BIOLOGICAL STUDIES ON INTRACELLULAR BACTERIA. 135 



the exact conditions existing in the mycetocytes or mycetome 

 have not been sufficiently reproduced. 



16. The number of symbionts per host is enormous. For Cero- 

 plastes rusci, Berlese computed the number per host to lie be- 

 tween sixty and seventy thousand. 



17. The reasons for the belief that the intracellular organisms 

 are symbionts and not commensuals or parasites are the fol- 

 lowing : 



(a) Every individual of a species is infected. 



(&) The infection produces changes in the host cells, but these 

 are harmless. 



(c) The infection routes and methods of localization, while 

 different in different hosts and symbionts, follow very definite 

 courses within a species. 



(o?) The microorganisms are numerically controlled by the 

 host, never increasing up to a point where they may prove fatal. 



(#) The microorganisms within the insects obtain nourish- 

 ment, and protection from drastic temperature and draught con- 

 ditions. 



1 8. While numerous theories have ben advanced, the symbiotic 

 relationship, if any exists, has not been established upon any 

 scientific basis. ^ 



Buchner (1912) in an extensive paper, together with original 

 morphological investigations, gave an excellent review of all of 

 the literature dealing with the subject of the intracellular sym- 

 bionts of insects. For this reason, another extensive review of 

 the literature would be merely a duplication of something that 

 has already been admirably accomplished. 



In beginning this investigation, I was primarily interested in 

 the physiological aspect of the subject. If the intracellular or- 

 ganisms are really of some use to the host, they may possibly 

 assist in the metabolism of reserve and other foods. This can 

 only be accomplished through the secretion of enzymes by the 

 microorganisms. I therefore thought it well to select one or two 

 species of insects, to attempt to cultivate the intracellular organ- 

 isms, then to make a systematic study of their enzyme activities, 

 and later to attempt to correlate such activities with the metabo- 

 lism of the host's diet. 



