MALARIA EXPEDITION TO THE GAMBIA 
29 
Action of Tidal Water on Mosquitoes 
Experiments were undertaken to see how far mosquito larvae would thrive in 
tidal water; larvae of A. costalis of various sizes were taken and were placed in tidal 
water in large glass jars, and supplied with food. Various percentages of salt water taken 
from the beach at high tide were added, at the same time control batches of similar 
larvae were kept in vessels containing the water in which they were breeding. It was 
found that many or the young and medium-sized larvae died in six to eight hours in 
the jars containing seventy-five per cent, of sea water, below this percentage they 
remained alive. 
In one experiment in which larvae were placed in undiluted tidal water, one 
large larva remained alive and changed into a pupa in three days after the experiment 
started. A garden tub in which mosquitoes had been breeding was emptied and 
cleaned, and sea water, taken as the tide was coming in, placed in it, the other tubs in 
the garden being covered with mosquito netting. In four days afterwards a batch of 
small Anopheles larvae was discovered in the water, which subsequently hatched out 
into adult mosquitoes {A. costalis) seven days later. This experiment was repeated 
with the result that first eggs of Anopheles and also Culex appeared in one or two days 
after the tidal water had been placed in the tub, and subsequently adult insects 
hatched out from them. From these experiments it would thus appear that 
certain kinds of mosquitoes can breed in tidal water if it is not disturbed, 
and subsequently when the dry season had fully set in, I found larvae in suitable tidal 
pools, namely, as I have already mentioned, in the drains near the sluice gates in which 
tidal water had soaked in through the gates. In this water, which contained 1038-5 parts 
of chlorine per 100,000 parts, I found a few larvae of A. costalis and large numbers of 
Culex halassios. On another occasion, in December, I found these mosquitoes breeding 
in a small hole from which shells had been taken, close to the edge of the water at the 
mouth of Oyster Creek. The Culex were subsequently hatched out from this tidal water, 
but the Anopheles larvae were nearly all infested with a fungus (not identified) which gave 
them a woolly appearance, and I failed to hatch out any of them. I observed A. costalis 
breeding in a similar salt-water pool during a period in which neap tides occurred. 
The tidal water in an arm of the central channel in Box Bar, running from the sluice 
gates to the cemetery, had become converted into a series of small pools by partial evapo- 
ration of the water, though at every tide some water leaks through the sluice gates 
into this channel, but during this period it was not sufficient to replenish this small 
branch drain. Anopheles larvae were found in great numbers in these pools. It is 
interesting to note, also, that at this time two of the largest pools which were situated 
close by, and have been described as swarming with mosquito larvae, had been rilled 
in with sand. Samples of water taken from various parts of the town were examined 
for the amount of chlorine present in them, the result is given in the following table. 
