118 SECTIONAL ADDRESSES. 
the present day—‘ that suctorial insects do not come under consideration 
in connection with the spread of plague.’ Further observations, how- 
ever, soon showed this conclusion to be erroneous. Liston found in 
Bombay in 1903 that the common rat-flea was Pulex (Xenopsylla) 
cheopis, that it was present in houses in which rats had died of plague 
and in which some of the residents had become infected, that the plague- 
bacillus could multiply in the stomach of this flea, and that the flea would 
—in the absence of its usual host—attack man. These observations 
pointed to the importance of this flea in the dissemination of plague, 
and the Second Plague Commission, which was appointed and began 
work in 1905, definitely proved that Xenopsylla cheopis is the trans- 
mitter of the plague-organism from rat to rat and from rat to man. 
The mechanism of transmission of the plague-bacillus was worked out 
by Bacot and Martin in 1913. They showed that in a proportion of 
these fleas fed on the blood of septicemic mice the plague-bacilli 
multiply in the proventriculus—which is provided with chitinous pro- 
cesses that act as a valve to prevent regurgitation of the blood from 
the stomach—and a mass of bacilli is formed which blocks the proven- 
triculus and may extend forward into the cesophagus. Fleas in this 
condition are not prevented from sucking blood because the pharynx 
is the suctorial organ, but their attempts to obtain blood result only in 
distending the cesophagus. The blood drawn into the cesophagus is 
repeatedly forced backwards into contact with the mass of plague-bacilli 
and on the sucking action ceasing some of this infected blood is expelled 
into the wound. The transmission of plague depends on the peculiar 
structure of the proventriculus of the flea and on the extent to which, 
in certain examples, the plague-bacilli multiply in the proventriculus. 
Such “blocked ’ fleas being unable to take blood into the stomach are 
in a starved condition, and make repeated attempts to feed, and hence 
are particularly dangerous. 
Until 1913 it was believed that all the fleas of the genus Xenopsylla 
found’on rats in India belonged to one species—cheopis, but in that 
year L. F. Hirst reported that the rat-flea of Colombo was X. astia, 
which had been taken off rats in Rangoon, and described by N. C. Roth-. 
schild in 1911. Hirst ascertained that this flea did not readily bite man’ 
if the temperature were above 80°F. A collection of 788 fleas from’ 
Madras City proved to consist entirely of X. astia, and Hirst suggested 
that the explanation of the immunity of Madras and Colombo from 
plague was the relative inefficiency of X. astia as a transmitter. Crage’s 
examination (1921, 1923) of 23,657 fleas obtained from rats in all parts 
of India shows that they include three species of Xenopsylla—namely, 
cheopis, astia, and brasiliensis. This last species is common in the’ 
central and northern uplands of peninsular India, but its’ bionomics 
have not yet been investigated: Cheopis is the predominant species 
in the plague areas, while astia is the common flea in those areas which 
have ‘remained free from plague or have suffered only lightly. In 
Madras City, for instance, during the twenty-one years, 1897-1917,' 
plague has occurred in twenty of these years, but the average mortality .. 
was only .013 per thousand—that is, though the infection has been. 
repeatedly introduced there, it failed each time to set up an epidemic. 
