40 BRITISH INSECTS 



nymphs, however, the thoracic spiracles are open 

 during the latter stages of development, and the insect 

 then obtains at least some of its oxygen by raising the 

 front part of its body above the surface of the water. 

 As the time for the final moult draws near, the now 

 full grown nymph ceases to feed, and quits the water 

 by climbing up some convenient stem or post. There 

 is no definite pupal stage. The nymph remains 

 passive for a brief period. Then its skin splits above 

 the thorax, and the imago begins to extricate itself. 

 When the head and thorax are free, the insect throws 

 itself backward, and rests for about half an hour, sup- 

 ported by the hinder segments of the abdomen, which 

 remain in the nymphal skin. The meaning of this 

 period of inactivity, as suggested by Mr. Latter, is 

 probably that the legs are as yet too soft and feeble for 

 use. When they have become dry and firm, the insect 

 jerks itself upward, and grasps with them the upper 

 part of the nymphal skin. With the purchase thus 

 gained, it draws out its abdomen. Some hours must 

 still elapse, however, before the wings have developed 

 and become hard enough for flight, while the full 

 colouring and sheen of the integuments are not gained 

 until two or three days later, With respect to the 

 dragon-fly's duration of life, Mr. Lucas remarks : " It 

 used to be thought that some of the larger species 

 required three years to reach the final stage. They 

 may take as long when the food-supply is scanty, but 

 in all probability it will be found that the majority 

 complete the cycle of existence within the year, even 

 if one or two of the small species have not a second 

 emergence about September." The life of the imago 

 appears seldom to exceed three months. The adults 

 never hibernate after the manner of some other insects. 



