528 Journal of Agricultural Research voi. xiii, no. h 



It will be noticed that aqueous solutions other than soap solutions do 

 not penetrate the tracheae. It was interesting to note that both nico- 

 tine and absolute alcohol are able to penetrate the tracheae, but when 

 greatly diluted with water they are no longer able to enter. Those sub- 

 stances which in an aqueous solution exhibit surface viscosity, such as 

 saponin, gelatin, and casein, do not penetrate the tracheae. Those mate- 

 rials have frequently been employed in preparing emulsions {4, 20, 21). 

 It is apparent from the table as a whole that compounds which are 

 soluble in ether or are capable of dissolving fats or oils are able to spread 

 over the body of the cockroach and penetrate the tracheae. Such sub- 

 stances as acetic acid, furfurol, and nicotine are not particularly good 

 fat solvents, and it may be noted that they did not penetrate any great 

 distance into the tracheae. Compounds with a high viscosity, such as 

 lubricating oil, failed to penetrate far into the tracheae; but if these 

 viscid substances had been given a longer time they would undoubtedly 

 have penetrated much farther. Some soap solutions, such as ivory, 

 gel at room temperatures in dilutions ordinarily used in spraying 

 (i : 150 or 200), and are therefore unable to penetrate to any great extent. 

 The penetration of such a soap solution is increased when its cohesion 

 is reduced by raising the temperature of the solution. Soft soap and 

 yellow soap were liquid at the same dilution and penetrated better than 

 Ivory soap, while Castile soap, manufactured from the liquid oleic acid, 

 penetrates very well. 



According to Morgulis {15) the exact chemical composition of chitin 

 is still questionable. From our results its composition must be such 

 that it is easily wetted by oils and oil solvents, or perhaps the surface of 

 the chitin itself contains or is coated with an oily or a fatty substance. 

 These results can not be applied to all insects, as some have special 

 coverings of wax over the chitin. The spreading of sprays on such 

 insects can, however, be determined by our methods with little trouble. 



RELATION OF VISCOSITY AND VOLATILITY TO THE PENETRATION OF 



THE TRACHEA 



The foregoing results have been primarily concerned with the pure 

 materials. Inasmuch as the pure materials are seldom applied in actual 

 practice, the question arises as to what takes place, when an emulsion 

 of an oil is used in spraying. By placing drops of the emulsion on the 

 wings, and also on the bodies of cockroaches, it was found that the oil 

 droplets were completely surrounded by the emulsifier and did not 

 come in contact with the body of the insect. The ability of such a 

 spray to adhere to the insect, to spread over the body, and to penetrate 

 the tracheae depends, therefore, on the character of the emulsifier and 

 not on the emulsified oil. Three distinct types of emulsions were noted : 

 First, emulsions made with gelatin or saponin formed a round drop and 

 tended to roll ofif the insect, just as did pure aqueous solutions of these 



