72 MEDICAL ENTOMOLOGY 



the lateral walls. The membraneous area at the junction of the tho- 

 rax with the neck has already been noted. Posteriorly there is a more 

 restricted area between the meso-epimeron and the meta-pleural plate, 

 and another narrow strip running vertically down the wall, between 

 the posterior margin of the thorax and the abdomen. 



On the internal aspect of the thoracic wall there are certain chitinous 



structures which are termed collectively the endo-skeleton ; they are, of 



course, mere ingrowths from the exo-skeleton, and are 



Endo-skeleton of designed to afford attachment for the muscles and to 

 Thorax 



give additional stability to the wall. In the prothorax 

 the existence of such a structure has already been mentioned (Plate XV, 

 fig. 6). It consists of two strong fibrous bands which run outwards 

 and forwards from the sternite to the pleural plates, binding the 

 whole together and serving to compensate for the looseness of the wall in 

 the region of the attachment of the neck. The anterior of these is 

 a broad and thick transverse band which arises from the whole length of 

 the line between the praesterna and the sternum, and forms a wide 

 arch parallel to and behind the borders of the latter. Laterally it 

 terminates in a stout rounded cord. The posterior part arises from 

 the lateral borders of the sternellum, and contracts to a similar cord, 

 the two uniting above the articulation of the fore leg. The fibrous 

 band so formed spreads out in a fan-shaped manner, and becomes 

 continuous with the anterior and internal edges of the episternum of 

 the prothorax, which are turned inwards and backwards to meet 

 it. 



In the mesothorax there is a structure of a different kind, designed 

 solely for the attachment of muscles. It consists (Plate XV, fig. 4) 

 of a pair of wing-like expansions set on a Y-shaped stalk, which lies 

 in the groove between the two lateral divisions of the mesosternum. 

 Each expansion consists of two thin plates of chitin, separated from one 

 another behind by a smaller plate perpendicular to them, but converging 

 towards one another anteriorly, so that they form the boundaries of a 

 boat shaped cavity. Behind the large anterior cavity there is a smaller 

 one, the anterior boundary of which is formed by the same transverse 

 plate which forms the posterior limit of the anterior cavity. Important 

 muscles arise from the inner surfaces of these plates, and the structures 

 which pass through the thorax rest between the arms of the Y-shaped 

 strand of chitin to which they are attached. 



In the mesothorax there is a much more important structure, the 

 mesophragma (Plate XV, fig. 1), which forms a most formidable obstacle 



