GASTRIC CECA OF THE HETEROPTERA. 145 



tant part in the digestion of the insect extremely small, even 

 though they had been found to secrete important digestive 

 enzymes. 



The deficiency in enzyme production shown by these organisms 

 will be taken up in connection with the culture work, and will 

 not be discussed here; although it might be mentioned that when 

 the caeca, together with a considerable section of the intestine 

 was removed, as in the Pentatomidse, and dropped into a tube 

 of bouillon, the tissues would remain white and apparently nor- 

 mal in every way for several weeks, or longer, although crammed 

 full of the csecal bacteria. This would show, at least, that the 

 caecal bacteria do not secrete a proteolytic enzyme. The same 

 thing was also observed in cultures from Anasa tristis, the 

 tissues remaining unchanged even after the cultures had been 

 growing vigorously for a week or more. 



In the case of certain of the intestinal bacteria of the higher 

 vertebrates, there is a well-known association existing between 

 these organisms and the host that is remarkably similar in many 

 ways to the one just described for the Heteroptera, although as 

 might be expected, it is complicated, in this case, by a vast 

 number of factors that do not have to be taken into considera- 

 tion in a treatment of this relation in the much more simple 

 insects. 



Practically all the higher animals harbor certain varieties 

 of intestinal bacteria which have become so intimately associated 

 with the host that they are generally referred to as the "normal " 

 intestinal bacteria; and there has been a tremendous amount 

 of work done in the attempt to determine the exact functional 

 importance of these organisms from the standpoint of the host. 

 As might be expected, by far the greater part of this work deals 

 directly with those forms peculiar to man and the higher verte- 

 brates, but as many of the principles which have been worked 

 out for the association in these animals throw a great deal of 

 light on conditions as they exist in the Heteroptera, the following 

 very brief summary of the pertinent work that has been done 

 on this subject should not be out of place. 



