ON THE SUBJECT OF PLAGUE 
867 
Mus norvegicus, the sewer -rat. The distinguishing features 
between these two are that in the sewer-rat, M. norvegicus , 
we have the ears barely reaching the eyes when laid forward, 
and the tail rather shorter than the length of the head and 
body together ; whereas with M. rattus, the house-rat, the 
extended ear covers or reaches beyond the middle of the eye, 
it has a sharper nose, thinner ears, and a tail which is longer 
than the length of the head and body combined by 25 per 
cent. Mus alexandrinus is a variety of M. rattus, and the 
common mouse is known as M. musculus. In the United 
States the ground-squirrel, Cytellus beechyi, acts as a reservoir 
of plague, and has as its flea Hoplopsyllus anomalus. 
Insects which have been implicated in the Spread of Plague . — ■ 
The insects which have been specially studied in connection 
with plague are flies, bugs, mosquitos, pediculi, ants, cock- 
roaches, and fleas. 
They are of more or less importance in this connection in 
relation to their habits of sucking blood and of transferring 
themselves from one host to another, as, for example, the flea, 
biting first the rat and, later, man. Great interest also attaches 
to the problem as to what becomes of the plague bacillus in 
the alimentary canal of such insects, and especially whether 
it is capable of multiplying there, and either increasing or 
decreasing in virulence. 
Flies have been proved to be capable of conveying the 
bacilli on their legs and thus infecting food material. Plague 
bacilli may remain virulent in the intestines of a fly for forty- 
eight hours. 
Bugs were found to have plague bacilli in their intestines 
after sucking the blood of rats suffering from plague ; but it 
was found impossible to transmit plague through the agency 
of these insects. 
Mosquitos also have not been implicated. 
Pediculi have been shown by the experiments of de Raadt, 
in 1915, to be capable of transmitting plague infection. Both 
the body-louse and the head-louse have been implicated. 
Ants of a certain species ( Monomorium vastator ) can trans- 
mit plague, and they often succumb after feeding on plague 
rats. This is on the authority of Dr. Hankin. 
