252 
THOMPSON YATES LABORATORIES REPORT 
but it is essential that it be very efficiently and thoroughly carried out ; and that the 
breeding-places should be intelligently searched for. 
Further, there are, no doubt, other conditions to which a drainage system 
could be applied, which have not been included in these remarks, and perhaps others 
which have not been encountered by us. 
II. Destruction of Larvae by the Use of Culicicides 
The general use of ' culicicides ' as a really efficient method is impracticable. 
From observations already made, it is clear that the very varying conditions under 
which Anopheles breed do not permit of the general application of any substance which 
will destroy larvae or prevent their development — applicable either regularly to the 
surface, as kerosene, tar, etc., or as a substance, such as lime, as has been suggested, 
which, once introduced, might render puddle, pool, or other collection of water use- 
less for breeding purposes in the future. 
Such a duty — in the carrying out of a general practical measure — as the dis- 
covery and treatment of Anopheles breeding-places, could not be relegated to a native 
official or number of officials, even under the supervision of a European sanitary 
officer. The varying and unlimited conditions under which Anopheles do and may 
breed, require the constant attention of one with some special knowledge and previous 
experience. 
In the report of the previous expedition, the use of ' culicicides ' was advocated 
for extensive trial in Sierra Leone, as a result of the observations of the expedition 
during their visit. And this serves as a striking example of the variability of the 
conditions at different seasons of the year, for on the visit of the members of the 
Royal Society's Commission in the dry season, those conditions for which the treat- 
ment was suggested had disappeared, and a new set, producing innumerable breeding- 
places, had arisen for which, in the opinion of the members of the commission, the 
operations previously suggested were not applicable. 
Under certain rare circumstances, apart from any general consideration, the 
use of ' culicicides ' would have to be resorted to as the only method of treatment. 
Kerosene regularly applied appears to be the most reliable of a number of 
proposed culicicidal re-agents : the cheap ideal substance which will have the effect 
of rendering pools permanently uninhabitable for the larvae is still undiscovered. 
From a number of experiments performed by us in West Africa with various sub- 
stances, we are not able to recommend any such substance with the desired properties ; 
further, we deem lime and gas lime, which have been suggested as a possible ideal 
re-agent, of no value for the purpose. 
Our attention having been directed to a statement in the medical journals 3 that in 
the country in the neighbourhood of Lake Chad, no mosquitoes occur, and 
