THE THORACIC EXO-SKELETON. 173 



spiracle and the paratreme ; below and in front it is continuous 

 with the plastron, for a short distance ; its other relations are 

 sufficiently indicated in the figures. Near the anterior extreinitj' 

 of its superior border there is a small tubercle, which articulates 

 with a corresponding depression in the paratreme ; this articu- 

 lation is the anterior dorso-pleural diarthrosis, it forms the 

 anterior extremity of the dorso-pleural syndesmosis. 



Many different views have been held as to the homology of 

 the lateral plate — several of these have been already alluded to. 

 Hammond attempts to identify it with the parapteron of 

 Audouin, but a more careful study of Audouin's writings would 

 have convinced him of his error. 



In many insects the corresponding plate is, apparently at 

 least, quite removed from the plastron of the mesosternum by 

 the extension of the pleuron forwards ; I believe, however, a 

 narrow neck will be found on careful examination uniting this 

 plate with the plastron. 



The Manubrium (I'late VIII., Figs, i and 2, ,>-) probably represents 

 the body of the prosternum. It is a narrow quadrilateral plate, somewhat 

 wider in front than behind ; it articulates with the plastron by the manu- 

 brial suture. Its inner surface exhibits three sub-parallel ridges — one in the 

 median line, and one on each of its edges ; these terminate behind in the 

 manubrial suture. The lateral and anterior edges of the manubrium are 

 continuous with the fle.vible syndesmotic integument of the prosternal area. 



There is a small pouch-like projection on the external surface of the 

 manubrium, close to the manubrial suture, and a corresponding hollow on its 

 internal surface. 



The manubrial suture forms an inflection on the inner surface of the 

 ventral thoracic wall, from the extremities of which a pair of strong fusiform 

 processes extend to the lower borders of the anterior spiracles. These pro- 

 cesses may be termed the 'hypotremata.' 



The Hypotreme (Plates VII., Fig. 2, and VIII. Figs. 2 and 4, j/) extends 

 obliquely across the anterior coxo-sternal foramen, and terminates in three 

 ridges— two ascend and surround the spiracle, and the third curves down- 

 wards and forms the external edge of the epitrochlea. 



Tiie hypotremata give great strength to the anterior part of the thoracic 

 wall, acting as a kind of stretcher, keeping the external wall of the thorax 

 from being drawn towards the middle line by the action of the muscles which 

 move the head and the anterior legs. As they extend from the suture between 

 the pro- and mesosternum, they may be regarded as the internal continuation 

 of this suture. The spiracle is situated between the pro- and mesothorax, 

 and the suture which forms a ridge in front and behind it is also continuous 

 with the hypotreme. 



