436 



ZOOLOGY. 



at their perfect state ; but here the larva differs only from the 

 perfect insect by the absence of wings, and the state of nymph 

 is only characterized by the growth of these organs, which, at 

 first folded and concealed under the skin, become then free, 

 but acquire all their development only at the epoch of their 

 last moulting. 



Fig. 458. Ephemera; Day-fly. 



We may cite as examples of insects presenting this kind of 

 metamorphoses the grasshopper and the ephemera (Fig. 458). 

 These last present even a remarkable peculiarity ; for in 

 general insects change their skin for the last time when they 

 pass from the stage of nymph to their perfect state, whilst 

 the ephemera experiences another moulting before 

 becoming completely adult, although in this state 

 it lives but a few hours. The larva of this 

 ephemera lives in water, and differs but little from 

 the adult, excepting in the shortness of its limbs, 

 the absence of wings, and by the row of Iamina3 

 or plates which it has on each side of the abdomen, 

 and which it employs as organs of respiration and 

 of swimming. The nymph (Pig. 459) only 

 differs from the larva by the presence of sheaths 

 enclosing the wings. At the moment when these 

 organs are to be developed,~ l the insect quits the 

 water, and after having vaulted in the air for some 

 minutes, proceeds to rest upon an elevated object, 

 when it abandons itself to violent movements, by 

 Fig. 459. means of which it throws off its tegumentary 

 membrane ; it is then only that its limbs ac- 

 quire all their length, and its body the colours it is after- 

 wards to preserve. 



535. Some insects, although they undergo considerable 

 changes when young, do not pass through the complete series 

 of transformations of which we have just spoken; they seem, 



