358 
MALARIA 
malaria, reduction, naturally only the basic 
principles have been mentioned. It should 
be borne in mind that every Anopheles con¬ 
trol problem is a new one and in each case 
a study of all factors affecting production 
should be made. Analysis of the data will 
indicate to a competent malariologist which 
measures are most appropriate. Usually a 
combination of methods is required. 
Occasionally it develops that malaria con¬ 
trol by anopheline reduction is not prac¬ 
ticable. Rural areas are often in this class. 
It is regrettable that efficient, practical, 
cheap malaria control has not yet been de¬ 
veloped for such use; for here the need is 
greatest. 
In connection with Anopheles control 
activities it must be borne in mind that 
complete elimination of the vectors is un¬ 
necessary, for below a certain minimum 
density malaria will disappear of its own 
accord. The minimum number of vectors 
necessary to perpetuate malaria depends 
on many factors and no formula is available 
from which this number may be deter¬ 
mined. Suffice it to say that in the case of 
A. quadrimaculatus, reduction to the point 
where only occasional adults are found in 
the suitable diurnal shelters, is effective. 
Desirable Future Studies Affecting 
Anopheles Control 
Basic measures for the control of Anoph¬ 
eles production were developed by Howard 
and by Ross and were first applied on a 
large scale during the construction days of 
the Panama Canal. Most of the ensuing 
study has resulted mainly in a refinement 
of measures there used. The lining of 
ditches, while new in the United States, 
was done at Panama more than 20 years 
ago and by the British in the Federated 
Malay States over 30 years ago. 
Dust larvicides and the pyrethrum lar- 
vicides represent recent developments, but 
do not open any new or basic approach to 
the problem. Likewise, screening recently 
developed to a high degree for rural com¬ 
munities in the United States, was recog¬ 
nized as a valuable malaria control method 
in the construction days of the Panama 
Canal. 
The gross biology of anophelines is 
known but the minimal factors necessary 
for rapid production require detailed 
study. It is probable that a careful study 
of such factors as chemical content of the 
water, plankton, daily and seasonal changes 
in pH of the water, and associated phe¬ 
nomena either singly or collectively, may 
offer an answer to the question, Why do 
Anopheles breed in some waters while 
others, apparently similar, never produce 
them? 
Further investigation may disclose the 
operation of some factors during larval 
development profoundly affecting the ca¬ 
pacity of the adult insect to develop or 
transmit plasmodias. 
It is also desirable that there be studies 
directed at the possible control of anoph¬ 
elines without seriously interfering with 
other forms of water utilizing life, upset¬ 
ting as little as possible the delicate bio¬ 
logical balance set up by nature in all 
water deposits. 
