SUMMARY AND CONCLUSIONS 257 



In connection with the distribution of bacteria, the alimentary 

 canal and its appendages constitute the most important system 

 in the internal anatomy of the fly. The alimentary canal is long 

 and complicated, consisting of the oesophagus, proventriculus, 

 ventriculus, intestine and rectum. A branch of the cesophagus, 

 the crop-duct, passes to the crop, a very distensible blind sac, 

 situated in the anterior part of the abdomen. The salivary 

 glands are greatly developed (Chapter IV). 



The oral lobes situated at the end of the proboscis of the fly 

 are provided with minute channels, the pseudo-tracheas, into which 

 liquid food is sucked through a form of grating, which prevents 

 most particles of large size entering the oesophagus. Occasionally 

 when feeding on fa:cal material containing parasitic ova, to 

 which flies are greatly attracted, great exertions may cause 

 larger particles, such as ova, to enter the mouth direct, without 

 the filtering action of the pseudo-tracheae coming into action 

 (Chapter V). 



The crop acts as a reservoir into which fluid food is passed 

 previous to its entering the intestine. The fluid in the crop is 

 only passed into the intestine gradually through the proventri- 

 culus, which acts as a valve. When feeding on soluble substances 

 this fluid in the crop, which is often highly contaminated with 

 bacteria, is regurgitated through the proboscis in order to 

 moisten and dissolve the food (Chapter vi). 



Flies can travel for considerable distances, up to lOOO yards^ 

 at least in open districts, but probably do not often travel far 

 in towns. In houses they walk over everything, including 

 food, and deposit both faecal material and ' vomit,' or fluid 

 regurgitated from the crop, on every article over which they 

 walk. After feeding they habitually regurgitate a portion of 

 the crop contents, and other flies suck up the vomit, and infect 

 themselves with any bacteria which may be present in it. 



Flies defaecate very frequently, especially when well fed, 

 many masses of faeces being deposited in the course of the day 

 (Chapter Vll). 



Outside houses flies congregate on fermenting and decaying 

 refuse of all sorts, carcases of animals and faeces. 



If flies are provided with a sufficient quantity of liquid food 



G.-s. 17 



