GASTRIC C^ECA OF THE HETEROPTERA. Ill 



greatly elaborated in structure. There is somewhat better reason 

 for believing, however, that the caeca were originally independent 

 organs which finally came to be occupied by the normal bacteria, 

 and this subject will be taken up under the discussion of the caeca 

 themselves. 



That the infection is regularly transmitted through the egg 

 from generation to generation is very evident from the above 

 facts; but this does not necessarily mean that the organism is 

 so closely bound up with the host as to exclude the possibility 

 or even the importance of infection from without. One can hardly 

 believe that reinfection by way of the mouth, whether an im- 

 portant factor in the association or not, is not constantly taking 

 place, at least from the excrement of other individuals if not also 

 from some undiscovered form of the organism that may exist 

 normally as a free saprophyte. 



As is well known, certain pathogenic, blood parasites, as the 

 spirochsetes, are regularly transmitted through the egg from one 

 generation to succeeding generations of the intermediate host, 

 which is a tick in the case of Spirock&ta duttoni. It has been 

 established, in this particular case, not only that the mother that 

 swallows the infected blood may transmit the parasites to her 

 young, but that their descendants, although allowed to feed 

 only on clean animals during their entire lifetime, may also pass 

 this second-hand infection on to their young. Transmission 

 may not stop even here, as it has been clearly established that it 

 may continue in this way for at least three generations of ticks. 



Just why this process should not go on indefinitely, when once 

 the infection is acquired by the tick, is not clear; but evidently 

 it does not, as only a comparatively small per cent, of the indi- 

 viduals of Onrithodorns moubata are able to transmit the disease 

 to man. 



In order for the association to continue, repeated infections 

 with the blood-inhabiting forms of the spirochaetes must ap- 

 parently take place. It may be that conditions in the arthropod 

 host are unfavorable in some way to the spirochaetes, and that 

 they gradually degenerate in passing through a number of genera- 

 tions of the host, so that for a continuance of the association 

 a periodical reinfection with a more vigorous strain from some 



