176 The Effect of Various Chemicals on Blow-Fly 



than are the old larvae. This shows that it is undesirable to carry 

 out experiments with larvae of mixed ages, and suggests that in 

 preventive measures against the blow-fly pest it is necessary that the 

 remedy should be applied before the larvae hatch, or in the very earliest 

 days of the larval stage. 



The higher mortality of the young larvae may possibly be explained 

 as follows. Young blow-fly larvae secrete a fluid which digests tissue. 

 The resulting liquid is then re-absorbed. The toxic effect of a toxic 

 compound, e.g. arsenic sulphide, on young larvae may therefore be 

 conceivably due to an interference with their digestive faculties, and, 

 in this case, the compound would be acting as a stomach poison, without, 

 however, entering the larva. The results with young larva would thus 

 be comparable to those obtained with caterpillars, when e.g. their 

 digestive organs had been affected. Old larvae, ceasing to feed, no 

 longer secrete digestive fluid, so that the effect of the compound on 

 these would be confined to that of a contact poison, and would chiefly 

 act upon the respiratory organs. 



Table V gives the results of further experiments which only differ 

 from those recorded in Table IV by the fact that the powder under 

 investigation was mixed with sufficient sand to reduce the propor- 

 tion of toxic agent to one-tenth of the original strength. A definite 

 number of larvae were placed in the mixture of sand and powder. As 

 before, the mixture was kept in glass jars covered with muslin. The 

 series contains experiments on young, and old, larvae. 



The experiments on the old larvae were for the most part vitiated 

 by the Chalcid infection, to which reference has already been made. 

 A great mortality, sometimes amounting to 100 %, was observed in 

 the control experiments and can only be explained by this fact. Only 

 those experiments in which the controls are not apparently affected 

 appreciably by the parasitic fly are therefore recorded. 



The higher susceptibility of the young larvae is most marked and, 

 considering the small percentage of toxic substance present, many of 

 the substances used gave results which lead us to believe that they 

 are worthy of a practical trial in the field. Arsenic sulphide, nitro- 

 benzene, methyl salicylate, cedarwood oil, ^-nitraniline, borax, picric 

 acid, dimethylaniline, quinoline, as in the previous series, have all 

 given highly satisfactory results ; whilst, in addition, copper carbonate, 

 oil of cloves, turpentine, /3-naphthol, creosote, green oil, boracic acid, 

 fusel oil, sinapis and aniseed oil, all seem to have a poisonous effect on 

 the young larvae. 



