MALARIA EXPEDITION TO THE GAMBIA 
l 9 
side of McCarthy's Square. In these drains I was never able to find 
mosquito larvae during the months of October, November, and the first 
part of December, although surface water from one-half to three feet 
deep was always present in them. There is no doubt that the absence 
of larvae in these drains was due to the large quantities of small fish 
which are continually swimming up and down the channels in small 
shoals of from thirty to sixty. The sides of the drains being perfectly 
smooth, no protection is afforded to the larvae. As the dry weather sets 
in the water gradually sinks lower and lower in the drains, until water 
is only present for a distance of twelve to fifteen yards from the sluice 
gates. This amount of water is present probably throughout the dry 
season, as it is tidal water which oozes into the drains through the sluice 
gates. In the process of drying-up, the fish die out or are killed 
when the drains are cleaned at the end of the wet season. The 
disappearance of the fish was followed by the presence of mosquito 
larvae in the water. At the end of December and in January I found 
innumerable quantities of mosquito larvae in the water in Hill Street 
drain, near the sluice gates, later, in Alglesea Street drain, and, just before 
1 came away, in Picton Street and Blucher Street drains. These mos- 
quitoes were principally Culex iba/assios, Culex duttoni, and a few Anopheles 
costalis. 
Shallower drains, made of brick or stone, either partly cemented or not at 
all, varying from one to one-and-a-half feet deep ; these drains either 
discharge at right angles to the larger drains or are continuations of 
them into the centre of the town. They vary with regard to their 
fitness to act as breeding-places of mosquitoes according to the seasons 
of the year. In the wet months they are probably free from mosquito 
larvae, owing to the amount of water passing along them, and thus the 
small fish would be able to swim up from the main drain. At the 
beginning of the rains and end of the wet season mosquitoes can breed 
in them. In October and November I found that after the rain small 
pools of water collected, owing to the unevenness of the cemented bed of 
the drain, after the main mass of water had passed away. This condition 
occurs in the drains of the following streets : Lemon, Hagan, and the 
town end of Blucher and Picton Streets. The other condition present 
in these drains, and of more importance than the irregularities in the bed 
of the drains, is the fact that many of these shallow drains are not com- 
pletely lined with cement, so that they allow the ground water to percolate 
into them. This water I found formed a very slow stream along the 
course of the drain, about one-half to one inch in depth, and was very 
