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 that, 

 in young larvae of yEschnidae, the thoracic spiracles 

 are not pervious, though those of adult larvae, almost 

 ready for transformation, are. In boiled water young 



/Eschnid 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 



FIG. 98. Extremity of abdomen of larva ^Eschnid larvae die wheil 

 of ./Eschna. The plates are separated ,1 -111 



to allow the admission of water. the TCCtUm IS Closed 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 ^Eschnidae. 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 by 

 the spiracles is not yet developed. When placed in 

 boiled water they 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 efficiency of their 



