XI] DEVELOPMENT IN MOSQUITO 187 



with an equal quantity of water the virus is even capable of 

 passing through a Chamberland Filter B, 0-033 c.c. of the 

 filtrate producing a typical attack when injected into a non- 

 immune person. 



The virus is extremely fragile, for a drop of infected blood 

 may be placed on an excoriated part of the skin with impunity, 

 as the slightest desiccation destroys the disease agent. More- 

 over, it is very susceptible to heat, for at a temperature of 

 55° C. it loses its virulence within five minutes. 



Development within the mosquito. Under ordinary cir- 

 cumstances when a Stegomyia has fed on the blood of a yellow 

 fever patient during the first three days of an attack, it 

 becomes infective after an incubation period of about 12 days. 

 The conditions of development within the mosquito are very 

 uncertain, however, for it often happens that mosquitoes remain 

 uninfective, even though fed on infected blood. 



The effect of temperature is very important and probably 

 accounts to some extent for the restricted range of the disease. 

 If a mosquito is kept at a temperature of about 22° C, instead 

 of becoming infective 12 days after an infected feed, the incu- 

 bation period is prolonged to three or four weeks. At 20° C. 

 the virus is incapable of developing within the mosquito and 

 thus the latter does not become infected. 



If an infected mosquito, capable of transmitting the disease, 

 is exposed to a low temperature, it ceases to be infective, but 

 recovers this power on again being warmed. It is possible 

 that changes in temperature alter the virulence of the disease, 

 for during the cool season the cases of yellow fever are often 

 benign, the most severe ones occurring during the hot weather. 



Under certain conditions it seems that the infection may 

 be transmitted to the offspring of an infected mosquito. 



Marchoux and Simond kept a female Stegomyia, which laid 

 its eggs 16 days after an infective feed. These eggs were kept 

 at a temperature of about 28° C. and developed in a com- 

 paratively short length of time into the adult insects. The 

 latter were found to be infected and produced a typical attack 

 of yellow fever when fed on a non-immune subject. Although 

 no one else has succeeded in repeating this experiment, the 



