40 
REPORT OF THE MALARIA EXPEDITION. 
27. Operations for Reducing the Numbers of Anopheles. — Such operations will be 
divided into : — 
(i.) Destruction of adults or larvae. 
(2.) Measures to prevent the insects breeding. 
Wliere the larvae cannot be found — as at Wilberforce — destruction of the adults can be 
resorted to. The gorged females, that is, the insects which are particularly liable to be dangerous, 
can often be killed while seated asleep in the daytime on the walls of a house. The same thing 
may be done on ships ; and it is quite possible that many infections may be prevented in this 
manner. Native servants become very expert at the work. But, of course, such a measure 
will not be possible on a large scale. 
The most vulnerable point in the history of gnats is when they are larvje ; they can then 
be destroyed wholesale. In the case of larvae in vessels, it suffices simply to pour out the water 
on the ground ; but it is more difficult to deal with larvae in pools, drains, cisterns, wells, streams, 
drinking fountains, and rice-fields, etc. Even here, however, very simple measures may often 
suffice. 
Thus small puddles can be hrushed out with a broom, the larvae dying on desiccation if 
the operation be done in dry weather. We judged that in Freetown 80 °/o of the puddles could 
be treated in this manner with little labour. 
Other pools can often be evacuated^ or filled up with earth by means of a few strokes of 
the spade. The former, however, may be impossible in perfectly level country. 
Where these simple measures are impracticable recourse may be had to " culicicides" The 
most popular of these is kerosene oil (paraffin). We found that this can be most easily applied by 
means of a rag fixed on a stick. In this manner a number of puddles can be " painted " in a few 
minutes at the cost of but little oil ; but enough oil must be used to make a fairly permanent film. 
Aaron states that for five dollars sufficient oil can be purchased to make five applications to 
100 acres of water [12]. Strachan recommends olive oil mixed with a very small proportion of 
turpentine, especially for cisterns of drinking water [27]. Nuttall quotes writers as saying that 
copperas, and even rusty iron, are culicicides ; certainly larvae are not commonly found in iron 
vessels, except when water runs into them — as from a tap. Numerous other culicicides are 
suggested by the work of Celli and Casagrandi [26]. 
It is obvious that by these means we can destroy a large number of larvae at any time ; 
and it is even possible that by repeatedly dealing with every pool in a given locality we may 
ultimately be able to exterminate, or almost exterminate, the insects in that locality. But there is 
one serious difficulty. Such operations carried out on a large scale will require constant care, 
involving a great deal of trouble and some expense ; while untrustworthy agents will be very 
likely to neglect their work. This will especially be the case if the Anopheles pools are very 
numerous. It will therefore be generally preferable to deal with the insects by preventing them 
breeding altogether. 
Two ways of doing this suggest themselves — 
