34 INSECTS AND HUMAN WELFARE 



every year, over half of which terminate fatally. At the pres- 

 ent time it is most widespread and abundant in tropical 

 countries, although by no means confined to them, and is 

 excluded from our own country only by dint of repressive 

 measures administered with the greatest thoroughness. 

 Within the past few years it has appeared only sparingly in 

 the United States, but at several times has given rise to a 

 temporary apprehension lest it pass beyond control. That it 

 has not done so shows the probability of future danger is re- 

 mote. 



Nevertheless there are other good reasons why we should 

 spare no efforts in reducing the number of rats. They are 

 said by Nelson to destroy annually $200,000,000 worth of 

 our foodstuffs and other property ; they constitute a fire- 

 menace, and besides, they can hardly be considered as de- 

 serving our hospitality from an esthetic standpoint. In short, 

 war against rats is important for many reasons, one of which 

 is the security against plague which it entails, and gradual 

 repression through individual effort is much easier than in- 

 tensive campaigns necessitated by the advent of plague in a 

 community. 



The relation of the flea to the transmission of plague is due 

 to the fact that rats are regularly infested by fleas that may 

 become infected with the bacillus of plague, if it be present 

 in the blood of the host upon which they are feeding. These 

 bacilli remain in a viable condition for some time in the gut 

 of the flea and may be transferred to a human subject bitten 

 by an infected flea. Thus, when a rat dies of plague, its fleas 

 leave it to search for a new host; if they attach themselves to 

 a rat, that animal is liable to infection, or if they feed upon a 

 human being, as they frequently do, the disease may become 

 transferred to man. Two species of fleas are commonly con- 

 cerned in the transfer, one in tropical and subtropical regions 

 and another in temperate regions. The tropical rat-flea, 

 Xenopsylla cheopis (Fig. 15) is thus of greatest importance in 

 the warm countries where it is most abundant, and the other, 

 Ceratophyllus fasciatus (Fig. 16), in cooler countries. Both 



