396 



ZOOLOGY. 



way, it suspends itself as it were to the surface of the water, 

 with the head downwards, and we see it at short intervals 

 renew its arrangement. The nymph continues to live in 

 water and to move in it, but instead of breathing like the 



Fig. 374 Larva of the 

 Gnat. 



Fig. 375. Gnat, magnified. 



larva, it ob'tains the air which it requires through the medium 

 of two tubes placed under the thorax. It floats on the surface 

 of the liquid, and after having accomplished its metamor- 

 phoses the perfect insect (Fig. 375), employs its cast-off 

 covering whilst a nymph as a barge or boat, until its long 

 limbs and wings have acquired sufficient solidity to permit it 

 to walk on the surface of the water or to fly away ; for if its' 

 body happened to be submerged, as occurs often when the 

 wind upsets its frail embarkation, it would infallibly be 

 drowned. 



534. The insects which undergo a semi-metamorphosis, 

 also pass through the state of larva and nymph, before arriving 

 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. 



We may cite as examples of insects presenting this kind of 

 metamorphoses, the grasshopper and the ephemera (Fig. 376). 



