observation the entire life-cycle of the parastic organism from the time it enters the circulation through the beak of the mosquito until it reaches its limit of growth in the human body. We have observed the blasts and sporozoits entering a red blood corpuscle; we have observed the gradual growth in this corpuscle; we have followed the gradual breaking down of the cells and have observed their rupture and the discharging spores. The development of the gametes has been observed; the flagellation and conjugation of the micro-and macro-gamete and the development in the Anopheline stomach from vermicule to zygote. All this has been demonstrated, and so little guess-work is there about it that the elimination of Anopheles breeding places is now the first step in dealing with an outbreak of real malarial trouble.
The relationship between rats, fleas and plague has also been practically established, and elimination of the disease in man is sought by the destruction of rats. I have already mentioned, in another connection, that the cat and dog flea would, when opportunity served,
bite human beings; the statement may be made more generally that almost any flea will, when there is occasion, bite almost any warm blooded animal upon which it finds its way. So the fleas upon plague stricken rats, leaving their natural hosts, may and do infest men instead of other rats when men herd where rats abound.
In more torrid or tropical countries insect-borne diseases are more numerous than in our more temperate clime, and there the mosquitoes, bearing a much greater variety of fevers, also transmit organisms of much higher character than "germs." Filariasis is a disease caused by minute thread-worms or filaria, and these also require the mosquito as an intermediate host to complete their development.
In recent years the records of studies made of certain tropical diseases have been largely records of studies in insect transmission of
