100 MEDICAL ENTOMOLOGY 



it is probable that the total capacity of the body cavity is diminished 

 by the contractions of these muscles, with the result that the air is 

 expelled ; inspiration is accomplished by the elastic recoil of the spiral 

 thread, which restores the trachea to its original form. The air sacs 

 would appear to play an important part in this mechanism. 



The chitinous nature of the wall of the tracheae and air sacs is of 

 the greatest importance from the point of view of technique. Ordinary 

 fixatives will not penetrate through chitin,.and, even if the fluid were to 

 find its way through the spiracles, it is still shut off from the soft tissues. 



THE ALIMENTARY CANAL 



The alimentary canal in its simplest form is a straight tube com- 

 mencing at the mouth and running backwards through the body to 

 terminate at the anus, which is situated at the extreme posterior end 

 of the abdomen. Some parts of this are specialized for one function 

 and some for another, and in most Diptera the tube has outgrown 

 the body cavity in which it lies, and has become twisted and coiled 

 upon itself. It is held in position by the tracheae which supply it 

 with air, and lies in the median position, that is, below the dorsal 

 vessel and above the ventral nerve chain. 



A proper conception of the morphology of the alimentary tract 

 can only be obtained by reference to its mode of origin. In the 



developing embryo insect the tract consists of three 

 Development ,.,..,. 



parts, which arise in different ways. At each end 



of the embryo there is an invagination of the cuticle, which proceeds 

 until two deep pits are formed ; that at the anterior end is termed 

 the stomodaeum, and that at the posterior end the proctodaeum (Plate 

 I, fig. 3). Between them a cleavage occurs in the mesoblast, and this 

 develops into a cavity, which in time becomes continuous with the 

 stomodaeum and the proctodaeum, so that a complete canal is formed 

 opening at the anterior and posterior ends of the body. The distinction 

 between these three parts influences both their structure and their 

 function, and should always be borne in mind. The invaginations 

 of the cuticle are lined with a chitinous layer which is continuous with 

 the integument, and can take no part in the actual processes of digestion, 

 whereas the middle 'portion, or mesenteron, has no such layer, and is 

 the true stomach or digestive chamber. It is lined with a specialized 

 epithelium, and may be otherwise modified in connection with the 

 digestive process. 



