21 I 
and keep these filled with water. After a certain 
number of daps they may contain young Anopheline 
larvae if the adults are present in the houses. (It is 
necessary to be sure one’s water supply does not 
contain young larvae or eggs). The absence of the 
larvae in the pools does not necessarily mean, however, 
that adult Anophelines are not present in the houses. 
3. In the conditions just described observe the 
pools made by the first shower of rain of the on-coming 
6 rains.’ Note after three days have passed the presence 
of larvae in many of these. The distribution of 
Anofhelines at the end of the dry season will usually 
be found to correspond to that of native huts. 
The Prevalence of Malaria 
If we proceed to ascertain to what extent malaria 
prevails in a district we may attempt to do so in 
several ways. 
1. We may consult hospital statistics and returns 
of death from malaria. This method is open to 
such grave error that it is extremely doubtful whether 
it is worth the labour bestowed upon it. 
2. We may determine to what extent en¬ 
largement of the spleen occurs. This method has 
been largely used. 
Precautions Necessary in Applying the 
Spleen Test* 
A he Age of the Individuals Examined. — 
The enlargement of the spleen due to ordinary 
malarial infection tends to disappear once the in¬ 
dividual has ceased to suffer from malarial infection. 
In very malarious countries, after childhood, the 
# In countries like India, where kala-azar exists, enlarged spleens from this and 
other causes may entirely invalidate the 1 spleen test ’ for malaria. 
