So far as can be determined, life goes on in the usual way, and its duration in the insect is not shortened.
The nature of the parasite which produces yellow fever is unknown, for it belongs to the filterable viruses; the infectious material, however, has been shown by inoculation to exist in the blood, and the disease is transmitted by a mosquito of another species, the _stegomyia_.

The development cycle within this takes a period of twelve days, which time must elapse after the mosquito has bitten before it can transmit the disease.

Here again the mutual interdependence of knowledge is shown.

Nothing could have seemed less useful than the study of mosquitoes, the differentiation of the different species, their mode of life, etc., and yet without this knowledge discoveries so beneficial and of such far-reaching importance to the whole human race as that of the cause and mode of transmission of malaria and yellow fever would have been impossible; for it could easily have been shown that the ordinary _culex_ mosquito played no role.

The role which insects may play in the transmission of disease was first shown by Theobald Smith in this country, in the transmission by a tick of the disease of cattle known as Texas fever.