INSECTA 



441 



to the latter without the intervention of the blood. These end tubes, 

 as may be seen in Fig. 309, are of the smallest calibre and their lumen 

 is intracellular. The chitinous lining, which in the main tracheae is 

 strengthened, forming the spiral threads which prevent collapse of 

 the tubes, in the tracheoles is thinned down so much that gaseous 

 diffusion can take place easily between the cell fluid and the lumen 

 of the tube. 



The system is further elaborated to secure regular circulation of 

 air in the main passages. Thus the stigmata are oval slits which can 

 be closed and opened in various ways — usually by valves operated by 



Fig- 309. Tracheal end cell and tracheoles from silk gland of caterpillar, 

 Phalcra bucephala. From Imms, after Holmgren, c. tracheoles ; e. end cell ; 

 t. trachea. 



special muscles. Respiratory movements can easily be observed in 

 such insects as wasps and grasshoppers. They are effected by the 

 alternate contraction of the abdomen in its vertical axis by tergo- 

 sternal muscles and recovery to the original form usually by the 

 elasticity of the abdominal sclerites. Abdominal contraction with 

 open spiracles results in expiration, but if the spiracles are closed the 

 air already in the system will be forced into the finer capillaries where 

 the oxygen pressure is thus increased. 



In some Orthoptera it has been found that certain stigmata are 

 normally inspiratory and others expiratory. Thus, in various grass- 



