i9i3" I 9 I 4-] The Common House Fly. 117 



chyle stomach, a crop, a ventriculus or true stomach, a fore 

 and a hind intestine. The proventriculus has muscular walls, 

 and probably acts as a valvular organ and a pumping stomach 

 also. The food swallowed almost certainly goes directly to 

 the crop. This is a double organ, the walls of which are com- 

 posed of non-striped muscle situated in the abdomen, and acts 

 merely as a temporary storehouse of food, for the latter 

 remains unchanged during its stay here. Pumped back again 

 by the proventriculus, the food now passes as required to the 

 true stomach, which is in the form of a tubular structure, with 

 greatly folded walls and saccules, which runs directly from the 

 thorax backwards to the abdomen and ends just over the 

 dilatation of the crop. The small intestine carries on the food, 

 and after coiling on itself four or five times it leads into the 

 large intestine, and so to the rectum. If the fly is feeding 

 abundantly it first fills its crop, and then continuing to feed 

 the food passes directly into the true stomach. If a fly 

 be examined three minutes after it has fed, all the food is 

 found in the crop. After ten minutes, the food is just begin- 

 ning to pass into the stomach. After fifteen minutes, the 

 upper third of the stomach is full. After two hours, the 

 stomach is three-quarters filled. After a single feeding, the 

 crop may remain full as long as four days, so providing a 

 supply against any future scarcity of food. 



After feeding, the fly behaves much like another familiar 

 but somewhat larger domestic pet. It proceeds to clean itself 

 by rubbing its legs together and by passing them over its 

 head and wings. At intervals it ejects large drops of liquid 

 from the proboscis, which may grow until they are equal to the 

 fly's head in size. The droplet may be withdrawn and ejected 

 several times, or it may be deposited on the surface, forming 

 the familiar ' fly blow ' on mirrors or window-panes. When 

 flies have fed on food infected with micro-organisms, in these 

 ' vomit ' stains the organism can be found. 



In the head is the large cephalic ganglion which corresponds 

 to the brain, and through a small opening in which runs the 

 oesophagus. Another large nerve mass lies in the thorax and 

 is joined to the brain by a median nerve cord ( = spinal cord). 

 From each of these ganglia nerves run to the various organs 

 and limbs. Thus from the front of the cephalic ganglion two 



