170 The Effect of Various Chemicals on Blow- Fly 



As a result of this series of experiments, it is evident that a very 

 considerable number of chemicals are deterrent to the flies and are 

 capable of protecting horse-flesh from their ravages. Later in the 

 summer, when the flies were much more numerous, this series of experi- 

 ments was repeated with the same powders. In this case, all controls 

 became blown in one day, and the protective effect of the powders was 

 not nearly so marked as in the first series. Most of the pieces of flesh 

 became blown in one to four days, immunity being only conferred by 

 the powders containing the following chemicals: copper carbonate, 

 nitrobenzene, borax, picric acid (calcium carbonate), creosote, sinapis 

 oil, and aniseed oil. 



The exact means by which this immunity is conferred is unknown. 

 Clearly the sense organs of the fly itself are affected. Assuming the 

 fly selects a spot for egg-laying from the smell, e.g. of decaying meat, 

 i.e. to be guided by chemotaxis affecting " smell organs," these compounds 

 influenced the smell of the horse-flesh by the flies. The deterrent 

 compounds may have acted solely as preservatives and so prevented 

 the decay of the horse-flesh that the flies failed to recognise it. On 

 the other hand, either the taste or the smell of the compounds them- 

 selves may have been obnoxious to the flies. 



It may be pointed out that the experiments in this section deal with 

 chemicals obnoxious to the adult fly. The following sections have to 

 do with chemicals obnoxious or toxic to the larvae. The results there- 

 fore are not comparable. 



B. Experiments with Compounds, 'presumably toxic to the larvae. 



The very large scale on which a successful insecticide would have 

 to be applied precludes the use of a pure chemical and necessitates 

 its dilution either by solution, emulsion in a suitable vehicle or admixture 

 with an inert powder. Obviously the vehicle must be cheap and 

 non-injurious to the sheep or its fleece. The following suggest them- 

 selves as being suitable in this respect: water, or paraffin as liquid 

 vehicles, and some such material as precipitated chalk for powders. 

 Many chemicals soluble in an oil but insoluble in water might be applied 

 in the form of an emulsion. On account of its property of "creeping," 

 paraffin might prove valuable. It is a solvent of many chemicals and 

 its price would not be prohibitive, especially if it were applied in a fine 

 spray or as an emulsion. 



Precipitated chalk also appears to be quite practicable and has 



