CHAPTER IX 

 MOSQUITOES AND DISEASE 



In many parts of the world mosquitoes are even more impor- 

 tant than house flies as carriers of disease germs, but this has 

 been known for only a few years. Two of the most dangerous 

 of all diseases, malaria and yellow fever, are transmitted from 

 one person to another only by mosquitoes, and several other 

 diseases, such as dengue and elephantiasis, are spread, at least 

 in part, by these insects. Besides this, mosquitoes are probably 

 responsible for the transmission of germs with which we are not 

 yet acquainted. 



How Germs are Carried. — The mosquito differs from the 

 house-fly in several important respects. In the first place, its 

 mouth parts are fitted for piercing (Fig. n, A), and it is thus 

 able to penetrate the skin and suck blood directly from the body. 

 Any germs that chance to be in the blood of the victim are thus 

 taken into the alimentary canal of the mosquito and may be 

 injected into the blood of the next person bitten. The house 

 fly carries germs upon its body or in its alimentary canal, and 

 is not itself diseased; it is called a passive carrier. The mos- 

 quito, on the other hand, is an active carrier. Its blood stream 

 becomes filled with the germs, which are transported to the 

 salivary glands, where they are stored up until the insect bites; 

 then they pass with the saliva into the wound and infect the 

 person bitten. 



What the Germs Are. — These germs are not bacteria, as are 

 those carried by the house fly , but minute animals called Protozoa. 

 (See Chapter XXV.) Those which cause malaria are visible 

 with the compound microscope, but the germs of yellow fever 



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