274 THE SOCIAL LIFE OF ANIMALS 



mately exhausted the juices from one food plant the 

 next generation appears with wings; in flying about, 

 some of them will usually find a new and suitable 

 food plant where they can settle and carry on. With 

 some species one of the most effective ways of keep- 

 ing wings from developing is to isolate the individ- 

 ual aphids and, conversely, one of the best recipes 

 for obtaining winged forms is to allow them to be- 

 come crowded. (104) 



These distinctly different types of grasshoppers 

 and aphids roughly suggest the structural differences 

 between the castes of social insects, just as compari- 

 son was suggested between the structural differences 

 of caste and of sex. The resemblance is so close that 

 the line cannot be drawn between its manifestations 

 in social and infrasocial animals. Not only that, but 

 the mechanisms by which the castes are produced 

 appear in many instances to be like those which may 

 occur when animals are aggregated together, even 

 though the aggregations are below the level usually 

 regarded as marking the lower limit of truly social 

 life. 



And since no one has yet demonstrated the exist- 

 ence of truly asocial animals it is impossible to define 

 the lower limits of sub-social living. All that can be 

 found is a gradual development of social attributes, 

 suggesting, as has been emphasized throughout this 



