66 DDT — Killer of Killers 



the leg, and ii the leg is severed before the poison has reached 

 the insea's body, the fly will live. Of course, the other fly 

 who does not undergo the amputation will soon join his an- 

 cestors. 



It has been mentioned previously that single doses ^f 

 Ko gram of DDT per kilogram of body weight are harm- 

 ^less. To produce symptoms of heavy poisoning or death re- 

 quires a minimum of from 0.15 to 2 grams per kilogram, de- 

 pending upon the individual as well as on the species. These 

 figures, however, refer only to warm-blooded animals. Per 

 kilogram of weight, it takes only from 0.0006 to 0.002 grams 

 of DDT to kill an insea. And since it takes thousands of 

 flies to equal a kilogram of weight, the amount of DDT nec- 

 essary to kill a single fly must be considerably less than a 

 billioneth of a gram. And that is why DDT is such a good 

 residual inseaicide. 



Flies can pick up fatal doses by walking over surfaces 

 covered with an almost unbelievably thin film of DDT. In 

 an experiment carried out by Geigy, a box made of wooden 

 frames and glass panels was sprayed with a %o % solution of 

 DDT in acetone, and after the solvent had evaporated, each 

 pane of glass was wiped 200 times with a woolen cloth. Flies 

 were then introduced into the box, and they picked up enough 

 DDT by walking over the polished glass to kill them within 

 a few hours. Obviously, when residual sprays are applied to 

 wooden or plastered walls, much greater amounts must be 

 used because a lot of the material penetrates into the pores 

 and does not remain on the surface where insects can get in 

 touch with it. 



