CLASS INSECTS. 



435 



and which are so harassing to man by their envenomed 

 sting, live in water whilst in.x their larva state. They are 

 then vermiform, without limbs, and have the abdomen ter- 

 minated by bristles and appendages disposed in rays (Fig. 

 456) ; finally, the last ring but one gives origin to a tube 

 sufficiently long (t), by means of which the animal draws 

 from the atmosphere the air it requires. To breathe in this 

 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 

 larva, it obtains the air which it requires through the medium 

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



Fig. 456. Larva of the 

 Gnat. 



Fig. 457. Gnat, magnified. 



of the liquid, and after having accomplished its metamor- 

 phoses, the perfect insect (Fig. 457) 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 

 IFF 2 



