Respiration and Metabolism 



265 



which in water inspires through the last pair of spiracles and expires through 

 all the lateral apertures, but in air, inspires and expires through the lateral 

 spiracles. 



Ingenious experiments have been devised to show the amount ot air which 

 can pass through the tracheal system. The locustid, Chortophaga viridifasciata, 

 circulates on the average 0.222 cc./gm./min. at 28° C.-"'" The vital capacity 

 of insects is known to be generally high, 60 cu. mm./gm. animal weight in 

 Dytisctis.-'^*' In exceptional experimental conditions the air flow may be 

 reversed, passing in through the posterior spiracles and leaving through the 

 anterior ones.-"'" 



Coordination of the insect respiratory movements lies in segmental nerve 

 centers, responsible for the ventilation pattern in each segment. They in turn 

 receive impulses from central coordinating thoracic centers, which are usually 

 more sensitive to respiratory stimulation from carbon dioxide excess and oxygen 

 lack. Rhythmic potentials corresponding to the breathing cycle have been 

 recorded in ganglia isolated from the water beetle and cockroach.'- -^^ In the 

 dragonfly, Aeschna, only the secondary (thoracic) centers are very easily 

 stimulated by low oxygen and high carbon dioxide tension, but in the stick 

 insect, Dixippiis, both types of centers respond to these stimulants.''-^''' It is well 

 known that isolated abdominal segments and decapitated insects may display 

 respiratory movements for considerable time. There is general agreement that 

 both carbon dioxide increase and oxygen want stimulate ventilation move- 

 ments in insects (Table 47). McGovran--"'*' has increased air flow through 



TABLE 47. RESPIRATORY RESPONSE TO O. WANT AND CO. INCREASE 



+ =positive stimulation. — =::no effect. ( ) ^questionable effect. 



