102 MEDICAL ENTOMOLOGY 



The oesophagus is a simple tube, which connects the pharynx in the 

 head with the proventriculus in the thorax. Immediately after it leaves 

 the pharynx it passes backwards through the brain, and after reaching 

 the thorax it divides into two branches of about equal size ; one of these 

 passes into the proventriculus, while the other is continued onwards 

 through the thorax as the duct of the crop or food reservoir. The latter 

 is a thin-walled sac, capable of considerable distension, situated in the 

 abdomen. The proventriculus, when present, lies either in the posterior 

 end of the thorax or at the anterior end of the abdomen, and often has 

 a considerable amount of muscle in its wall. It is the homologue of the 

 gizzard of other insects, but as the Diptera have no solid food to grind 

 it has no internal teeth, and appears in many forms to function as a 

 valve. The mid-gut is the main digestive part of the alimentary tract, 

 and is lined by a columnar secreting epithelium. It is capable of great 

 distension in some forms. The separation between the mid-gut and the 

 hind-gut, in the less specialized forms at least, is marked by the inser- 

 tion of the Malpighian tubes into the wall of the canal. Posterior to the 

 mid-gut the canal may be coiled and twisted to a considerable extent, 

 owing to the disproportion in length between it and the cavity in which 

 it lies. The upper part is generally the narrowest, and may for the sake 

 of convenience be termed the ileum, the lower and wider portion being 

 known as the colon, though it is important to note that these are mere 

 terms of convenience, having no definite morphological significance, and 

 cannot be applied in all cases. In the Muscid flies other terms are in 

 common use to distinguish the different regions. The last portion of 

 the canal is dilated to form a pear-shaped rectum, in which there are 

 certain curious trumpet-shaped bodies called the rectal glands or rectal 

 papillae. 



There is a good deal of variation in the shape and structure of 

 the alimentary canal in the two main divisions of the Diptera, and it 

 will be convenient to treat them, to a certain extent, separately. The 

 modifications found in the higher forms are due mainly to the increased 

 length of the canal, and to its more uniform calibre. 



Tabanus will again serve as an example, as it is fairly easy to obtain 

 and has a simple alimentary tract (Plate XXI). 



The oesophagus commences at the posterior end of the pharynx, 

 and at once narrows down to a thin tube. It first passes straight 

 backward through the middle of the brain, emerging at its lower border. 

 It then passes through the neck, lying at first between the two main 

 nerve trunks which pass from the brain to the thorax, and later upon 



