YELLOW FEVER 163 



nets and wire windows and doors are a sufficient 

 check on the access of Anopheles to man. If they 

 could only be kept permanently apart, we might hope 

 for the disappearance of the parasite from our fauna. 

 In relieving man from the pest, all lovers of animals 

 will rejoice that we are also relieving the probably 

 far more acute sufferings of one of the most delicate 

 and beautiful insects that we know. 



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. Like the Culex, 

 but unlike the Anopheles, Stegomyia has a hump- 

 backed outline, and its larva has a long respiratory 

 tube at an angle to its body, from which it hangs 

 suspended from the surface-film of its watery home. 

 It is a very widely distributed creature ; it girdles the 

 earth between the Tropics, and is said to live well on 

 shipboard. It breeds in almost any standing fresh 

 water, provided it be not brackish. The female is 

 said to be most active during the warmer hours of 

 the day, from noon till three or so, and in some of the 

 West Indies it is known as the ' day-mosquito.' 



The organism which causes yellow fever has yet to 

 be found. It 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 inoculating man 

 with the disease for nearly two months. The period 

 during which a man may infect the mosquito, should 

 it bite him, is far shorter, and extends only over the 

 first three days of the illness. 



Very careful search has hitherto failed to reveal the 



II — 2 



