E. R. Speyer 125 



(6) Collection of insects from the traps. 



After an experiment the insects contained in the traps were killed 

 by the emersion of the whole trap in hydrocyanic acid gas, generated 

 from potassium cyanide and sulphuric acid (dil.) in a small crucible 

 placed at the bottom of a large metal vessel, used generally for the 

 removal of manure from stables. After about two minutes exposure 

 the insects were shaken on to a sheet of paper and collected in glass vials. 



Special note was taken of any flies found dead in the traps (by careful 

 removal of the cone) before exposure to the gas, and also of insects found 

 dead in the bait-dishes. 



(c) Disposition of traps. 



In these preliminary experiments, six traps were placed upon the 

 floor of a stable at the Government Agricultural Experiment Station 

 near Arlington, Washington D. C, U.S.A. 



In this stable were two horse-boxes where large numbers of house- 

 flies were breeding. Six traps, A, B, C, D, E and F were used for the 

 experiments, located as follows: — The distance between A and B, 

 C and D, E and F, in each case was 6 inches, the distance between 

 B and C, D and E was 10 feet respectively, and the distance from 

 Fto A 15 feet. Traps B. C and E had, for baits, various attractive 

 substances, whilst A, D and F had, in the first series of experiments, 

 a control substance — the same in each trap — of moderate attractive 

 power: in the second series a control substance of great attractive power. 



After a convenient interval, the traps B, C and E were interposed 

 with their baits, so that they occupied each position for three equal 

 periods of time, and the contents of each trap in its three positions were 

 added together. 



(d) Sources of error and their compensation. 



1. Owing to the nature of the traps, a number of flies might escape 

 from the entrance to the trap, after being attracted to a specific sub- 

 stance, and fly to another trap. 



All the traps in an experiment would be affected to a similar degree, 

 so that the error is of little magnitude. 



2. The position of some traps was certainly more favourable than 

 that of others, apart from the power of the attractive substances used 

 in them. 



This was fully compensated for by the interposition of the traps 

 described under (c). 



