282 GENERAL BIOLOGY 



hastening the secretion of the desired substance. 

 The ants will usually fiercely attack trespassers, and 

 the aphids are thus assured of a protection which 

 nature has denied them, in return for which they 

 supply their keepers with food. An instance of the 

 persistence with which the ants look after their 

 " cattle," is afforded by a discovery of Professor 

 Forbes. In the Mississippi Valley, a member of 

 the aphid family called the " corn-root louse " 

 infests the roots of maize. Its eggs are laid in the 

 ground in autumn and hatch out the following 

 spring. Rotation of crops and other methods of 

 combating the pest were unsuccessful, until it was 

 discovered that the little brown ant (Lasius brnn- 

 neus) takes the aphids when they hatch and 

 before there are corn-roots to feed on, and " with 

 great solicitude carefully places them on the roots 

 of certain kinds of knot-weed (Setaria and Poly- 

 gonum) which grow in the field, and there protects 

 them until the corn germinates." 



In other cases the association is not so advantage- 

 ous. Certain species profit by the industry of others 

 without giving an adequate return. A classic 

 example is the cuckoo, which lays its eggs in other 

 birds' nests, to be brooded and reared vicariously. 

 The bee-moth, if it can escape the sentry at the door, 

 enters the hive and lays its eggs, appropriating room 

 and food that the bees need for themselves. In 

 such a case the " host " is injured, but only indi- 

 rectly. It is only a step, however, to a condition 

 in which the host is directly injured by the activity 



