerous small crabs called pea crabs {Pin- 

 notheres) which live habitually insidfe the shells of living mussels. 

 The mussels and the crabs tolerate each other, perhaps to their 

 mutual benefit. 



The relations between ants and aphids (plant lice) are 

 often referred to in popular natural histories and books about 



Fig. 230. — The hermit crab, Pagurus hernhardus, in snail shell covered with 



Hydractinia. 



insects as examples of symbiosis of unusual interest. Un- 

 fortunately, however, not enough cdreful study has been given 

 to many of these apparently true examples of symbiosis to 

 enable us to be certain of the truth of the alleged care and 

 guarding of the ant-cows, as Linnaeus called these aphids, by 

 their milkers, the ants. That ants do swarm about the aphids 

 to lap up the "honey dew" excreted by them is wholly true, 

 and the very presence of the sharp- jawed and pugnacious 

 ants must keep away many enemies of the defenseless plant 

 lice, toothsome morsels for the ladybird beetles, fiower-fly 

 larvae and other predatory insects. 



In the case of the interesting relations between the corn 

 root aphid, Aphis maidisradici, of the Mississippi Valley States 

 and the little brown ant, Lasius hrunneus, however, we 



