124 Dr Stephens, The Prevention of Malaria. 
parasite in Anopheles as Ross had previously done for birds. 
It need only be added that up to the present time it has not 
been shewn that any Gulex is capable of transmitting malaria. 
And further, only those mosquitoes ( Anopheles ) which contain 
sporozoites (i.e. the final stage in the development of the parasite) 
derived from a previous case of malaria are capable of trans- 
mitting malaria. Seeing then that Anopheles solely are concerned 
in the transmission of malaria, it becomes very important to know 
where they are found, and where they breed, and generally to 
study the habits of this genus. Much work has been done by 
Ross, by the Italians, by Koch, and by others on this point. We 
may give shortly here our results in tropical Africa. We in- 
vestigated the distribution in Freetown itself and in the surround- 
ing bush country. 
Freetown. 
In many well-lighted houses it was impossible to detect 
Anopheles when we visited them early in the morning, but 
it was quite possible that they had been present and had flown 
away to seek some dark retreat, so that we devised the method 
of making artificial pools of cement in the streets which were 
kept filled with water. In these Anopheles laid their eggs and 
the larvae hatched. By this means we detected the presence of 
this mosquito in many districts where search in the houses had 
been negative. 
The distribution of Anopheles in Freetown is in the main this ; 
it is dependent on the distribution of the water there, streams, 
drains and natural pools. Jn the huts and houses bordering on 
these the mosquitoes occur in large quantities, especially in the 
dark huts, but our pools shewed that Anopheles also lurked in 
the houses, gardens, and vegetation of districts remote from these, 
but in regions where, during the rains, pools had existed. 
Anopheles larvae constantly occurred in the rock pools of the 
hill-streams of Sierra Leone, though it was impossible to detect 
the larvae elsewhere in the stream. 
Adult insects were also caught by us occasionally when 
camping out in situations remote from human dwellings. They 
were very few, however, and often in the heart of the bush no 
mosquito of any kind was caught. 
In mangrove swamps Anopheles larvae are extremely difficult 
to find, but we have found them in a few instances at the edge 
of shallows. 
In the bush wherever it is unusual to meet with shallow pools, 
Anopheles are certainly uncommon, but there exists a set of 
conditions under which they may be found in quantity. Wherever 
in a clearing in the bush we find a collection of native huts it is 
safe to predict that here Anopheles will be detected by careful 
