546 



STATE BOARD OF AGRICULTURE. 



TABLE II. 

 Comparative action of a few substances on insect-adivity. 



DO dlL INSECTICIDES AND GASES PASS INTO THE TISSUES OF THE INSECT-BODY? 



It had now been shown definitely that the oil-sprays, pyrethrum 

 powder, and such common contact insecticides could not possibly be 

 so effective as they are, through plugging the tracheae alone, and that 

 their vapors at ordinary temperatures were often as effective as the 

 liquid or powder itself. The question naturally arose, then, as to 

 whether these insecticides might not become mainly effective after hav- 

 ing been absorbed into the body tissues. So very little of the carbon 

 disulphide vapor or hydrocyanic acid gas is necessary to bring about 

 death to most insects that it might seem very reasonable to suppose they 

 are absorbed. Equally reasonable, then, would it be to believe that 

 the vapors of kerosene, and other volatile insecticides are absorbed, 

 but direct evidence may be had. From the gases which are very poison- 

 ous to insects, hydrogen sulphide was selected for a test to determine 

 whether it becomes absorbed by the tissues before death results. .This 

 gas gives abundant colored precipitates with the salts of certain metals 

 in solution. For instance, with lead acetate a black precipitate of 

 lead sulphide results and in a solution of cadmium chloride, the sul- 

 phurated hydrogen throws down yellow cadmium sulphide. A method 

 of using this test on insects, therefore, readily suggested itself. In- 

 sects were placed in an atmosphere containing a large percentage of 

 the poisonous gas and left there until nearly dead. They were then 

 removed and a warm solution of one of the salts named was quickly 

 injected into the body tissues by means of a fine hypodermic needle. 

 The presence of the gas in the body Avas proven very decisively by the 

 precipitate that resulted. The hot lead acetate and cadmium chloride 

 solutions, when used as killing agents on insects that had not been 

 just treated with H .S, did not blacken the tissues. If the .white grub 

 of a beetle were used, the rapid progress of the precipitation during the 



