106 HUGH GLASGOW. 



Illinois, the bacteria, although characteristic in shape, often 

 averaged decidedly longer and stouter than those from most of 

 the other localities; but this difference in size was not constant 

 in all the specimens of Mnrgantia from these states, and it was 

 doubtless due to some slight difference in the metabolism of the 

 particular individuals examined. 



Mnrgantia is the only species that I have studied in any 

 numbers with this idea in mind, from outside Illinois; but 

 hundreds of specimens of other species have been examined from 

 different parts of this state, and all of these show, without 

 exception, that in their morphology the type of caecal organism is 

 absolutely constant for a given host species. 



ORIGIN OF THE C/ECAL INFECTION IN THE HETEROPTERA. 



At an early stage in the work it became evident that one of the 

 first things to be determined was the exact time and manner in 

 which the infection with the caecal bacteria normally takes place. 

 Assuming that the normal bacteria would develop readily in 

 artificial cultures, the next essential step would be to differentiate 

 between this organism and any of the common saprophytic or 

 parasitic forms that are so frequently met with in the alimentary 

 canal of most insects, and which would naturally be expected 

 to appear also in cultures from the caeca. Until the identity of 

 the normal species in culture could be established beyond ques- 

 tion, it would clearly be useless to undertake any time-consuming 

 study of the physiology of any of the organisms that might be 

 isolated in an attempt to correlate this with the digestive proc- 

 esses in the insect; and at the commencement of the work it 

 appeared that the only possibility of certainly differentiating 

 between the normal bacteria and the contaminating forms that 

 might appear, was in some way co secure insects free from the 

 infection, and then by direct feeding experiments to determine 

 which were really the "caeca!" bacteria. 



The normal bacteria are present, not only in the caeca, but 

 also in considerable numbers free in the alimentary canal of the 

 host and, as might be expected, they also occur in the brownish, 

 liquid excrement of the insect. It is easy to imagine from this 

 how the surroundings might be very highly contaminated with 



