THB NEBVOUS SYSTEM. 73 



by a small muscle, which arises from the margin of the 

 spiracle below, and is inserted into a pair of thin plates, close 

 to the hinge of the peritreme. The structure of the posterior 

 thoracic spiracles differs only in detail from that of the anterior. 



The posterior thoracic spiracle may be said to be triangular, with 

 rounded angles ; its superior border is longest and straight, the other sides 

 are slightly curved, the posterior edge is shortest. It is surrounded 

 internally by a deep ridge, derived from the posterior margin of the 

 meta-sternum, which is especially marked at the posterior inferior angle 

 of the spiracle. 



The pseudo- valves, or peritreme, consist of a pair of valve-like plates, a 

 small posterior, and a large anterior valve. These are attached beneath to 

 the meta-sternum, which forms the inferior boundaries of the spiracle ; 

 the upper margin of which is formed of the lateral plate of the meta- 

 thorax, and this is so deeply notched behind, that the halter arises 

 quite close to the posterior superior angle of the spiracle. The pseudo- 

 valves are strengthened by arborescent nervures similar to those of the 

 peritreme of the anterior spiracle. 



The valve or epitreme arises from the posterior inferior border of the 

 spiracle : its hinge resembles that of the epitreme of the anterior spiracle, 

 except that the plates into which the muscle is inserted are beneath 

 the hinge. Its action has not been observed during life, owing to its 

 depth from the surface being greater than that of the epitreme in the 

 anterior spiracle. It undoubtedly entirely closes the opening, but it is 

 uncertain whether its opening and closing is synchronous with that of 

 the anterior epitreme. 



The main trachea of the thorax extends on each side from the anterior 

 to the posterior thoracic spiracle, lying close to the thoracic viscera. It sends 

 a large branch which curves inwards and forwards into the head, and another 

 smaller branch into the abdomen. This latter swells out on each side into 

 the great abdominal pulmonary sac. More properly, perhaps, the anterior 

 and posterior portions should be called continuations of the lateral tracheae 

 and the portions uniting it with the spiracles should be looked upon aa 

 branches. A large trunk from the posterior thoracic spiracle follows the 

 wall of the thorax upwards and inwards ; it sends a large branch forwards 

 in the mesial line between the muscles and the back. This gradually be- 

 comes narrower and supplies the dorsal muscles, and sends a prolongation 

 backwards which fills the whole cavity of the post-scutellum. (Plate I. & 

 Plate V. fig. 6). 



A number of small sacculi, connected with each other in a chain, extend 



