juneio, i9i8 Physical Properties Governing Contact Insecticides 531 



passage of a liquid, as it is quite possible that the stain could penetrate 

 by being absorbed by the walls of the tracheae without any penetration 

 of the oil. If a few pieces of chitin are placed in kerosene stained with 

 picric acid, they are capable of absorbing practically all the stain from 

 the oil. On the other hand, it is quite possible that the amount of the 

 insecticide which might penetrate and kill the insect would be so small 

 that the amount of stain carried with it could not be detected. Further, 

 it would be impossible to determine the penetration of a vapor by a stain 

 in the insecticide. Chemical tests for insecticides in the tissues in many 

 cases would not be delicate enough for the certain detection of their 

 presence or absence. Finally, after a number of trials, it was found that 

 the best method was to use an indicator for dead tissues. Trypan blue 

 is a water-soluble colloid which does not penetrate the living tissues, but 

 is able to penetrate and stain dead tissues. This was selected as the 

 most suitable indicator. Larvae of the wax moth were used in these 

 experiments. If a living, untreated larva is opened and the tissues covered 

 with an aqueous solution of trypan blue for a period of two minutes and 

 the stain then removed and the larva examined under water, it will be 

 found that there are particles of the stain adhering along the midintestine, 

 more or less on the silk glands, and along the nerve cord. This does not 

 seem to be a true staining of the tissues, but rather the adherence of 

 particles of the stain, which are difficult to remove by washing (ad- 

 sorptions) . 



When an insect has been treated with an effective contact insecticide 

 and opened and stained just before it dies, the tissues which have been 

 killed are stained a deep blue, thus indicating the point of entrance of 

 the poison. No effort was made in this work to determine which tissues 

 were primarily efifected by the chemicals. As a counter stain, to show 

 to what extent the materials penetrated the tracheae, Sudan III was 

 used with the oils and resin with the aqueous solutions. The penetra- 

 tion into the insect of the different insecticides are given in Table III. 



Table III. — Penetration of materials as indicated by Sudan III, with trypan-blue 



tnethod 



FUMIGATION V.'iPORS 



