INSECTS AND ENTOMOLOGISTS 215 



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 dis- 

 charging 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 prac- 

 tically 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, 



Fig. 6. A rabbit flea. 



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 dis- 

 eases have been largely records of studies in insect transmission of * 



