THE THORACIC EXO-SKELETON. [85 



Tlie Mesopliragma {j;g) is a partial concavo-convex septum, 

 which projects downwards and forwards into the thorax from 

 the lower edge of the shield of the post-scutellum. It is convex 

 behind, and attached above to the post-scutal shield. Its 

 free inferior edge is deeply notched in the middle line, and 

 separated from the lateral walls of the thorax by a narrow 

 fissure on each side, which transmits several small muscles. 



The mesophragma has a small, almost square, process, 

 directed forwards and outwards near the upper edge of the 

 posterior spiracle ; it arises at the junction of the free and 

 attached margin of the mesophragma. 



Internal Ridges. — The internal surface of the postero-lateral region of 

 the thorax may be readily examined by making a section in the vertical plane, 

 through the junction of the dorsocentral and lateral parts of the dorsum. 

 This region exhibits two sets of ridges, or inflected sutures — one set radiating 

 from the posterior thoracic diarthrosis, and the other from the inferior angles 

 of the post-scutellar shield. 



One ridge is common to the two sets ; it is the suture between the shield 

 and the lateral plate of the post-scutellum. Two other ridges radiate from 

 the posterior thoracic diarthrosis. The upper one is the great alar apophysis, 

 the lower the 'tympanic ridge.' The tympanic ridge crosses the tympanic 

 notch, and forms the straight margin of a transparent semicircular plate — 

 the mirror. The convex margin of the mirror is attached to the thoracic 

 wall, dividing the tympanic bulla into two parts ('j). Its plane is nearly 

 at right angles to the membrana tympani major, from which it is separated 

 by a narrow space above its free margin. It resembles the mirror of the 

 Cicadit. 



The tympanic ridge extends as far forward as the costal edge of the 

 epimeron. 



The ridges from the lower centre are the common one already mentioned, 

 that between the post-scutellum and the tympanic bulla, and the upper 

 margin of the posterior spiracle. The one between the tympanic bulla and 

 the post-scutellum ends in the tympanic ridge, which it joins at a right 

 angle immediately behind the insertion of the great tympanic membrane. 



The Morphology of the Post-scutellar Region. — The morphology 

 of this region has given rise to very great differences of opinion. 

 Brauer first correctly ascribed it to the mesothorax. Ham- 

 mond follows Brauer, but incorrectly includes the metasternum 

 in the mesothorax. 



Adopting Brauer's view, the posterior spiracle lies between 



