IX 



DRAGON-FLIES 



339 



fleshy folds, corresponding in position to the three 

 valves. Within these is a somewhat capacious 

 chamber (the rectum) whose wall exhibits a compli- 

 cated and interesting structure. Six thick longitu- 

 dinal bands, separated by thin and flexible membranes, 

 seem to be intended to allow great distention without 

 risk of stretching the delicate epithelium or cellular 

 layer. Each longitudinal band bears a double row 

 of transverse folds, which 

 enormously increase the 

 epithelial surface, and at 

 the same time lodge 

 tracheal branches. Oustalet 

 estimates that there are 

 no fewer than 24,000 of 

 these folds. The tracheal 

 branches enter larger and 

 regularly arranged air- 

 tubes, which in turn open 

 into the main trunks run- 

 ning the length of the body. 

 A large volume of water 



can be sucked into the rectum at pleasure, and from 

 this the tracheae draw a fresh supply of oxygen. 

 The vitiated water can either be expelled gently, or, 

 when the larva requires to propel its body forwards, 

 with considerable force. An Agrionid larva, Calop- 

 teryx, has similar rectal gills, besides external 

 tracheal gills. 



Under strictly natural conditions, Dragon-fly larvae 

 may increase the effectiveness of their respiratory 

 organs by means already mentioned in connection 



Z 2 



FIG. 101. A small part of one leaflet, 

 highly magnified, showing many 

 fine tracheal branches. The por- 

 tion shown is marked by a small 

 circle in Fig. 100 (lower left-hand 

 corner). 



