SIGNIFICANCE OF STEGOMYIA FASCIATA IN WEST AFKICA. 230 



weeks in the holds, galleys, engine -rooms, or bunks of a ship. It is for this 

 reason that it is so essential to screen ships which trade in rivers in yellow 

 fever countries, or to insist that they shall be moored several hundred yards 

 from shore. 



With regard to the appearance of the mosquito itself, it is very readily 

 recognised. On the wing, it appears grey in colour, and it glides from point 

 to point just like a small bit of ' fluff.' On account of its colour and 

 markings, it is known as the ' Scots Grey,' or the ' Tiger ' mosquito. When 

 it alights, the two long banded white hind legs continually waving up and 

 down are very characteristic. 



As I have said above, this species is usually, though by no means always, 

 a clean-water breeder ; this habit is doubtless due to the fact that, being 

 essentially a house-frequenting mosquito, it naturally seeks out the water 

 nearest at hand, and this is, of course, the domestic drinking, cooking and 

 washing supply. It has therefore come to be known as a clean-water breeder ; 

 so much so, that its presence in water is taken as evidence of the good quality 

 of the latter. From this belief has sprang a further deduction, namely, that 

 the presence of the larva? in water is beneficial ; for it is supposed that they 

 feed on harmful bacteria, and therefore tend to purify the water. I have 

 made experiments to determine whether there is any truth in this belief, and 

 I have found, as indeed might be expected, that the contrary is the case, 

 and that water containing larvae becomes much more crowded with bacteria 

 than water without larvae*. The natural food of the larva? appears to consist 

 of minute algae. 



Stegomyia fasciata bites in the daytime as well as at night f, and in my 

 experience it is noiseless. When it has bitten a person sufiPering from yellow^ 

 fever in the infectious stage, the virus, whatever its nature, requires 12 to 

 13 days to mature in the body of the mosquito before the latter is capable of 

 transmitting the infection. This period is known as the '^ extrinsic incubation 

 period." When once the mosquito is infected, all evidence points to the fact 

 that it retains the innection for a very long period ; three months has been 

 noted, but it is quite [)ossible that the infection lasts as long as the liCe of the 

 mosquito, it is precisely because of the length of duration of the infection 

 in the Stegomi/ia that it is possible to explain the well known sporadic out- 

 breaks of cases of yellow fever, which frequently occur long after an epidemic 

 is supposed to have disappeared. There is lu) conclusive evidence that the 

 infected female Steyomyia transmits the virus to its eggs and larvae. 



* ^The effect of Mosquito larvae upon drinking water/ Boyce & Lewis, Annals of 

 Tropical Medicine k Parasitology, March 1910. 



t [Goeldi (' Os Mosquitos no Para,' p. 103) states that S. fasciata, in Parjl, bites pei- 

 sistently by day, and that while it does bite also at night, such cases are certainly 

 exceptional. — Eu.j 



