The fly population may be taken as the sanitary index of a place.

The discovery, too, that insects are porters of disease has led to a great extension of our knowledge of their life history.

Early in the nineties, when Dr.Thayer and I were busy with the study of malaria in Baltimore, we began experiments on the possible transmission of the parasites, and a tramp, who had been a medical student, offered himself as a subject.

Before we began, Dr.
Thayer sought information as to the varieties of mosquitoes known in America, but sought in vain: there had at that time been no systematic study.

The fundamental study which set us on the track was a demonstration by Patrick Manson,( 3) in 1879, of the association of filarian disease with the mosquito.