106 DRAGONFLIES [CH. IV 



whose members may be at once known by the fact that the 

 imagines spread their wings flafc when at rest, respiration 

 is carried on by the hinder portion of the intestine, the 

 rectum, which alternately receives and expels water through 

 the anus. This latter is surrounded by five valves, the 

 three larger of which can be brought together or widely 

 separated so as to guard or expose the terminal opening 

 of the rectum. Six longitudinal bands of muscle run in 

 the rectal wall, each carrying several thousand transverse 

 folds which very largely increase the superficial area and 

 also receive fine branches from the tracheal tubes. The 

 intervals between the six bands are occupied by very thin 

 and flexible cuticle. The whole arrangement permits of 

 considerable distension so that a quantity of water can 

 be drawn into the rectum. The oxygen dissolved in the 

 water passes through the walls of the rectum into the 

 tracheal branches and so is distributed over the body. It 

 is by the forcible expulsion of this water of respiration 

 that the larvae can, if they wish, propel themselves forward 

 through the water at a moderate speed. 



The Agrionidce have, in place of the five valves, three 

 thin blade-like lamellae of considerable length ; these, as 

 already mentioned, are employed in swimming, but they 

 also take an important part in respiration. Each lamella 

 possesses a close network of tracheal tubes which doubt- 

 less absorb air that is dissolved in. the water and pass 

 it on to the main tracheal branches, and so supply all the 

 tissues of the body. Some Agrionids, and perhaps all, 

 can also take water into the rectum for respiratory pur- 

 poses. In the later stages of larval life the spiracles are 



