of the perfect insect can be developed during one season. 

 In some cases these changes are well marked ; in others, the 

 transformations are less complete, as in the case of plant 

 lice and grasshoppers." (Bulletin IV., Delaware College 

 Agricultural Experiment Station.) 



Insecticides. 



"Insecticides, or the substances used for destroying 

 insects, may broadly be divided into two classes ; (1) inter- 

 nal poisons, or those w^hich take effect by being eaten along 

 with the ordinary food of the insect; and (2) external 

 irritants, or those which act from the outside, closing the 

 breathing pores, or causing death by irritation of the skin. 

 Besides these, however, various other substances are used 

 in preventing insect attack, — keeping the pests away because 

 of offensive odors, or acting simply as mechanical barriers. 



"The most important insecticides are the poisons. Of 

 these the most popular are the various combinations of 

 arsenic, known as Paris green, London purple, slug-shot, 

 and a large number of patent insecticides sold under various 

 names." (Bulletin, Vol. II., No. 1, Ohio Agricultural Ex- 

 periment Station, March, 1889, Weed.) 



Other insecticides are kerosene, petroleum, whale oil soap, 

 resin or arsenical soaps, the oils of tansy, mint, pennyroyal, 

 cedar, tar or cade, thymol, cresylic acid, carbolic acid, alco- 

 hol, pyrethrum, hellebore, sulphur, quassia, tobacco, etc. 

 All are valuable ; but for most insects, arsenic in some form 

 is most eflScacious. 



Principal Insecticides. 

 Arsenic. — Pure white arsenic or arsenious acid was suc- 

 cessfully used for the destruction of the canker Avorm as early 

 as 1875, and is still sometimes recommended for the destruc- 

 tion of insects but, fortunately, is rarely used. Whether 

 it can be advantageously applied in cold water is still a ques- 

 tion as it is so heavy it quickly settles to the bottom of the 

 vessel. AVhen we add to this fact the increased danger to 

 human life that is incurred by having in the house so deadly 

 a poison, that cannot be distinguished in color from flour. 



