336 NATURAL HISTORY OF AQUATIC INSECTS ch. 



that the process of rectal respiration (to be explained 

 later on) is more efficient than tracheal respiration. 

 In other experiments, either the spiracles or the 

 rectum was closed by collodion. 



By such means as these Dewitz ascertained tliat, 

 in young larv'ae of /Eschnidae, the thoracic spiracles 

 are not pervious, though those of adult larvse, almost 

 ready for transformation, are. In boiled water young 



yEschnid larvae always 

 bring the tip of the abdo- 

 men to the surface, older 

 ones either the tip of the 

 abdomen or the thora- 

 cic spiracles. Young 



KiG. 98.-Extremity of abdomen of larva ^Schuid lnVVSt dlC whcU 

 of /Eschna. The plates are separated .1 , • 1 1 -L 



to allow the admission of water: the rCCtUm IS ClOSCd by 



collodion ; older larvae 

 endure the stoppage of either rectum or spiracles, 

 but die if both are stopped. 



In Libellulidae the fore pair of thoracic spiracles 

 are developed and become pervious much earlier than 

 in ^schnidae. In old larvae the spiracles are more 

 important as a means of respiration than the rectum. 



Young larvae of Agrionidae have the thoracic 

 spiracles pervious, but not fully functional. Dewitz 

 thinks that the muscular apparatus for inspiration b}- 

 the spiracles is not yet developed. When placed in 

 boiled water the}' crept up the stick, but never 

 pushed any part of the body out of the water. The 

 surface-layer was apparently sufficiently aerated for 

 their purpose. Sometimes they waved the abdomen 

 to and fro, as if to increase the efficienc}' of their 



