NA TURE NO TES 7 7 



to the disease, and perish in great numbers. They are active agents in the 

 spread of cholera ; and the history of the late wars definitely shows that flies 

 play a large part in carrying the bacilli of enteric fever from sources of in- 

 fection to the food of man, thus spreading the disease. 



The diseases already mentioned are caused by bacteria. But flies also 

 play a part in the conveyance of a large number of organisms which are not 

 "bacteria but which, nevertheless, cause disease. In considering the part 

 played by flies in disseminating diseases not caused by bacteria, we can neglect 

 all but a very few families, those flies which suck blood having alone any 

 interest in this connection. From the point of view of the physician by far 

 the most important of these are the gnats or mosquitoes. 



The parasite which causes malaria is conveyed by an insect, and, so far 

 as we know, by one genus of mosquito only, the Anopheles. The malarial 

 parasite lives in the blood-cells of man, but at a certain period it breaks up 

 into spores which escape into the fluid of the blood, and it is at this moment 

 that the sufferer feels the access of fever. Their presence and growth within 

 the blood cells result in the destruction of the latter, a very serious thing to 

 the patient if the organisms be at all numerous. If the spores be sucked up 

 by an Anopheles, they undergo a complex change, and ultimately reproduce 

 an incredible number of minute spores, each capable of infecting man 

 again if it can but win entrance into his body by a mosquito bite. 



Another elegant little gnat Stegomyia fasciata, closely allied to Culex with 

 which until recently it was placed, is the cause of the spread of that most 

 fatal of epidemic diseases, the yellow fever. The organism which causes 

 yellow fever has yet to be found. Jt seems that it is not a bacterium, and 

 that it lives in the blood of man. It evidently passes through a definite 

 series of changes in the mosquito, for freshly infected mosquitoes do not at 

 once convey the disease. After biting an infected person it takes twelve 

 days for the unknown organism to develop in the Stegomyia before it is 

 ready for a change of host. The mosquitoes are then capable of inoculat- 

 ing man with the disease for nearly two months. The period during which 

 a man may infect a mosquito, should it bite him, is far shorter and extends 

 only over the first three days of the illness. 



Of the great obstacles which have for generations succeeded in keeping 

 Africa, except at the fringes, comparatively free from immigrant3, thrre, and 

 these by no means the least important, are insignificant members of the order 

 Diptera. 



The third fly we have now to do with is the tsetse fly (Glossina), which 

 communicates fatal diseases to man and to cattle, and to domesticated animals 

 of all kinds. The members are unattractive insects, a little larger than our 



