260 THE RELATION OF FLIES TO SUMMER DIARRHOEA 



remains excessive Hamer brings forward two considerations : first, 

 that the comparatively early fall in the amount of diarrhoea is 

 observed in years of very low mortality as well as in years of 

 excessively high mortality in which such an explanation might 

 hold good. Secondly, the hypothesis does not adequately account 

 for the almost identically similar behaviour of the two curves both 

 in their ascent and descent. The close correspondence between the 

 curves, Hamer believes, accords better with the view that both 

 are dependent upon variations of temperature than with the 

 hypothesis that diarrhoea stands in direct causal relation to fly 

 prevalence. In his last report and in the light of three years' 

 records (1907 — 1909) Hamer is still unconvinced that the evidence 

 available can be considered to support a causal relation between 

 summer diarrhoea and flies and a critical attitude is still main- 

 tained. 



In regard to Hamer's chief criticism, namely, that while the 

 number of flies is still excessive, the diarrhoeal curve has begun 

 to decline, I have previously pointed out (1910) that a considera- 

 tion of the habits of the house-fly will probably afford an ex- 

 planation of this difficulty. Flies are very susceptible to changes 

 of temperature. When the temperature falls flies become less 

 active and retire into the shelter of houses and other buildings, 

 although their numbers, as indicated by captures in traps set 

 indoors, may still be considerable. In so far then as the activity 

 of the flies is associated with the temperature, the temperature 

 curve should be studied in addition to the fly and diarrhoea 

 curves. If this is done, it is usually found that a fall in the 

 number of flies is preceded by a fall in the temperature and that 

 these two curves are associated somewhat closely, that is, the 

 numerical activity of the flies — since the numbers caught are 

 more indicative of their numerical activity than of their numerical 

 abundance — is dependent upon the temperature and also, I have 

 found, upon the state of the weather and sky. Therefore, if the 

 flies become less active, they will be less liable to transmit the 

 organisms causing summer diarrhoea, and although the numbers 

 caught in the houses may exceed in numbers those caught earlier 

 in the season when the diarrhoea curve was rising, those which 

 are very active will be less in number and consequently instead of 



