6 MEDICAL ENTOMOLOGY 



taken into the stomach of the flea increase in numbers and do 

 not become attenuated, but pass out with the feces or even in 

 undigested blood per anum in a virulent condition; the direct 

 inoculation must be through a "rubbing in" process onthe part 

 of either the host or the flea. ^K: 



The greatest complexity is involved in such cases in which the 

 carrier is a necessary factor in the life history of the pathogenic 

 organism, e. g. the Anopheles mosquito has piercing mouthparts, 

 is bloodsucking, and an intermittent parasite, in which a given 

 period of time must elapse before it can transmit the causative 

 organism of malaria, once it has become infected. This period 

 of time corresponds to the time required for the Plasmodium to 

 pass through its sexual cycle and find its way back into the 

 proboscis, i. e. into the salivary glands, ready to be reinoculated. 



INSECT MOUTHPARTS 



Importance of mouthparts. It becomes evident that an 

 insect possessing mouthparts capable of piercing the skin of the 

 higher animals must be looked upon as a possible carrier of 

 blood infection, although it may in actual experience never 

 attack other animals. If the insect is provided with mouthparts 

 of the usual biting type it cannot relate to infection introduced 

 through the circulation except by rare accident through a pre- 

 ^fcwisly inflicted open wound. 



The mosquito would be harmless as far as malaria and yellow 

 fever are concerned if the mouthparts were of the mandibulate 

 or biting type. These insects together with certain other species 

 such as the stablefly, Stomoxys calcitrans, the tsetse flies and the 

 ticks are important because of the power which they possess of 

 piercing the skin of higher animals and introducing parasitic 

 organisms into the blood. The housefly on the other hand cannot 

 introduce organisms directly into tjie circulation because its 

 mouthparts are not of the piercing type. These creatures are 

 attracted by and often breed in excrementous matter, are then 



