that spread Disease. 19 



of the infected insect at the time of the l)ite. Yellow fever is 

 primarily a disease of the West Indies and the East Coast of 

 Mexico, but it has been spread by shipping to many other parts of 

 the world. 



The causal agent of yellow fever is not known with certainty, 

 although the disease has been attributed to an extremely minute 

 protozoon found in the blood. That it is a living organism and 

 not a chemical substance is evident from the fact that some time 

 (about three days) elapses between the bite of the mosquito and 

 the onset of the febrile symptoms, and a mosquito is not capable 



Fig. 4.— A Mosquito that spreads Yellow Fever, 

 Stegomyia fasciata, female, X 4. 



of infecting until about twelve days after it has sucked the blood 

 of a yellow-fever patient. These intervals of time are clearly 

 periods during which an organism is undergoing some parts of its 

 developmental process. 



Specimens of Stegomyia fasciata Fabr. are shown in the series 

 of mosquitoes on the West side of the table-case that stands 

 at the entrance of Bay I. The species occurs in tropical and 

 subtropical regions throughout the world, and is also found in 

 Southern Europe and other countries bordering the Mediterranean 

 Sea, and in the southern parts of the United States of America. 



c 2 



