94 MEDICAL ENTOMOLOGY 



appendage upon the head. The muscles of the legs pass from above 

 downwards, those of the first joints arising in the ventral part of the 

 thorax from apodemes or chitinous protuberences, and passing into 

 the tubular space enclosed by the integument of the leg. In the 

 remaining joints the muscles arise above, and are inserted below, the 

 articulation on which they act. 



The muscles of the body wall are arranged primarily in two sets, 

 acting, so far as the shape of the body is concerned, in opposition to 

 one another. One set is more or less circular, and connects the 

 separate plates of the segment in which it lies ; by approximating them 

 it reduces the transverse diameter of the body, with a corresponding 

 increase in the long diameter. The other, which is internal to this, 

 connects the segments with one another, running in an antero-posterior 

 direction, so that its contraction diminishes the length of the body 

 by approximating the segments. This primitive arrangement is altered 

 in a characteristic manner in the three regions of the body. 



In the head the exo-skeleton of all the segments has become welded 

 to form a chitinous box, the walls of which permit of no movement. 

 The muscles of the body wall have, therefore, disappeared altogether. 

 There are a few small bundles in the lateral walls of the neck, which 

 suffice to move the head on the thorax. 



In the thorax the muscles are very greatly enlarged, and modified 

 in their arrangement, in order to provide the motive power for flight, 

 and it is on account of their large mass that the thorax in the Diptera 

 is so conspicuous. They fill up almost the whole of the cavity, the 

 various structures which pass through it being compressed into the 

 ventral angle. (Plate XX, fig. 2.) 



Flight is accomplished by rapidly repeated alterations in the vertical 

 and longitudinal diameters of the thorax, which impart a vibratory move- 

 ment to the wings, and since the wings of the Diptera 

 are mesothoracic outgrowths, it is the muscles of the 

 mesothoracic segment which have been specially developed. It was 

 pointed out in connection with the anatomy of the thorax that this 

 segment has become increased to such a disproportionate extent as to 

 reduce the segments in front and behind to mere rings of chitin, and 

 that it is separated from the metathorax by a diaphragm-like in- 

 growth, the mesophragma. It is in the space anterior to the 

 mesophragma that the main mass of the muscle is contained, the 

 muscles of the other two segments, except those of the legs, being 

 reduced to very thin sheets. The muscles of the mesothorax con- 



