ALIMENTARY TRACT OF TABANUS 103 



the single trunk formed by the fusion of these, a point which is well 



demonstrated in serial sections. After entering the thorax it divides 

 almost at once into two branches, one dorsal to the 



other, the dorsal branch being in the same line with A ' imentar 



. . _,, of Tabanus. Oeso- 



the cervical portion of the oesophagus. The two phagus 



branches are equal in calibre, but differ greatly in 

 length. The dorsal one runs straight through the thorax as the duct 

 of the crop, while the ventral one terminates after a very short course 

 by entering the proventriculus just behind its anterior end. The oeso- 

 phagus, in fact, appears to end undivided in the dorsal surface of the 

 proventriculus, and the duct of the crop to leave it at the same point. 

 The two openings are situated on a slight elevation in the dorsal wall of 

 the proventriculus. 



At the commencement of the oesophagus no cellular lining can be 

 distinguished, and the wall appears to consist only of an unstainable 

 chitinous membrane, but as the tube leaves the brain it comes to have 

 a layer of thin and flattened cells underneath the chitin. As the tube 

 passes through the neck the cells become more numerous and more 

 regularly arranged, until there is a regular layer of squat cubical cells, set 

 on a distinct basement membrane, while the chitinous intima is at the 

 same time reduced in thickness. The basement membrane of the cells, 

 which are, of course, hypodermal in origin, is of considerable thickness. 



The duct of the crop is precisely similar to the thoracic portion of the 

 oesophagus, except that its lumen is a little narrower. 



The crop is a small bi-lobed sac with an extremely thin wall. This is 

 composed of a single layer of very small flattened cells, external to which 



there are many small muscle fibres arranged in an 



Crop 

 irregular network. From what has been said with 



regard to the relations of the two divisions of the oesophagus it will 

 be evident that the crop is in a direct line with the posterior end of 

 the pharynx in the head. 



The function of this sac is a little difficult to determine. It often 

 contains a few small bubbles of gas, and is as frequently empty and 

 collapsed, when it assumes the shape shown in the figure. (Plate XXI, 

 fig. 1.) The present writers, in the course of several hundred dissec- 

 tions, have never found it to contain fresh blood even in flies killed 

 while in the act of feeding, though on rare occasions a little pigment, 

 presumably derived from blood, has been seen in it. In freshly-killed 

 flies, taken in the act of feeding and with the mid-gut filled with blood ; 

 its walls can often be seen to contract in a peristaltic manner, such as 



