128 SANITARY ENTOMOLOGY 



by the regurgitation of food from the crop, as proven by Graham-Smith, 

 who fed flies upon .carmine colored food, and found carmine stains on 

 semi-fluid material upon which these flies later fed, for 22 hours. 



The intestinal canal is composed of pharynx, esophagus, crop, pro- 

 ventriculus, ventriculus or chyle stomach, proximal and distal intestine 

 and rectum. The esophagus passes from the pharynx through the cer- 

 vical region into the thorax, in the anterior part of which it opens into 

 the proventriculus, and from this same point a duct which is continuous 

 with the esophagus passes back into the abdomen to the crop which is a 

 bilobed sac, capable of considerable distention. This crop serves as a 

 food reservoir. The fly feeds until it has engorged the crop, and often 

 will continue feeding, the food then passing directly into the proventri- 

 culus. The opening of the proventriculus into the esophagus is ventral. 

 This organ is circular, flattened dorsoventrally. The ventriculus is 

 tubular, narrowest in front and narrowing again in passing through the 

 thoraco-abdominal foramen. The proximal intestine is the longest region 

 of the gut, being considerably coiled. The distal intestine begins at the 

 entrance of the Malpighian tubules, and is only curved once. It is sep- 

 arated from the rectum by a valve. The rectum is composed of three 

 parts, the intermediate of which is swollen to form the rectal cavity into 

 which the four rectal glands empty. 



Food may remain in the crop for several days, and even when no 

 further food is given, it requires many hours to empty the crop com- 

 pletely. After feeding the fly usually retires to a quiet spot and cleans 

 its head and proboscis. It frequently regurgitates its food from the crop 

 in the form of large drops of liquid which are subsequently slowly drawn 

 up again and probably pass into the proventriculus. These drops of 

 regurgitated food frequently are deposited, often for the purpose of 

 moistening sugar and similar dry foods. 



We may now see how easy it is for a fly which has fed on infected 

 substances to contaminate other substances for days by regurgitation 

 from the crop, as well as through fecal deposits. Experimental evidence 

 ha§ proven contamination by both the feces and the vomit. 



The fly's body is externally constructed so as to further aid in 

 disease carriage. There are numerous hairs or setason the body, espe- 

 cially on the legs. The last joint of the tarsus of each leg bears two 

 claws and a pair of membranous pyriform pads or pulvilli. These pulvilli 

 are covered beneath with innumerable, closely set, secreting hairs by means 

 of which the fly is able to walk in any position on highly polished sur- 

 faces. These sucker-like pads or pulvilli and the setas of the legs are 

 excellent bacteria carriers, and not infrequently larger organisms as 

 mites, worm eggs, etc., are thus carried. 



The sexes of the house fly are about equal in number. Copulation 



