SYMBIOSIS 57 



Under conditions precluding the possibility of infection by dejecta, 

 it was found that two species of rat fleas, Xenopsylla cheopis and 

 Ceratophyllus fasciatus, fed upon septicsemic blood, can transmit plague 

 during the act of sucking, and that certain individuals suffering from 

 a temporary obstruction at the entrance to the stomach were responsible 

 for most of the infections obtained, and probably for all. In a 

 proportion of infected fleas the development of the bacilli was found to 

 take place to such an extent as to occlude the alimentary canal at the 

 entrance to the stomach. The culture of pest appears to start in the 

 intercellular recesses of the proventriculus, and grows so abundantly as 

 to choke this organ and extend into the oesophagus. Fleas in this 

 condition are not prevented from sucking blood as the pump is in the 

 pharynx, but they only succeed in distending an already contaminated 

 o?sophagus, and, on the cessation of the pumping act, some of the blood 

 is forced back into the wound. Such fleas are persistent in their 

 endeavours to feed, and this renders them particularly dangerous. 



The point of particular interest here is that the more 

 intense the morbidity of the flea, the more " persistent it gets 

 in its endeavours to feed," which renders it "particularly 

 dangerous." If we have here apparently only a special case, 

 if it be pleaded that it is only accidental that in this instance 

 the alimentary canal becomes occluded at the entrance to the 

 stomach, causing extreme thirst of the flea, our reply must be 

 that all " in-feeders " eventually find themselves in a similar 

 plight and confronted by similar precarious circumstances. All 

 "in-feeders" are, and are bound to become increasingly 

 fastidious. The fastidiousness of the flea is such that 

 it requires warm blood for its sustenance. " He only sucks the 

 tasteful blood." How is it to acquire the modicum of bio- 

 chemical substances which its parasitic habits have precluded 

 it from obtaining direct from their true (symbiotic) source, 

 except by having increasing recourse to dishonest means, i.e., 

 by preying on warm-blooded mammals? 



If the flea again typical of all " in-feeders " provides 

 no positive share of biological symbiosis, it provides, per contra, 

 the temptation and the soil for invasion by pathogenic micro- 

 organisms. When the fleas suck infected blood into their 

 stomach, the plague germs multiply enormously within them. 



