THE THORACIC EXO-SKELETON. 183 



pared it with the long wing feathers of the birds of paradise. Its use is 

 unknown, but it is probably a sensory appendage. 



The Paxapteron (Audouin), seen from the exterior of the thorax (Plate 

 VII., 77], is a strong heart-shaped nodule, which lies in the mesopleural syn- 

 desmosis, just below the dorsoplcural suture. It is the head of a powerful 

 lever-like apodeme, which gives insertion to the anterior wing muscles. 



The Post-scutellar Region, or Dolium (Plate VIII., Fig. 5), 

 forms the greater part of the posterior aspect of the thoracic 

 wall. It is bounded above by a syndesmosis, which unites it 

 with the scutellum, and by the posterior alar apophysis, and 

 the insertion of the wing scales ; and below by the upper 

 margins of the posterior spiracles and of the thoraco- 

 abdominal opening and mesophragma. 



The sclerites which form this region are those of the post- 

 scutellum and the tympanic bullae. They are united by 

 symphyses, which project internally as strong ridges, and 

 form a kind of sub-hemispherical cap, the dolium, which is 

 supported by the lateral plates of the metasternum, with 

 which it is firmly connected by strong internal ridges. It is 

 also articulated with the scutellum by the posterior thoracic 

 diarthroses and by syndesmotic integument. 



The Post-scutellum (Plates VII. and VIII., 22) consists of a 

 median shield and two large lateral plates. 



The median shield is a very massive sclerite, sub-quadrate in 

 outline, with a concavo-convex centre, convex on its outer 

 surface. Its superior angles are prolonged to form the con- 

 vex heads of the posterior thoracic diarthroses, and articulate 

 with corresponding concave articular surfaces at the roots of 

 the alar apophyses of the scutellum. Its inferior angles pro- 

 ject backwards and upwards into the abdominal cavity and 

 support the first abdominal ring. 



The upper edge of this plate is united with the scutellum by 

 syndesmosis, and is strengthened by a thick ridge. Its lateral 

 edges articulate with the lateral plates of the post-scutellum. 

 Its inferior edge has a rim on its posterior surface, the post- 

 dorsal arch, and its edge articulates with the mesophragma. 

 The OS cornutum of Jurine appears to be the upper edge of the 

 post-scutellum. 



