268 ANIMAL BIOLOGY. [Part II. 



The Traclieal System. Aerial respiration is, in the cock- 

 roach, on quite a different plan from that which is found in the 

 air-breathing vertebrate. There the blood is carried to a special 

 sac, the lung, in which it is aerated ; and by the blood the 

 oxygen thus absorbed is distributed through the system. In 

 the insect a number of minute tubes (trachece) ramify throughout 

 the body and carry air directly to the various organs. 



From the spiracles there pass inwards tracheal trunks. Those 

 in the abdominal region give off, (1) anterior and posterior 

 branches, which unite together to form longitudinal lateral 

 vessels ; (2) dorsal branches, which pass up into the pericardial 

 cavity and branch right and left, thus giving rise, by union of 

 the branches, to dorso-lateral arches ; (3) ventral vessels running 

 transversely, but coming into relation with ventro-lateral vessels, 

 one on each side of the nerve-chain ; (4) median branches which 

 ramify upon the alimentary and reproductive viscera. From 

 the thoracic stigmata several ventral and dorsal transverse 

 trunks take their origin, which are joined to each other by 

 longitudinal connecting vessels, there being also a well-developed 

 connection between these thoracic tracheae and the anterior 

 ventral transverse vessel in the abdominal region. From the 

 anterior thoracic stigmata four large vessels, two dorso-lateral 

 and two ventro-lateral, pass to the head. 



Examined under the microscope the tracheae are seen to 

 be transversely striated, and in some cases, where a tube has 

 been torn in mounting, a spiral thread of chitinous material 

 is seen to be drawn out from the trachea. This chitinous 

 lining strengthens the tubes, keeps them open, and gives them 

 elasticity. 



The thoracic stigmata are provided with external valves. 

 Those in the abdominal region have internal valvular slits, which 

 can be closed by the action of occlusor muscles on a chitinous 

 bow developed along its anterior edge. Thus the tracheal 

 system can be shut off from communication with the exterior, 

 enabling the insect to live for some time in a noxious atmo- 

 sphere. 



Inspiration and expiration are effected by movements of 



