206 



HARDWICKE'S SCIENCE-GOSSIP. 



Externally, the chitinogenous cells rest upon a 

 delicate basement-membrane. 



Where a number of branches are given off together 

 the tracheal tube may be dilated. Fine branches, 

 such as accompany nerves, are often sinuous. In 

 the very finest branches the tube loses its thread, the 

 chitinogenous cells become irregular, and the intima 



border, which protects the opening. The air-tube 

 divides into two primary trunks immediately within 

 the orifice. The second thoracic spiracle is simpler 

 and smaller ; its valve is nearly semicircular, and the 

 orifice, when slightly open, has the figure of a horse- 

 shoe. The abdominal spiracles are obliquely trun- 

 cated papilla; without valves and directed back- 



Fig. 122. — Tracheal System of Cockroach. Side view of head seen from 

 without, introducing the chief branches of the left half. X 15. 



Fig. 124. — Second Thoracic Spiracle of Cockroach (left side). X about 35." 



is lost in the nucleated protoplasmic mass which 

 replaces the regular epithelium of the wider tubes.* 



The spiracles of the cockroach offer much intricate 

 detail, and it is far from easy to master their structure 

 and mode of action. The first thoracic spiracle is 

 the largest in the body. It is closed externally by a 

 large, slightly two-lobcd valve, attached by its lower 



* It has been supposed that these irregular cells of the 

 tracheal endings p.ass into those of the fat-body, but the latter 

 can always be distinguished by their larger and more spherical 

 nuclei. 



wards ; their openings are vertical, and oval or 

 elliptical. 



We have already pointed out that the wall of the 

 air-tube for a .short distance from the spiracular 

 orifice has a tesselated instead of a spiral marking. 

 The tesselated border is most regular in the case of 

 the second thoracic spiracle. The chitinous cuticle 

 within the opening is crowded with fine setiv, while 

 the setae of the external integument arc often 

 arranged so as to form a fringe on one side of the 

 aperture. 



