THE BREEDING OF INSECTS. 



331 



Fig. 292. — Duckweed. 



Watercress, Nasturtium officinale. 

 Stoneworts, Chara and Nitella (several species of 

 each). 



Frog-spittle or water-silk, Spirogira. 

 A small quantity of duckweed, Lemna (Fig. 292), 

 placed on the surface of 

 the water adds to the 

 beauty of an aquarium. 

 When it is necessary 

 to add water to an aqua- 

 rium on account of loss 

 by evaporation, rain wa- 

 ter should be used to 

 prevent an undue ac- 

 cumulation of the mineral matter held in solution iq 

 other water. 



The Constant-level Siphon. — Certain insects 

 that live in rapidly flowing streams require a con- 

 stant change of water. Some of these are extremely 

 difficult to breed in confinement, but others can be 

 kept alive easily if placed in an aquarium 

 which is prepared as described above, 

 and through which there is a stream of 

 water constantly flowing. 



The water can be admitted to the 

 aquarium from a faucet, and the surplus 

 water removed by a device which may 



be called a constant-level siphon. This 

 Fig. 293. —Con- . , . . . „. 



stant- level IS represented m operation in rig. 291, 



^^^ °°' and separate in Fig. 293. 



The siphon can be made of small lead pipe. It 

 differs from an ordinary siphon in being bent up at 

 the outer end (the last bend shown in the figure is 



