METHOD OF MARKING FLIES 71 



the flies' ability and desire to travel. The range iA' liight of Hies 

 in cities has a direct bearing on the pcjssibilitics of the carriage of 

 infection and location of breeding places and nuisances. Accord- 

 ingly in the summer of 1911 I arranged a series of experiments in 

 the city of Ottawa which were carried out, under my direction, by 

 Mr G. E. Sanders, a field officer of the Division of Entomology, to 

 whom belongs the credit of devising the excellent method of 

 marking the flies. 



The point of liberation of the flies was on Porter's Island, a 

 small island about 1000 feet long, Ijnng in the Rideau River, which 

 runs through a part of the city and is a tributary of the Ottawa 

 Rivei' which it joins a short distance further along its course. The 

 surrounding district forms a portion of the north-eastern part of 

 the city and consists chiefly, especially on the northern side of the 

 river which is knoAvn as New Edinburgh, of working-class dwellmg 

 houses. On this island which is connected with the bank by a 

 small bridge, small-pox cases were isolated in a small wooden house 

 used as a hospital, or in tents. The land rises gently from the 

 river on the southern side. 



The flies used in the experiments were obtained and marked 

 in the following manner : Stable refuse, in which the larvae were 

 found in large numbers, was placed in breeding boxes, provided 

 with a circular aperture at the top and balloon fly-traps were 

 placed over these apertures. The flies, on emerging from the 

 pupae, entered these cages and were thus obtained in a healthy 

 and uninjured condition. They were marked by spraying them 

 while in the wire cages with a solution of rosalic acid (rosaurin or 

 methyl-aurin Cjo Hie O3) in 10 per cent, alcohol, applied by means 

 of a fine spray. This method of marking insects was devised by 

 Mr Sanders, who first used it in experiments with ants. It is 

 simple, harmless and reliable as a means of detection. The presence 

 of a marked fly on a sticky fly-paper is indicated by its producing 

 a scarlet coloration when the paper is dipped into water made 

 slightly alkaline. In these experiments the flies were reared and 

 marked in the Division of Entomology, w^hich is situated about 

 three miles from Porter's Island, to which they were carried in the 

 cages and liberated on arrival. " Tanglefoot " fly-papers were 

 placed in as many as possible of the houses in the neighbouring 



