INCUBATION IN INSECTS. 81 



When the mosquitoes of malaria and yellow in 

 fever suck diseased blood a definite incubation insects. m 

 period must elapse before they are able to convey 

 infection to other individuals by their bites. Fol- 

 lowing the ingestion of malarial blood by the mos- 

 quito (various species of Anopheles), the parasite 

 undergoes a sexual type of proliferation in the in- 

 sect's stomach, the product of which, the sickle- 

 shaped bodies, reach the salivary glands only after 

 eight or ten days. These are the pathogenic forms 

 of the parasite which are inoculated into man by 

 the mosquito. In the intermediate stages of this 

 cycle the parasite is not infective. The mosquito 

 (Stegomyia) which transmits yellow fever is not 

 infective until at least twelve days after it has 

 sucked diseased blood. And it now appears also 

 that a similar incubation period occurs in the 

 tsetse fly in its role as the carrier of sleeping-sick- 

 ness. After the insects are once infected, however, 

 they continue virulent for a long time. Hence a 

 primary non-infective stage is succeeded by a pro- 

 longed infective stage in this type of transmission. 



On the basis of the conditions in malaria the 

 temptation has been a strong one to conclude that 

 a primary non-infective stage incubation period 

 in the insect carrier is indicative of a sexual 

 cycle in the development of the parasite; hence 

 also of the protozoan character of the parasite. 

 Thus it is anticipated in many quarters that the 

 micro-organism of yellow fever, at present un- 

 known, will prove to be a protozoan, because of 

 the incubation period in the mosquito, mentioned 

 above. Novy, however, has very aptly pointed out 

 that an incubation period in the insect might well 

 occur in the case of a bacterial disease as well as 



