THE ALIMENTARY CANAL AND ITS APPENDAGES. 55 



branched extremities, This coat is thickest near the opening of the 

 oesophagus, and the fibres of that portion of the organ which lie in the 

 mesial line are mostly circular. Numerous nerve cells belonging to the 

 sympathetic system are scattered over the external coat, and form a 

 pair of large patches, near the entrance of the oesophagus. The cavity 

 of the sucking stomach is lined with pavement epithelium, and 

 frequently contains a beautiful mould, allied to Botrytis, which often 

 causes the death of the insect by its rapid development. 



The proventriculus is a small globular cavity at the commence- 

 ment of the chyle stomach ( Plate IV, Fig. 4,) ; it has thick 

 glandular walls, and secretes the gastric fluid ; it opens 

 directly into the chyle stomach, where absorption commences. 

 The epithelium of the proventriculus consists of several layers 

 of clear nucleated cells, those upon the surface being angular 

 and closely adherent to each other ; the deeper layers are oval, 

 and the most external float freely in fluid. They difier remark- 

 ably from the epithelium of the chyle stomach, which consists of 

 a single layer of conical cells. This is especially noticeable in the 

 larva, where the epithelium of the chyle stomach is opaque with 

 granular matter : whilst the gland structure of the proventricu- 

 lus is perfectly transparent. The proventriculus of the larva is 

 larger and more oval than that of the imago. 



The proventriculus may be said to consist of an intussusception of 

 the oesophagus, the muscular coat disappearing in the middle of the 

 organ, and the membranous coat being reflected back upon it and 

 swelling out to make room for the thick glandular lining. It is again 

 constricted at the commencement of the chyle stomach by a muscular 

 sphincter. The tracheal vessels, which ramify in a close net work over 

 the membranous coat, dip down into the interior with the oesophagus ; 

 and then take a radiating direction. It has a larger supply of tracheas 

 than any other part of the alimentary canal. 



The analogy between the functions of the crop proventriculus 

 and chyle stomach, and the crop rennet and paunch of Ruminants 

 is too obvious to need enlarging upon. But the resemblance of 

 the digestive act in the two cases is even closer, for the fly 

 frequently brings its food back into the mouth, probably for the 

 purpose of mingling it with the saliva. I have repeatedly 

 observed flies exude a large drop of fluid from the oral cleft 



