THE BUTTERFLY VIVARIUM. 19 



structure, but at the same time furnish, in the necta- 

 ries of their blossoms, food for the Butterflies which 

 have reached their perfect state, during the short 

 time that they can be preserved in the Vivarium. 



In insect Vivaria, in which the rearing of water 

 insects forms part of the plan, the same principles 

 must be applied, in order to keep the water clear 

 and pure, as those employed in fresh-water Aquaria, 

 namely, the addition of water plants and Alga, such 

 as the favourite Valisneria Spiralis, and one or two 

 species of Chara, or some of the Oscillatorice, the 

 curious spasmodic movements of which are exceed- 

 ingly interesting. These plants serve to aerate the 

 water according to the principle first clearly an- 

 nounced by Ingenhauss in the last century, when he 

 stated that "plants immersed in water, when ex- 

 posed to the action of light, emit an air known as 

 oxygen." 



Another approach towards the condition of na- 

 tural ponds, and one almost equally necessary to 

 preserve the purity of the water, is the introduction 

 to the miniature lake of some of the Water Snails, 

 whose function it is to consume those portions of 

 decaying vegetation which would otherwise rapidly 

 poison the water, and render it unfit for the support 

 of any kind of animal life, as previously stated. 



