98 MEDICAL ENTOMOLOGY 



spiracle gives rise to a short trunk, which divides almost at once into 

 a number of branches, some of which break up for the supply of the 

 adjacent viscera and the body wall, while others pass backwards and 

 forwards to anastomose with the tracheae from the adjacent spiracles. 

 One branch passes straight across the body immediately below the dorsal 

 plates, there anastomosing with its fellow of the opposite side to form a 

 loop below the heart. 



In the great majority of the air tubes, at least in the Orthorrapha, the 

 diameter is uniform, and is reduced in a simple manner at each bifurcation, 



but in certain parts there are dilatations, in which, as 



Air Sacs . .. . . 



will be seen presently, the specialized structure ot 



the wall of the tube is not found. In Tabanus there are two of these air 

 sacs in the anterior end of the abdomen, which, on account of their 

 white colour and large size are very conspicuous when the abdomen is 

 opened without damaging them. They always contain air, but are not 

 distended to such a degree as to make their walls taut. The tracheae of 

 the head have always a large number of air sacs on them, especially in 

 the region of the brain, and these have an important function in relation 

 to the mechanism of feeding, as already described. In the Cyclorraphic 

 flies (Plate XX, fig. 6) almost all the main branches of the thoracic 

 tracheae are dilated to form sausage-shaped sacs, from which small 

 tracheae are given off, these breaking up at once into a large number of 

 branches for the supply of the adjacent tissues. One of the branches 

 from the anterior spiracle on each side passes into the abdomen, and 

 there dilates into a large sac, which occupies a considerable proportion 

 of the abdominal space. (Plate XX, fig. 6.) 



The structure of the tracheae and air sacs will be best understood by a 

 reference to their mode of origin. They represent invaginations of the 



ectoderm, and as such are lined, from the spiracle to 

 Structure of ..... . . ... . . , . 



Tracheae divisions, with chitin which is continuous 



with that of the exo-skeleton. Their outer surfaces, 

 that is, those which are in contact with the body contents, are lined 

 by a layer of cells which is continuous with the hypodermis or chiti- 

 nogenous layer of the exo-skeleton. The internal lamina consists 

 of a very thin but uninterrupted layer of chitin, but in all except the 

 most minute tracheae this is modified in a remarkable manner to form 

 the well-known spiral thread. (Plate XX, fig. 4.) When examined under 

 a high magnification, and preferably after some preliminary maceration 

 in potash, the wall of the tube is seen to be traversed by a large number 

 of extremely thin annular threads, which pass four or more times round 



