38 MEDICAL ENTOMOLOGY 



perhaps powerful enough to scrape off the dried clot from a raw surface, 

 to enable the fly to get at the serum and blood underneath. In those 

 flies which are now confirmed blood-feeders the teeth have become 

 powerful cutting weapons, and there is a specialization of the joints 

 and muscles of the proboscis in conformity with their increased size. 

 The whole proboscis, in fact, becomes altered in the direction of the pro- 

 duction of a piercing stylet. The course of the development of this 

 form of cutting apparatus can be traced with remarkable clearness in 

 the present day forms, as will appear from the subsequent descriptions. 



In view of these relations, the structure of the proboscis of Musca 

 must be understood before passing on to the blood-sucking Muscids. 

 The domestic species of Musca are moreover of sufficient im- 

 portance to the sanitarian as the possible transmitters of disease to 

 warrant a detailed description of their mouth parts, especially since 

 there is reason to believe that it is mainly through the proboscis, 

 which may be transferred from infected material to the food of 

 man very frequently, that infection is conveyed. At the same time it 

 should be noted that there are many species of Musca, which, although 

 they have no biting mouth parts, are confirmed blood-feeders, and are 

 theoretically as capable of transmitting pathogenic parasites as are true 

 biting flies. These will be dealt with in Chapter III. The follow- 

 ing description refers specially to Musca nebttlo, the common bazaar fly 

 of Madras, but the differences between species of like habit are so small 

 that it will apply to most of them. 



The proboscis of Musca, in the position of extension, hangs directly 

 downwards from the lower surface of the head, and is roughly cylin- 

 drical and blunt pointed. It consists of three parts, 

 Musca (Plate VIII , , , , . 7,77 



j.g ^ named from above downwards the rostrum, haustellum, 



and the labella. Of these the two latter correspond to 

 the proboscis and labella of the Orthorraphic flies. The rostrum, on the 

 other hand, is a part of the head cavity, which can be protruded at 

 will ; it contains the pharynx and the buccal cavity. 



The proboscis is completely retractile, and when not in use is with- 

 drawn ; the rostrum, reduced in size by the collapse of the air vessels 

 which it contains, passes back into the head cavity, and the haustellum 

 with its labella is concealed on the inferior surface. 



The rostrum is shaped like a truncated pyramid, with the narrow end 

 directed downwards. Its wall consists of a tough but flexible membrane, 



which is attached above to the margins of the epistomal 

 Rostrum . . . , 



orifice, and is continuous below, with the walls 01 the 



