22 ENTOMOLOGY FOR MEDICAL OFFICERS 



parasite from the blood of one of numerous species of 

 vertebrate animals in which the said parasite is naturalised 

 and innocuous, and may (after a definite period during which 

 the parasite multiplies) inject it into the blood of some alien 

 species of vertebrate to which it is fatal.^ Carriers of this 

 kind are the most dangerous of all, as it is impossible to 

 control all the original sources of their infection. 



(iv) Carriers like the ticks (and possibly^ Stegomyia 

 fasciata sometimes, in " carrying " the germ of yellow fever). 

 Here the carrier imbibes a specific parasite in the blood of a 

 victim and transmits it through egg, or larva, or nymph (see 

 p. 274), so that the tick in an after stage becomes infective. 

 Carriers of this kind are more than ordinary dangerous, since 

 the infection may remain latent for many months, during 

 which the infected eggs and larvae may be carried mechani- 

 cally to an enormous distance. 



It is best to restrict the term Carrier to these forms that 

 are necessary for the maintenance of any species of endo- 

 parasite. 



(c) Porters. 



Carriers of this kind are, roughly speaking, intermediate 

 between Casual Carriers and Adapted Carriers ; they render 

 definite assistance in distributing a specific microparasite, but 

 are not essential to its existence. 



Here are included certain common species of fleas (p. 190) 

 that imbibe the specific plague-bacillus from the blood of 

 one of several species of susceptible vertebrate animals, but 

 do not digest the bacilli, which pass away in the excreta of 

 the insect. When the flea, thus infective, resorts to another 

 susceptible vertebrate to feed again, it somehow or other— 

 probably by its excreta, which are freely expelled during the 

 act of sucking blood— infects the wounds that it inflicts on 

 its new host. There seems to be here no definite adaptation 

 of the bacillus to the flea, or vice versd ; and there seems to 

 be no doubt that though fleas very materially aid the dis- 

 persal of the bacillus, the bacillus can be disseminated in 

 many other ways. 



; This, of course, is w/to be understood as being the usual way in 

 which, m the case of man, Glossina and Stegomyia spread their soecifir 

 infections. i'cv.nn. 



