THE THORACIC EXO-SKELETOA. 171 



forms a convex fold, which supports the abdomen ; below the 

 shield of the post-scutellum a large mesophragma projects 

 downwards and forwards into the thoracic cavity, reducing the 

 thoraco-abdominal opening to a crescentic slit. The edges 

 of the mesophragma are connected with the anterior edge of 

 the metapleura, so that it separates the capacious mesothorax 

 from the small metathorax, the cavity of which is thus 

 rendered continuous with that of the abdomen. 



The halteres are seen between the upper extremities of the 

 metapleura and the lateral plates of the post-scutellum. Below 

 the halter and in front of it, the great posterior spiracle is very 

 conspicuous, and a convex plate is seen between the upper 

 margin of the spiracle and the lateral plate of the post-scutellum ; 

 this is the posterior part of the tympanic bulla. 



The scutellum articulates with the upper edge of the shield 

 of the post-scutellum by syndesmosis, and the superior angles 

 of the post-scutellar shield form strong articulations with 

 cavities on each side of the scutellum : this is the posterior 

 thoracic diarthrosis (Mi hi). 



The lateral parts of the metasternum and a portion of the 

 mesosternum are also seen in the posterior view of the thorax, 

 and the great wing scales project above the tympanic bullae and 

 the lateral plates of the post-scutellum. 



c. The Sclerites of the Thoracic Skeleton. 



The Mesosternum consists of the mesoplastron, the lateral 

 plates, and the mesothoracic entothorax. 



The Plastron is formed of two lateral halves, united by a strong 

 inflected suture the entothorax, which appears as a mere 

 line externally. Each lateral half of the plastron is sub- 

 quadrate and articulates, in front in the middle line, with 

 a quadrilateral plate situated between the anterior coxae, which 

 I term the manubriurn, from which it is separated by the 

 manubrial suture. 



On each side of the manubrium the plastron is bounded, in 

 front, by the anterior coxae and the epitrochlear sclerites, 

 laterally by the lateral plates and the sterno-pleural sutures, 



