34 Guide to Insects and Ticks 



resting phase, they become slowly transformed into fleas, which 

 finally emerge from their cocoons, and invade the fur of new 

 hosts. 



On the small glass table in the case are shown transparent 

 models of human red blood-corpuscles and plague bacilli, enlarged 

 '6,000 diameters (fig. 10). Bubonic plague, the disease that is 

 transmitted by the bite of various kinds of flea, of which 

 Xcnopsylla cheopis is the most important in tropical regions, so 

 that it is sometimes termed the plague-flea, is due to the presence 

 of minute bacterial organisms, known as Bacillus pestis Kit., 

 occurring in the blood. These organisms were first discovered 

 during the epidemic of plague in Hong Kong in 1894. The bacilli 

 occur in the fluid part (plasma) of the blood, and not in the 



Fig. 10. — Organisms of Plague, 

 Bacillus pestis, and red blood-corpuscles ; X 1,500. 



corpuscles ; they may have the form of small rods with rounded 

 ends, and sometimes show a constriction across the middle. 

 Occasionally they are found in chains or series set end to end ; 

 each bacillus repeatedly elongates and divides across, thus 

 increasing the total number at a rapid rate. 



On the floor of the case are shown actual specimens of the 

 tropical rat-flea and its larva, mounted on slips of glass. The 

 specimens are so small that no details can be seen ; they are 

 introduced into the series merely to give a correct impression of 

 the real size. 



Under the microscope thac is set upon the table at the 

 entrance to Bay X on the East side of the Hall, near the statue of 

 Prof. Huxley, are shown specimens of the tropical rat-flea, 

 XcnopsyUa cheopis, in five stages of its life-history. Slide A shows 



