472 



THE ANIMALS AND MAN 



be only wide enough to allow the insects under observation 

 to move freely through the soil. If it is too wide the insects 

 will be able to conceal themselves. Immediately outside 

 of each glass there is a piece of blackened zinc which slips 

 into grooves in the ends of the cage, and which can be easily 

 removed when it is desired to observe the insects in the soil. 

 "Aquaria. 'For the breeding of aquatic insects aquaria 

 are needed. As the ordinary rectangular aquaria are ex- 

 pensive and are liable to leak we use glass vessels instead. 



"Small aquaria 

 can be made of jelly- 

 tumblers, glass fin- 

 ger-bowls, and glass 

 fruit-cans, and larger 

 aquaria can be ob- 

 tained of dealers. 

 A good substitute 

 for these is what is 

 known as a battery- 

 jar (fig. 240). There 

 are several sizes of 

 these, which can be 

 obtained of most 

 dealers in scientific 

 apparatus. 



"To prepare an 

 aquarium, place in 

 the jar a layer of 

 sand; plant some 

 water-plants in this sand, cover the sand with a layer 

 of gravel or small stones, and then add the required 

 amount of water carefully, so as not to disturb the plants 

 or to roil the water unduly. The growing plants will keep 

 the water in good condition for aquatic animal life, and render 

 changing of the water unnecessary, if the animals in it live 

 naturally in quiet water. Among the more available plants 

 for use in aquaria are the following : 

 "Waterweed, Elodea canadensis. 



FIG. 240. Battery-jar aquarium. 



