16 



, In the influence of variations in temperature on the rate of multi- 

 plication, on the activity and on the duration of life of the mosquito 

 we have a satisfactory explanation of the peculiarities of seasonal 

 prevalence of the disease in endemic foci, of the decline of epidemics 

 with the advent of cool weather, and of their abrupt cessation on the 

 occurrence of a severe frost. The occurrence of cases even after a 

 killing frost is explained by the fact, already mentioned, that the 

 S. calopus is peculiarly a house mosquito, and it is for this reason 

 occasionally able to escape the full rigors of the climate. 



Aerial conveyance. — On this subject I can do no better than quote 

 Carter (1904), who has given it a great deal of attention. 



Although direct observations on this problem are few, yet there are certain indi- 

 rect ones bearing, however, entirely on the aerial conveyance of the Stegomyia 

 ■ infected with yellow fever. It is notorious that yellow fever is usually conveyed but 

 a short ways aerially ' ' across the street ' ' or more often " to the house in the rear, ' ' 

 which is about as far as it was expected to be thus conveyed. This represents a 

 distance of about 75 yards. The two longest distances recorded in recent times of 

 aerial conveyance, one of 225 meters (Melier) and one of 76 fathoms — 456 feet (the 

 writer) — are entirely exceptional. So much for the distance which the "infected" 

 Stegomyia is conveyed — or, rather, usually conveyed — aerially. 



On the other hand, it is known that vessels moored in certain districts of the 

 Habana harbor did not develop yellow fever aboard except in those who had been 

 ashore or unless they lay close to other vessels which were infected. . This experi- 

 ment has been made on so large a scale, with so many vessels, and for so many years 

 that we must accept as a fact that an infected Stegomyia was not conveyed aerially 

 from the Habana shore to those vessels, or, allowing for errors, was very rarely so 

 conveyed. The distance which had been found safe was something over 200 

 fathoms — 1,200 feet. The prevailing wind was generally slightly on shore, but was 

 not constantly blowing. Whether there is any difference in the distance to which 

 infected or noninfected mosquitoes are conveyed is, of course, entirely a matter of sur- 

 mise. There is no apparent reason why there should be. Yet the infected Stegomyise 

 have almost certainly become so in a house, and with their very domestic habits must 

 be found out of doors, where they would be subject to conveyance by the wind in 

 much smaller numbers than the uninfected insects, and consequently a lesser number 

 of them would be conveyed aerially. Observation is needed on this subject — the 

 distance (across water) that Stegomyise are aerially conveyed. 



Conveyance by railroads and vessels. — The yellow fever mosquito 

 may be conveyed from one place to another in the railway car. I cap- 

 tured one in a day coach en route between Donalsonville and Bain- 

 bridge, Ga., in August, 1905. My experience in traveling by rail, 

 both in Mexico and in the southern part of the United States, leads me 

 to believe that the number thus conveyed is very small, so that the 

 chance of conveying one that is infected is probably very slight. 



Distribution by vessels may not infrequently b.e observed. They 

 have been found on steam vessels, but much more commonly and in 

 greater numbers on sailing vessels, because the latter are more likely 

 to present easily accessible breeding places. It can hardly be doubted 

 that the outbreaks of yellow fever in such northern cities as Balti- 

 more, Philadelphia, New York, Boston, and Quebec were due to the 



