CH XI.] THE GNAT. 193 



The gnat undergoes many metamorphoses. If 

 water be allowed for some time to stand still in a 

 bucket, or if a quantity of that fluid be taken from 

 a stagnant pool, it will be found to contain innu- 

 merable aquatic insects of the following shape {fig. 1) : 

 these are the larvae of the gnat ; they swim with the 

 head downwards, a position which, to most animals, 

 would be fatal ; they retain the longest tube, which 

 is their respiratory organ, on the surface ; the other 

 tube forms the anus. In this state they live on the 

 contents of stagnant waters, and change their skins 

 several times. 



After having thrice got rid of its skin, the gnat 

 appears in a new forni, for, instead of being oblong, 

 it is lenticular ( ^g. 2). The surface of the circle is 

 vertical to the water. In this, which is its nymphine 

 state, it is still capable of moving briskly after the 

 manner of a shrimp, by expanding and elongating 

 its body {fig. 3), and striking the water with the fins 

 at its tail. In this stage of its metamorphosis, it 

 has no organ for food, and it seems to require none 

 but a regular and abundant supply of air appears 

 indispensable ; it floats on the surface of the water, 

 and only descends by efforts made with its tail. 

 Two ears maybe observed sticking out at the thick- 

 est part ; these are its respiratory oigans, and afford 

 a curious instance of an import! rt part, being re- 

 l.—R 



