104 MEDICAL ENTOMOLOGY 



would result in expelling its contents. The view most consistent with 

 the anatomical appearances is that it acts as a primary receptacle for 

 the blood, which is passed into it direct from the pharynx as it is 

 sucked up, and is passed on into the mid-gut by the contraction of the walls 

 of the sac. The presence of the blood in the sac acts as a stimulus to 

 provoke its contraction, so that the blood never remains in it, but is 

 returned up the duct as fast as it is received. This is all the more 

 likely to be the case in that the duct of the crop is in line with the 

 pharynx, while the part of the oesophagus leading to the proventriculus 

 is bent at an angle. The straight course for the blood is into the crop. 



The term ' sucking stomach ', frequently applied to this structure both 

 in this and in other flies is a misnomer, for whatever its function may be 

 it certainly does nothing to assist in sucking up the blood, neither is it a 

 stomach, in the sense of having any digestive action. The term oesopha- 

 geal diverticulum is also undesirable, as hiding the essential similarity of 

 this structure with that in other insects. 



The proventriculus is a long flattened tube, extending from the thoracic 

 inlet to the commencement of the abdomen. In its anterior two-thirds it 



is about twice as broad in the transverse diameter as it 

 Proventriculus ... . , , , 



is in the vertical, so that its lumen is a transverse slit, 



but posterior to this it becomes rounder, with a small circular lumen, and 

 also much narrower. At the anterior end there are two broad lateral 

 expansions, the anterior and external angles of which project forwards 

 and receive minute nerves from the head. This part, when seen from 

 the dorsal aspect, resembles a butterfly with expanded wings. With the 

 exception of the posterior and rounded part the whole of the surface of 

 the organ is studded with small regularly arranged elevations, which give 

 it a mammilated appearance when examined under a low power ; there 

 are four rows of such elevations on each of the dorsal and ventral 

 surfaces, and two rows on each lateral border. 



The opening by which the oesophagus communicates with the pro- 

 ventriculus is situated in a small elevation on the dorsal surface, not at 

 the extreme end, but a little behind it. 



The minute structure of the wall of the proventriculus is essentially 

 the same as that of the oesophagus, but the appearance on section is pro- 

 foundly modified by the great increase in the thickness of the wall and the 

 bunching up of the cells to form the elevations referred to above. A section 

 through the middle of the tube shows a transverse lumen, with many 

 narrow fissures passing into the substance of the wall on all sides. The 

 lumen is bounded by a continuous sheet of finely granular material, 



