392 



THE INVERTEBRATA 



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 

 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 



a.s. 



Fig. 293. Tracheal system of the locust, Dissosteira Carolina. Modified 

 from Vinal. A, Side view. B, Dorsal view, the lower half to show air sacs, 

 the upper half to show tracheal supply to the alimentary canal, a.c. alimen- 

 tary canal; a.s. air sacs; l.t. longitudinal trunk; sp. spiracles. 



the abdominal sclerites. Abdominal contraction with spiracles open 

 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- 

 hoppers (Fig. 293), the first four pairs are open at inspiration and 

 closed in the expiratory phase, while the last six pairs are open in the 

 expiratory phase and closed at inspiration. It follows that an air 



