172 The Effect of Various Chemicals on Blow-Fly 



was cut into rectangular pieces 12 x 8 inches and these were nailed on 

 to a large board, wool uppermost, strips of wood being nailed between 

 the different pieces, to prevent the larvae from migrating from one to 

 another. The chemical to be tested was mixed with a definite propor- 

 tion of precipitated chalk, a piece of hide dusted with the medicated 

 powder and a definite number of larvae placed upon it. The hide 

 was then covered with a piece of muslin, which was tacked down all 

 round to the wooden partition strip, and the whole then placed aside 

 for the larvae to develop. Untreated pieces of skin were also set aside 

 to act as controls. The results are given in Table II. 



TABLE II. 



A definite number of larvae were placed on a piece of sheep-hide, the ivool 

 side of which had been previously treated with the active substance 

 diluted with precipitated chalk. 



The method was not successful; the pieces of skin dried up and 

 often putrefied. Further, the larvae ate their way through the skin 

 and crawling beneath it, out of the reach of the powders, pupated there 

 in a manner quite impossible under natural conditions. Considerable 

 difficulty was experienced in obtaining supplies of fresh skin suitable 

 for the purpose. The length of the fleece varied very greatly on different 

 skins, this, of course, introducing another undesirable factor. It was 

 evident that, for comparative results, this method was useless and 

 that some substitute for sheep-skin must be found. 



Though differing greatly from wool, sand and sawdust appeared 

 to be most convenient for the purpose. Definite quantities of these 

 can be employed, and they are readily mixed with the substance under 

 investigation. It was thought advantageous to make experiments both 

 with sand and sawdust, as they differ considerably in one very important 

 point, which might — and evidently did — have some effect on the results; 



