FACTORS IN INFECTION OF ANOPHELES 
109 
gametes, the zygotes must next penetrate 
the stomach wall. This is a critical period 
in the life of the zygote, or ookinete as it 
is now called. While as yet we do not 
know the relative importance of the fac¬ 
tors operating against the penetration of 
the stomach wall by the ookinete, we can 
be fairly sure that the most important 
factors are intrinsic, that is, are connected 
with the chemical composition of the 
stomach wall or with the physiological 
processes therein. After penetration of the 
stomach wall the ookinete now proceeds to 
develop into an oocyst on the outside of the 
mosquito’s stomach. Both extrinsic and 
intrinsic factors may conceivably operate 
in favor of or against development in this 
location. Upon maturation of the oocyst 
and liberation of the sporozoites through 
bursting of the former, the parasite now 
runs the additional hazards of migration 
through the hemocoele to the salivary 
glands, and of penetration of the glands. 
Once the glands are infected with viable 
sporozoites certain factors, for the most 
part yet unknown, may operate for or 
against the preservation of the sporozoite 
in a viable condition. It will thus be seen 
that the hazards encountered by the para¬ 
site during its lifetime in the mosquito are, 
in complexity and importance, comparable 
to if not greater than those encountered in 
man. While the mosquito may be thought 
of as a somewhat simpler organism, it must 
be remembered that such environmental 
factors as temperature may have greater 
effect upon the parasite in it than in man. 
It should be noted that greater changes 
take place in the structure and biology of 
the parasite while in the mosquito than 
while it is in man. 
As soon as the mosquito theory of ma¬ 
larial transmission had been established by 
Manson, Boss and Grassi, there arose the 
task of determining what mosquitoes are 
responsible for transmission. As already 
indicated, many factors play parts in con¬ 
tributing to the success of a given mosquito 
in transmitting the malarial parasites. Let 
us assume for the moment that all other 
factors are conducive to transmission pro¬ 
viding the mosquito is susceptible to infec¬ 
tion. Susceptibility—or the lack of it— 
might then play the all-important part in 
determining whether a given mosquito may 
act as a vector. A great deal of effort has 
been spent in testing various species and 
varieties of Anopheles to discover whether 
they are susceptible to one or another of 
the malarial parasites, and it is clear that 
a great deal still needs to be done in this 
direction.’ Species sanitation as a tech¬ 
nique for control has arisen from the recog¬ 
nition that species differ in their abilities 
to transmit malaria, and at least some of 
these differences are known to be attribu¬ 
table to differences in the susceptibility of 
the mosquitoes to the parasites. Very little 
attention has been directed to the under¬ 
lying causes of susceptibility or non-sus¬ 
ceptibility, especially in Anopheles. 
While it was early known that species 
differed greatly in their susceptibilities, 
Darling (1910) clearly showed that indi¬ 
viduals within a susceptible species vary 
greatly in their capacities for becoming 
infected. When uninfected individuals 
had previously been discovered within lots 
of mosquitoes fed on malarial patients, it 
was often assumed that the failure to be¬ 
come infected is to be explained on the basis 
of an insufficiency of parasites in the in¬ 
fecting meal. When the heavy infections 
obtainable in avian malaria were used 
(Huff 1927), however, the fact was ines¬ 
capable that some individuals are refrac¬ 
tory to infection. Double feedings of the 
same mosquitoes on the same strain of 
malaria resulted, with few exceptions, 
either in failure to become infected at all 
or in infection with two broods of parasites 
(Huff 1930). Many of the malariologists 
who have tested species of Anopheles for 
susceptibility to human malaria have failed 
to indicate the degree of infection in the 
patient. Therefore, one cannot be sure 
that their lack of infections in mosquitoes 
was not the result of an insufficient dosage 
of gametocytes in the blood meals. How¬ 
ever, King’s (1929) results and a large 
mass of data published by M. F. Boyd and 
his coworkers from the Tallahassee station 
