14 INSECTS : THEIR STRUCTURE AND FOOD. 



thorns, etc., with which plants have tried to render themselves safe. 

 Modern science has not yet discovered any method of altering the com- 

 position of plant sap so as to render it distasteful or poisonous to insects. 

 When this subject has been mastered, we shall be in a position to deal 

 more siicccssfnlly with insects by purely preventive methods. 



