FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 57 



deterrence, so that at 3 per cent, it is equal in its attraction to the water control, 

 and at 2 "5 per cent, approaches very neat to the control of sugar and water. The 

 continuous line represents the mixture which is finally recommended, 50 per cent, 

 lime water and 2*5 per cent sugar. It is equal to the water control at 5*5 per cent., 

 and then becomes m.ore attractive than water because the flies recognise in it a food, 

 while the formaldehyde is not repellent enough in weak solution to deter them in 

 large numbers. At 5 per cent, only 12 per cent, fewer flies drank than would have 

 done so if the solution were sugar and water only. At 2*5 per cent, formaldehyde, 

 only 3*5 per cent, declined to drink. The neutralisation of the formic acid and the 

 addition of the bait have at this point nearly counteracted the deterrent action of 

 the formaldehyde. It is of considerable interest to notice that the lines which 

 represent the formaldehyde with and without lime water are parallel in pairs, while 

 those with and without sugar converge in pairs at 2 '5-1 '5 per cent. This shows 

 the influence of the presence of formic acid in very dilute solutions. The formalde- 

 hyde used proved on titration to be -f 60 acid (Eyre) which is equivalent to 2*76 

 grams of formic acid per litre of formaldehyde. At 5'0 per cent, dilution this is 

 0*014 per cent, formic acid, and at 2'5 per cent, it is 0"007 per cent, formic acid. The 

 presence of sugar is sufiicient to counteract the latter but not the former. A factor 

 which has not been taken into consideration is that the formaldehyde vapour may 

 be deterrent and that fewer flies may taste stronger than weaker solutions. This 

 was not apparent in the open, but showed itself when the experiment was repeated 

 in enclosed spaces. There was then progressive deterrence as the strength of the 

 solution increased, as was shown by the smaller percentage of the available flies 

 which settled on it. The curve of this deterrence is represented by the dotted line 

 in Diagram 11. Consideration of it will show that its fall is so gradual that it would 

 not affect the argument even if its influence were as powerful in the open as in the 

 enclosed space, since the influence of the other factors is so much more potent. 



The Toxicity of Formaldehyde. 



In determining this factor, the solution used was 50 per cent, lime water and 2 '5 per 

 cent, sugar, only the percentage of formaldehyde varying. About 50 flies were 

 enclosed in each of several glass basins of capacity of 600 cc. The basins were then 

 placed over saucers containing the poison and a count was taken of the flies that 

 tasted and those that drank. In the first series the time allowed was fifteen minutes, 

 but it was found that all the flies that intended to drink did so in the first five minutes, 

 and the exposure was subsequently limited to this time. The poison was then re- 

 moved and the edge of the basin slightly raised to allow air to circulate. At the end 

 of one hour and of two hours the numbers lying inert were counted, the difference 

 between the two counts representing the recoveries, as formaldehyde seems to act 

 as an anaesthetic, and its effect in weak solution, or when a very little is taken, is 

 sometimes only temporary. No recoveries were noted after two hours. AU the 

 flies were then anaesthetised and counted. Four series of each percentage were thus 

 made. The complete results are summarised in Table XIII. This shows the total 

 and average numbers of flies exposed to each percentage of formaldehyde used (from 

 2*5 to 20 per cent.) ; the average, maximum and minimum numbers of flies which 

 tasted ; the average, maximum and minimum percentages of those which drank 

 (681) E 



