iv] FROM WATER TO AIR 27 



abdomen is relatively shorter and broader. The 

 larval head has, like that of -the imago, short feelers, 

 and the eyes are somewhat large, though far from 

 attaining the size of the great globular eyes of the 

 dragon-fly. But the third pair of jaws, forming the 

 labium, are most remarkably modified into a ' mask/ 

 the distal central portion (mentuin) being hinged to 

 the basal piece (sub-mentum) which is itself jointed 

 below the head. The mentum carries at its extremity 

 a pair of lobes with sharp fangs. Thus the mask can 

 be folded under the head when the larva lurks in 

 its hiding place, or be suddenly darted out so as to 

 secure any unwary small insect that may pass close 

 enough for capture. Dragon-fly larvae walk, and also 

 swim by movements of the abdomen or by expelling 

 a jet of water from the hind-gut. The walls of this 

 terminal region of the intestine have areas lined with 

 delicate cuticle and traversed by numerous air- tubes, 

 so that gaseous exchange can take place between the 

 air in the tubes and that dissolved in the water. 

 The larvae of the larger and heavier dragon-flies 

 (Libellulidae and Aeschnidae) breathe mostly in this 

 way. Those of the slender and delicate 'Demoiselles ' 

 (Agrionidae) are provided with three leaf-like gill- 

 plates at the tail, between whose delicate surfaces 

 numerous air-tubes ramify. These gill-plates are at 

 times used for propulsion. Thus air supply is en- 

 sured during aquatic life. But occasionally, when 



