47 



ON THE REASONS FOR THE VARIATION IN THE EFFECTS OF 



FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 



By Capt. Ll. Lloyd, D.Sc, R.A.M.C. (T. F.). 



The following is an account of an investigation carried out on behalf of the Hygiene 

 Department of the Royal Army Medical College at the suggestion of Prof. Maxwell 

 Lefroy, who, after his visit to the Mesopotamia War Area in 1916, was insistent 

 on the importance of the discovery of some poison for house-flies (Musca doinestica 

 and allied species) that could be used indoors with safety. The work was carried 

 out in the Entomological Department of the Imperial College of Science in the early 

 part of 1917. Large stocks of flies which were kept breeding in two rooms were 

 available for the purpose. 



Formaldehyde has been frequently recommended as a poison for flies, but those 

 who tested its effects gave varying accounts of it. Miss Lodge (1), who studied 

 it in some detail, was unable to account for these variations and stated that no 

 definite conclusions could be drawn from her work. An analysis of the various 

 factors which might cause such variations was made, and it was found that there 

 were three which play an important part. These are (1) impurities in the formal- 

 dehyde, (2) the humidity of the atmosphere, and (3) the strength of the solution 

 used. 



The following subjects will also be discussed, (4) the advantage of adding a bait, 

 (5) the most effective mode of administration, and (6) comparison with other poisons. 



" Formaldehyde " must be understood throughout to mean the 40 per cent, 

 solution in which it is usually sold, and all strengths are given as percentages of this. 

 " Formalin," the trade name of one brand, is often loosely used for this solution. 



Impurities in the Formaldehyde. 



Formaldehyde is prepared by passing a mixture of heated air and the vapour 

 of methyl alcohol over some heated contact agent, such as copper gauze or silver. 

 The methyl alcohol is crude or pure, but should be 90 per cent., and should not 

 contain more than 1 per cent, of acetone. The purity of the product depends largely 

 on that of the methyl alcohol, and on the freedom of the air from ammonia. 

 The following substances are liable to be present : trioxymethylene, methylal, 

 methyl and ethyl alcohols, acetone, hexamethylene tetramine, methylamine, and 

 formic acid. Prof. Baker, of the Imperial College of Science, provided samples of 

 formaldehyde to which certain of these substances had been added in proportions 

 in which they are liable to occur. Solutions of these, generally in water and soaked 

 up in clean sand, were exposed side by side with solutions of the control formal- 

 dehyde in the fly room, and left for twenty-four hours. The numbers of dead flies 

 in the immediate neighbourhood of each were then counted. Many flies left the 

 table to die and no account could be taken of them. There was always a definite 

 group dead in a circle of six inches radius around each jar, and the comparisons 

 were based on these. In order to obviate the influence of the varying numbers 

 of flies on different days, all numbers are giv^en reduced to percentages of the controls. 

 All decisions are based on a number of tests, with the positions of the various solu- 

 tions and the controls interchanged. 



