FLAT-WINGED GROUP 



3133 



aquatic larvae by the possession of a peculiar structure fixed under the head, 

 known as the mask. In their mode of respiration dragon fly larvae are also peculiar, 

 some being provided with external tracheal gills in the form of three leaflets 

 placed near the tail end which serve to assist in locomotion, while others 

 breathe by means of gills of an exceptional character. The latter are situated in 

 the hinder part of the intestine and consist of six longitudinal bands in its walls, 

 crossed by several transverse folds, supplied with numerous fine branches from the 

 tracheal trunks. Water can be sucked in at the opening, guarded by five valves at 

 the hind end of the body, and when it becomes vitiated can be squirted out again 

 either gently or with considerable force. When it is suddenly and violently 

 expelled, it serves to propel the insect forward at a rapid rate. The larvae live 

 about ten or twelve months, during which time they undergo several molts; rudi- 

 ments of wings appearing some time before the final transformation. When this is 



LIFE HISTORY OF DRAGON FLIES. 



i. Larval skin of a dragon fly ; 2. Larva with its mask exserted ; 3. Libellula depressa ; 4. Advanced larva 

 of a libellula ; 5. The same about to undergo its final transformation. 



about to take place, the larva leaves the water by climbing the stem of a plant, or 

 to some other dry spot. As the time approaches, its eyes, which were before dull 

 and opaque, become bright and transparent. Its skin dries up, and soon begins to 

 crack along the middle of the thorax; the thorax appears through the cleft, and 

 swelling up causes it to extend; the head is next disengaged, and the legs are then 

 drawn out of their sheath. The insect now throws its head farther and farther 

 back, and by this means gradually frees the hinder part of its body, with the 

 exception of the last few segments which still remain inclosed in the larval skin. 

 After a while, it suddenly bends its body forward, grasps the sides of the 

 sheath with its legs, and, doubling up its abdomen, finally extricates the rest 

 of its body. 



Dragon flies are divided into three families, of which the first two have more in 

 common with one another than with the third. The Libellulidce are distinguished 

 by their comparatively stout bodies; by the size of their eyes, which cover almost 



