10 H. O. HALVORSON 



various species may be put into a state of cryptobiosis by 

 dehydration, as mentioned earlier, and also by a lowering of the 

 temperature. Thus, many nonencysted forms of protozoa can 

 be preserved for long periods of time by being suspended in a 

 frozen medium. Here, there is a great deal of variation between 

 species, some are sensitive to freezing whereas others are very 

 resistant. Here, also, as with bacteria, the method of freezing, 

 the ultimate temperature, and the constancy of the temperature 

 at which they are stored can have an important bearing upon 

 the longevity of the individual cells. 



INSECTS 



The eggs of insects, for example, the eggs of mosquitoes, 

 have many properties in common with plant seeds and the cysts 

 of protozoa. Under proper conditions these can be kept for 

 long periods of time without loss of viability. Eventually they do 

 die, which may indicate there is a low level of metaboHsm which 

 uses up the reserve food. If stored at room temperatures, the 

 life span is materially shortened. 



The hatching of these eggs may be compared to the germina- 

 tion of seeds or the excystment of protozoan cysts. In most cases 

 the hatching process needs to be triggered by some external 

 factor, such as a favorable temperature, along with a lowering 

 of the oxygen tension^^. In nature, these eggs may thus be 

 preserved through the winter, but when spring comes and the 

 temperature gets high enough for substantial growth of bacteria, 

 the necessary lowering of O2 tension can occur through the 

 metabolic activity of these organisms, and, as soon as the 

 oxygen tension is brought to the proper low level, hatching will 

 take place. In the laboratory the same thing can be accomplished 

 by replacing the oxygen with nitrogen. Thus, persons who are 

 working with mosquito eggs can store them for a long time 

 without much loss of viability and hatch them at will. There are 

 some mosquito eggs, however, that have a more complex trigger 

 mechanism, in that they may require a change in temperature 



