FIELD BOOK OF PONDS AND STREAMS 



(Fig. 34, 2). The life-history of mosquitoes, larva, pupa 

 and adult (Fig. 34, i), can be watched in such a simple aqua- 

 rium; only this one should be kept covered. 



Fig. 34. — Aquaria in tumblers: i, Ciilex mosquitoes, 

 showing one larva and two pupae with their air-tubes 

 thrust through the surface film, floating eggs, adults; 

 2, snails and Elodea. 



The balanced aquarium (Fig. 35) is just what its name 

 says — a balance between plants and animals. The plants 

 use the carbon dioxide produced by the animals; the animals 

 use the oxygen given off b}'' the plants. A good aquarium 

 plant is one which affords a bountiful supply of oxygen. 



The best aquarium dishes are rectangular glass ones which 

 are as wide as they are high. Cylindrical ones distort the 

 vision; deep ones shut out the air so that the water gets little 

 oxygen. In making a balanced aquarium spread a layer of 

 mud over the bottom, being sure that it is free from car- 

 nivorous animals; cover this with a layer of sand and gravel. 



38 



