FEEDING AND BREATHING 



43 



and L. C. Miall (1886). The rhythmic contraction and 

 relaxation of the spiracular muscles and the elasticity of 

 their chitinous attachments cause the spiracular valves to 

 open and close alternately. The contraction of the 

 abdominal muscles, the details of whose arrangement vary 

 in different groups of insects, reduces the capacity of the 

 abdomen so that the organs press on the air-tubes ; when 

 the abdominal muscles are relaxed, the maximum capacity 

 is restored through 

 the elasticity of the 

 body- walls. Thus 

 it comes to pass that 

 with the expansion 

 of the body a small 

 volume of fresh air 

 passes in through 

 the open spiracles 

 to supplement the 

 residual air always 

 present in the 

 tubes. When the 

 spiracular vahv^es 

 close and the body 

 contracts so that 

 pressure is exerted 

 on the tubes, air is 



forced into the tracheoles. The opening of the spiracles 

 while the body is still contracting ensures the expiration 

 of a certain quantity of air, to be followed— when the 

 abdominal muscles relax— by the inspiratory action already 

 explained. The passage in and out of the inspired and 

 expired air-current is accompanied by a twofold diffusion 

 of gaseous constituents, oxygen passing from the fresh air 

 wherein its tension is high to the tracheoles and tissues 

 with their low oxygen tension, while the excess of carbon 

 dioxide diffuses from the tracheoles to pass out of the 

 spiracles with the expiratory air-currents into the atmo- 

 sphere with its low tension of carbon dioxide. 



Fig. 15. — Abdominal spiracle and its connec- 

 tion with the tracheal system in a Louse 

 {Haematopinus) . s, spiracle ; v, vestibule ; 

 /, lever ; cm, closing muscle ; om, opening 

 muscle ; st, spiracular trachea ; t, part of 

 longitudinal tracheal trunk. X 200. 



