FORMALDEHYDE AS A POISON FOR HOUSE-FLIES. 



61 



Advantages of adding a Bait. 



Reference to Table XII and to Diagram i will show that to get the best effect from 

 the formaldehyde it is necessary to add some bait to counteract the deterrent effect. 

 By this means a larger number of flies are induced to drink. Sugar is the most 

 attractive and most permanent bait, and 2'5 per cent., or half an ounce to the pint, in 

 the dilution is sufficient ; to make it stronger does not make it more effective, while at 

 1 per cent, it begins to lose its influence. An advantage of this substance is that it 

 has no interaction w^ith the formaldehyde. Glucose, maltose and lactose have also 

 been tried, but are less effective than cane sugar. Molasses will be found eft'ective. 

 An infusion of hay, boiled and strained, and a cold infusion of horse-dung have been 

 tested and found to be effective, but less so than sugar, and they are not recommended 

 for this reason. 

 The formula to be employed is : — 



40 % Formaldehyde, 50-60 cc. . . . . 1 fluid ounce. 



l pint. 



h ounce (dessertspoonful), 

 add to make 1 pint. 



Filtered lime water, 500 cc. . . 



Sugar, 25 grams 



Water, add to make 1,000 cc. 



Method of Administration. 



It is necessary to protect the formaldehyde from the action of air, and to prevent 

 flies from falhng into it, both of which factors make it acid, and soon cause the lime 

 water to be neutralised. A simple form of trap has been devised for this. The 

 poison is placed in a bottle, and the mouth is closed by means of a platform of absorp- 

 tive material from the centre of which a stem of the same material passes down into 

 the fluid. The top is whetted w^th the fluid at the commencement and is kept wet by 



Fig. 1. Form of trap for 

 use with formaldehyde. 



capillary attraction. Such an arrangement may easily be made from blotting paper. 



A circle of the paper two inches in diameter is cut out, and a small slit is made in the 



centre. Two strips of the paper of the same width as the slit are then passed through 



it and the ends are doubled outwards and downwards. The strips are then drawn 



down till the doubled-back portions rest on the platform, and the trap is ready for 



use (see fig. 1). 



