ALIMENTARY TRACT OF PHILAEMATOMYIA 113 



obviously be an advantageous one, for it exposes a larger surface of 

 digestive cells, and at the same time reduces the distance by which 

 the central particles of food are separated from the gut wall. The 

 increase in length, which is confined to the mid-gut, has necessarily 

 resulted in throwing the tube into many convolutions. The proven - 

 triculus has assumed a definite shape common to all, and forms a 

 valve separating the fore-gut from the mid-gut. 



The most striking development is the formation of a second tube 

 internal to the epithelium, by which the food contained in the gut 

 is separated from the digestive cells. This structure is known as the 

 peritrophic membrane. It is found in many other insects, and also in 

 their larvae, and indeed it is somewhat remarkable that it should be 

 absent in the Orthorraphic flies. 



The form of the alimentary canal in the Cyclorraphic Diptera 

 is remarkably constant in the different genera, such differences as there 

 are being mainly in the length of the tube and the complexity of the 

 coils. That of Philaematomyia will serve as a type ; the parts in 

 some other members of the group will be referred to subsequently. 



The oesophagus is a short tube which passes through the neck from the 

 posterior end of the pharynx to the anterior part of the thorax. At its 



commencement it is of considerable width, but it very 



, L , , . , Philaematomyia 



soon contracts to form a narrow tube which passes oesophagus 



through the brain and neck. The structure of the 



wall is the same as that of Tabanus, except that there is rather more 



muscular tissue. 



As it passes into the thorax the oesophagus lies ventral to the pro- 

 ventriculus, and comes into close contact with it. The two communicate 

 by an opening which is directed upwards and slightly backwards ; 

 immediately posterior to this point the duct of the crop commences, and 

 runs backwards into the abdomen ventral to the gut. The oesophagus 

 and the duct of the crop appear in fact to form a continuous tube, 

 closely pressed against the proventriculus ; the communication between 

 it and the proventriculus is only revealed by sections. 



The crop is a large and thin- walled sac like that of Tabanus, 

 situated in the anterior portion of the abdomen. When empty 

 it is contracted, but when the fly has just finished 

 feeding it is distended with blood, and may occupy 

 a considerable proportion of the abdominal cavity. Its wall is thicker 

 than that of the corresponding organ in the Orthorraphic flies, both 

 the muscular and cellular portions being more conspicuous. 

 15 



