THEIR RELATION TO EACH OTHER 127 



tions, and appear to forgive the intruders their tres- 

 passes against the Aphids. A still further special- 

 ization, decidedly more important from the economic 

 standpoint, is found among those ants that gather and 

 preserve the eggs of plant lice during the fall and winter, 

 and colonize them on suitable food plants in the spring. 

 The strawberry louse and corn-root louse are examples 

 of this kind, and both of these Aphids would find it 

 difficult if not impossible to maintain themselves were 

 it not for the assistance given by the ants. The economic 

 importance of the matter comes in when we consider 

 that, except for the ants, it would be easy to starve 

 out the Aphids by a mere rotation of crops. In some 

 instances, where the plant lice will not live under- 

 ground, the ants build protecting shelters around the 

 colonies on their food plants, and thus gain all the 

 advantages that other species get from their under- 

 ground forms. 



Indirectly, therefore, ants may become decidedly 

 injuriotis to a growing crop, even though they do 

 not themselves feed upon it, and the best way of 

 dealing with an injurious form may be through its 

 protecting ant. 



Besides plant lice, scales are often visited and here 

 again the protection accorded by ants in the destruc- 

 tion of forms inimical to the scales is the return ren- 

 dered for the food supply. It has been charged, indeed, 

 that the Sciitellista introduced into California to con- 

 trol the black scale, has been practically destroyed by 

 ants that obtain honey-dew from the scales. Scales, 

 however, are never really domesticated like plant lice, 

 and while they are of very great importance to some 

 ants they are never entirely dependent upon them for 

 existence. Some few species among the tree-hoppers 

 and frog-hoppers, also excreting honey-dew or waxy 



