DRAGON-FLIES 173 



of the "demoiselles," the commonest dragon-flies of fenny 

 lands, it is long and slender. 



Larvae which are completely submerged, like those of dragon- 

 flies, cannot breithe like most other insects by filling and 

 emptying their air-tubes through spiracles. Some dragon-fly 

 larvae suck up water into the intestine, extract from it part of 

 the dissolved air, and eject it again. This peculiar means of 

 respiration may be supplemented in the demoiselle-larvse 

 (Agrionidas) by three leaflets at the tail-end, which act as gills. 

 The sudden expulsion of water from the intestine is sometimes 

 used as a means of propelling the body. In older larvae the 

 air-tubes become distended with air, and spiracles open on the 

 thorax, replacing all other means of respiration. Young larvae, 

 when placed in boiled water, which contains little or no 

 dissolved air, rise to 

 the surface, and try 

 to take in a supply of 

 aerated water by the 

 intestine. Older larvae 

 under the same cir- 

 cumstances generally 



try to thrust the thorax ^'S- Q'-— Head of dragon-fly larva, with labium 



■', ^ ,. . extended. 



out of the water. 



There is no pupal or resting stage in the life-history of 

 a dragon-fly. The wings and other peculiar organs of the 

 imago develop gradually, becoming more evident at each of the 

 later moults ; at length the larva creeps out of the water, casts 

 the larval skin, and in an hour or so is transformed into a swift, 

 aerial creature, innocent and gay to look at, but still as hungry 

 and as cruel as in its dingy larval state. It is worth while to 

 observe with some care the escape of the fly, which can be 

 done most easily by keeping a number of full-fed larvae in a 

 large tank. Whenever a cast skin is found on the rim of the 

 tank, it is time to look out carefully for the emergence of the 

 flies. They creep out of the water, grasp a plant or some 

 other convenient object, and then rest till they are dry. At 

 last the larval skin splits along the back of the thorax, and the 

 soft imaginal body begins to protrude. As it becomes free, it 

 swells and enlarges the cleft. The thorax, and a little later, 

 the head are freed, the legs are drawn out of their sheaths, and 

 then the fly (of some species) throws its head backwards. 



