16 ENTOMOLOGY FOR MEDICAL OFFICERS 



subject. Some apply it indiscriminately to any animal that, 

 getting in any way contaminated with pathogenic micro- 

 organisms of any kind transports them in any manner to 

 a suitable station. In this view a house-fly that casually 

 settles on a baby's eyelid after casually crawling on pus is 

 a "carrier." 



(2) Others would restrict the term so as to include only 

 cases where the micro-organism is actually ingested by the 

 insect, multiplies by fission in its gut, and is passed with 

 undiminished virulence in its secretions or its excreta. In 

 this view a house-fly that casually swallows typhoid bacilli, 

 and afterwards casually voids them increased in number into 

 milk or on to food is a " carrier." 



(3) Others again would restrict the term still further, so 

 as to include only cases where a specific micro-organism is 

 ingested by a blood-sucking Arthropod, from which, after 

 proliferation, its progeny are expelled in such a way that 

 when the said Arthropod bites again, the wound that it 

 inflicts may somehow become specifically infected. In this 

 view a flea that has become infected from a plague patient is 

 a carrier of the plague bacillus. 



(4) Still others restrict the term to those cases where 

 there is a fixed and constant relation between a specific 

 parasitic micro-organism and a specific blood - sucking 

 Arthropod. In these cases the specific micro-organism 

 after being ingested resides for a definite period in the 

 specific Arthropod (or one of several closely-related species), 

 proliferates in a definite and distinctive manner in some part 

 of the Arthropod's gut, and ultimately colonises in a distinc- 

 tive manner a definite part or tissue (usually the mouth- 

 parts or salivary glands) of the Arthropod ; so that when, 

 after the lapse of a definite term, the said Arthropod bites 

 another customary victim, it perforce in the act of biting 

 transfers some of these colonists to a new host. The 

 infection thus " carried " by the Arthropod may expire with 

 the life of the individual Arthropod, or may be transmitted 

 through its eggs and larval stages to the following genera- 

 tion. 



It is for such cases, where a particular kind of blood- 

 sucking Arthropod is definitely utilised by a specific micro- 



