DISTANCE TRAVELLED HV FLTES 67 



localities in Manchester 8196 were M. domestica, 293 were F. caid- 

 cularis and 64 were other species. 



Ill Bii-minghani Robertson (1909) found the different species 

 of flies in the following proportions in a collection of 24,562 flies : 

 M. domestica 22,360, 91 per cent.; F. canicukms 1154, 47 per 

 cent.; the blow-fly C. eri/throcephala 840, 3'4 per cent.; and other 

 species, 218 or 9 per cent. 



From the foregoing flgures it may be taken that as a general 

 rule Masca domestica constitutes more than 90 per cent, of the 

 total fly population of a house. The proportions of the other 

 species vary, the variation depending largely on the situation of 

 the house, whether it is urban, suburban or rural and also upon 

 the sanitary conditions prevailing in the neighbourhood. 



Seasonal Prevalence of Flies. 



As a general rule house-flies are most abundant during the 

 hottest months of the year. In Europe and North America, 

 north of Mexico, they are most numerous during the months of 

 July, August and September. In South Africa they are abundant 

 from October to February. The fly season in Australia extends 

 from October to March. In more tropical countries they are 

 prevalent during a more extended period. 



Flight or Distance travelled by House-Flies. 



The distance that house-flies are able to travel either by their 

 own exertions or by the aid of the wind is manifestly an important 

 question in view of its connection with the spread of infection by 

 these insects and the location of their breeding places. We are 

 not concerned here with the ability of flies to travel in electric 

 street cars, trains or steamboats ; the fact that they are able to be 

 transported by these means is a matter of common observation. 

 The question I am about to discuss is : how far do flies travel 

 under natural conditions ? 



Normally they do not fly great distances. I have previously 

 compared them to domestic pigeons which hover about a house 

 and the immediate neighbourhood. On sunny days they may be 



5—2 



