CASE OF PETROPOLIS 269 
Furthermore the French commission admit that in capturing mosquitoes that 
came to bite in the daytime they obtained “un nombre trés considérable” (a 
great number) of Aédes calopus. However they try to demonstrate, in accord- 
ance with their contention, that these female calopus coming to bite in the day- 
time were all individuals but a few days old. They adopt as a criterion of age 
the condition of the vestiture and state that it is rare that a calopus more than 
15 days old is not much denuded. In fact, under normal conditions, calopus 
frequently retains its vestiture a long time. Denudation depends upon circum- 
sances in the insect’s life and we can not accept it as a criterion of age. 
We have already referred to the case of Petropolis, the inhabitants of which 
were said to escape yellow fever in their diurnal visits to Rio de Janeiro. It may 
almost be said that the French commissicn obliged the mosquitoes to respect the 
itinerary of the boats to Petropolis and abstain from the molestation of the 
passengers between their arrival at 9 a. m. and their departure at 4 p.m. We 
see the case of Petropolis in an entirely different light. There have been no 
epidemics of yellow fever at Petropolis for the simple reason that the average 
night temperature is unfavorable to the development of calopus, as the French 
commission themselves have shown. On this account Petropolis has been a 
health resort of the wealthy, native and foreign, during the hot months. 
As to the apparent impunity with which the inhabitants of Petropolis could 
visit Rio de Janeiro during yellow-fever epidemics, this was probably due to a 
variety of circumstances, not evident, but acting together. It will be safe to say 
that most of the inhabitants of Petropolis who spent the business hours at Rio, 
whether natives or foreigners, had previously lived elsewhere in Brazil. In the 
course of such residence they could not have failed to have had yellow fever and 
thus acquired immunity. On the other hand a large proportion of the employees 
of the business houses at Rio were non-immune foreigners who were obliged to 
live in the city for pecuniary reasons. It was among these that yellow fever was 
most in evidence. With the immunes, as the French commission have shown, 
immunity is strengthened and prolonged by the bites of infected mosquitoes and 
there is no reason to doubt that this process was in operation with the business 
men from Petropolis. That those who spent a single night in Rio became in- 
fected was not the rule. The notoriety of such cases is to be explained by the 
circumstance that the person attacked was generally a member of the foreign 
diplomatic service. Customarily the diplomatic corps did not leave Petropolis 
during the months of yellow-fever epidemic; if, by chance, one of them was 
obliged to visit Rio and remain over a single night he probably became infected 
in the early morning when calopus is most aggressive. 
The fact seems to be well established that non-immunes from Petropolis, when 
visiting Rio de Janeiro in the daytime, escaped infection. This is easily ex- 
plainable in that most of the mosquitoes would already have obtained their meal 
of blood before the arrival of the visitors from Petropolis, having had fully five 
hours of daylight to accomplish their purpose. Such mosquitoes as would bite 
later in the day would be mostly fresh individuals that had not obtained their 
first meal and were therefore harmless. The members of the second American 
