36 MEDICAL ENTOMOLOGY 



direction posterior to the prestomum ; it cannot enter from the front 

 without passing through the tubercles at the tip of the epipharynx, 

 for these are considerably distal to the prestomum, and would in any 

 case separate out any coarse particles. Now when there is a negative 

 pressure in the food canal the soft tip of the hypopharynx will be drawn 

 towards the epipharynx, and the size of the opening diminished, while 

 on the other hand it can be increased by the withdrawal of the labrum 

 by means of the muscle at its base. There is thus a balance maintained 

 between the size of the opening on the one hand and the pressure in 

 the canal on the other, and should any large particle impinge on the end 

 of the food channel it can be at once excluded by the valve-like action 

 of the tip of the hypopharynx. 



In the mosquito the mechanism is similar. Here again the tip of the 

 hypopharynx is soft (this is readily seen if fresh preparations are 

 examined under a high power) and falls a little short of the tip of the 

 epipharynx. Both organs are flattened at the tip, so that although the 

 food channel between them is circular, the prestomum is a transverse 

 slit, which can be closed by the pressure of the soft end of the 

 hypopharynx against the more rigid epipharynx. 



The structures in the buccal cavity and in the pharynx which have 

 been described as sieves are of very doubtful function. It is difficult to 

 see how the fly could get rid of a large particle which had once got so 

 far ; the only conceivable method would be a reversed peristalsis, which 

 is most unlikely to occur, 



Some such filtering mechanism as this would seem to be a necessity 

 for the fly, when one considers that the wound is made partly by the 

 rasp-like blades on the maxillae, which tear through the tissues, and are 

 very likely to loosen or dislodge particles of greater diameter than that of 

 the food canal. 



In the Cyclorraphic Diptera the mouth parts are much more differ- 

 entiated from the primitive arthropod type than in the forms so far 

 .considered. The mandibles and first maxillae are 

 absent ' and the whole function of obtaining food is 

 transferred to the second maxillae, which have in con- 

 sequence undergone profound changes. All that remains of the other 

 appendages is a pair of simple palps. In the non-biting flies the inner 

 surfaces of the labella are modified to form a pad by which fluid can 

 be sucked up from moist surfaces, while in the biting forms, for which a 

 cutting apparatus is necessary, a complex arrangement of teeth is deve- 

 loped in the same situation. The external appearance of the proboscis 



