152 



THE COCKROACH : 



The second thoracic spiracle (fig. 8G) lies in front of the meta- 

 thorax, between the bases of the second and third legs. It is 

 much smaller and simpler than the first. Its valve is nearly 

 semi- circular, and the free border is strengthened on its deep 

 surface by a chitinous rim, which terminates beyond the end of 

 the hinge of the valve in a process which gives insertion to the 

 occlusor muscle. 



Fig. 85. First Thoracic Spiracle (left side), seen from the outside. X 70. V, valve ; 

 /, setose lining of valve (mouth of tracheal tube) X 230. The occlusor muscle is 

 shown. The arrow indicates the direction of air entering the spiracle. In the 

 natural position this spiracle is set obliquely, the slit being inclined downwards 

 and backwards. (P. americana.} 



The abdominal spiracles present quite a different plan of 

 structure. The external orifice is permanently open, owing to 

 the absence of valves, but communication with the tracheal trunk 

 may be cut off at pleasure by an internal occluding apparatus. 

 The external orifice leads into a shallow oval cup, which commu- 

 nicates with the tracheal trunk by a narrow slit, or internal 

 aperture of the spiracle. The chitinous cuticle, surrounding 

 this internal aperture, is richly provided with setae, which are 

 turned towards the opening.* Fig. 87 C represents a spiracle 



* In the first abdominal spiracle the sette are developed only on that lip which 

 carries the bow. 



