CH. IV] DRAGONFLTES 105 



first maxillae, by which it is speedily cut in pieces. The 

 action of the " mask " may be well imitated with the 

 human arm : keeping the upper arm against the side, 

 place the palm of the hand against the shoulder, the 

 whole arm is now in the position of the mask at rest ; the 

 upper arm represents the proximal part, the elbow the 

 hinge, the fore-arm the distal part : the forceps may be 

 represented by separating the third and fourth digits; now 

 extend the arm straight above the head, bring third and 

 fourth digits in contact, and return the arm to its first 

 position. 



Some of the most interesting studies in natural history 

 are to be found in the means adopted by aquatic insects 

 for securing the oxygen necessary for respiration. Tracheal 

 tubes are essentially air-breathing contrivances; their 

 lateral apertures, the spiracles, constitute efficient and 

 convenient points for entry and exit of air in terrestrial 

 insects, but not so in those which have betaken them- 

 selves beneath the surface of the water. Hence in these 

 latter we find many beautiful modifications and ingenious 

 devices for maintaining respiration. 



The larvae of all Dragonflies have two pairs of spiracles 

 upon the thorax ; in some (Libelhdidce) the pair between 

 the first and second segments of the thorax can be seen on 

 the dorsal surface with the unaided eye. It is however 

 not until the later phases of larval life that these openings 

 become functional; indeed in the earlier stages they are 

 not really open but air is obtained by other means. In the 

 larvae of the two families, the Libellulidce and ^Eschnidce, 

 which include our larger and strong-flying species, and 



