INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 



117 



seems to be a distinct advantage accruing to both members of 

 the partnership sufficient to account for it. The relation 

 of the ants to the aphides or plant-lice which they capture may 

 be so described. The aphides, although captives, are nour- 

 ished, often at great expense of labor to the ant, on the food 

 which they most prefer, and in return the ants use the sweet 

 secretions of their bodies as food. Certain hermit-crabs, 



FIG. 56. 



FIG. 56. Argynnis cybele on thistle. Natural size. Photo by Folsom. 



Questions on the figure. For what purpose does the butter-fly visit 

 the thistle? What special adaptations does the butter-fly possess for this 

 mode of life? What is the gain to the thistle from the visits? 



whose habit it is to occupy gasteropod shells as a home into 

 which they insert the soft posterior part of the body, cultivate 

 friendly relations with a sea-anemone which becomes attached 

 to the shell, often with the active help of the crab. In this 

 case the anemone is supposed to conceal the hermit and to 

 help protect it by means of its nettling cells, and in return is 



