i84 THE INTEGUMENTAL SKELETON OF THE EM AGO. 



The lateral plate of the post-scutellum (Plate VIIL, Fig. 5, /^) 

 is sub-triangular, with the apex of the triangle in relation with 

 the inferior angle of the median shield of the post-scutellum, 

 to which its posterior edge is firmly united ; its anterior edge 

 articulates by symphysis with the tympanic bulla ; and its 

 superior edge, united with the posterior alar apophysis of the 

 scutellum, forms the inferior margin of the supra-tympanic 

 fissure. This edge is much thickened by a strong internal 

 inflection, which projects into the thoracic cavity ; it is con- 

 tinued forwards and downwards into the costal margin of 

 the epimeron (Plate VII., Fig. 2, 22). 



The posterior thoracic diarthrosis is greatly strengthened by 

 the inflected superior and posterior edges of this plate. 



The Tympanic Bulla {n), seen from the side, appears sub- 

 hemispherical, but from behind sub-triangular (Plate VIIL, 

 Fig. 5, tp). It is the segment of a cone with a convex base. 

 The apex of this cone is in relation with the insertion of the 

 halter. Its upper edge forms a symphysis with the post- 

 scutellum, and its lower edge the upper margin of the posterior 

 spiracle. The lower edge also articulates with the two pro- 

 cesses of the metasternum, which form the anterior and 

 posterior margins of the spiracle. 



The convex base of the tympanic bulla is deeply notched 

 above and in front, so that a semicircular opening is left 

 between it and a rod of chitin, which extends from the meso- 

 pleuron to the upper margin of the tympanic bulla. This is 

 a portion of a strong ridge, the tympanic ridge, between the 

 posterior thoracic diarthrosis and the costal margin of the 

 epimeron (Plate VII., Fig. 2, 21). 



The notch, or foramen, in the bulla is closed by a tense mem- 

 branous integument {ni), the membrana tympani major (Mihi), 

 which looks upwards and forwards beneath the great wing scale. 



The tympanic bulla, seen from above and in front, resembles 

 a kettledrum, the membrana tympani major being the drum 

 skin. The lowest point of the posterior wing root rests upon 

 the drum membrane. The inner edge of the membrane is 

 attached to the alar apophysis of the scutellum. 



