I08 DISTRIBUTION OF BACTERIA 



deliberately removed with the anterior pair of legs. The same 

 thing was frequently noticed in subsequent similar experiments. 



From all the observations which have been made it seems 

 highly probable that blow-flies frequently re-infect their limbs 

 when cleaning themselves. It is, however, almost impossible to 

 determine by actual experiments how frequently or to what 

 extent this occurs. Probably both the frequenc}' and the extent 

 vary with the consistency of the food. 



Measurements which were made of the contents of the crops 

 of recently fed blow-flies showed that more than o*02 c.c. of fluid 

 might be contained in them. On one occasion a blow-fly which 

 had recently fed on coloured syrup was held by the wing and 

 the material which it vomited collected and measured. It was 

 found that 0"0i c.c. of coloured syrup had been deposited. 

 Subsequently the fly was killed and dissected and 0"0i c.c. 

 of fluid obtained from its crop. 



All the experiments which have been made on this subject 

 show that blow-flies take up large quantities of fluid in feeding, 

 and are capable of vomiting at least half of the contents of their 

 crops. 



If flies are allowed to feed on fairly large volumes of milk 

 contained in watch-glasses they often fall into the milk. After 

 emerging the}- leave long trails as they crawl up the sides of the 

 watch-glasses. Frequently also they soil their limbs while feeding, 

 and in walking away leave trails like those just mentioned. On 

 several occasions these trails have been proved to be infected. 

 Other flies approaching the milk usually stop and suck at the 

 half dried trails and drops left by previous flies, and probably 

 infect their feet and take up micro-organisms into their crops. 



The experiments and observations on the feeding habits of 

 blow-flies all show that the individual h\o\w-fiy {C. erjthroccphala) 

 is capable of distributing greater numbers of bacteria, and 

 over a longer period of time, than the individual house-fly 

 {M. domestica). House-flies, however, are probably a greater 

 source of danger owing to their greater prevalence and to their 

 more frequent occurrence where food can be easih- contaminated. 



