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CKAB-HOLES, TREES, AND OTHER MOSQUITO SOURCES IN LAGOS. 



By J. M. Dalziel, M.D., B.Sc. (Public Health), F.L.S., 



Senior Sanitary Officer, Gold Coast. 



The occurrence of mosquitos in crab-holes is tolerably well known both in West 

 Africa and in other countries. It is doubtful, however, whether these sources 

 receive sufficient practical attention, and one hears too often when their presence is 

 demonstrated the opinion expressed that the insects only shelter there and are not 

 likely to use them as breeding-places when so many others are available. One has 

 even found medical officers whose unbelief had to be assailed by demonstration of 

 the larvae and pupae. Others again, though aware of the fact, perhaps think the 

 occurrence of too trifling importance to justify the necessary steps to eradicate them, 

 even when near a dwelling. 



The following notes from Lagos seem to support the opinion that where it can be 

 shown that the imagines caught in houses are of the same species as those found as 

 larvae in crab-holes in the vicinity, an effort to eliminate the latter is called for even 

 at the expense of some labour and cost. In many cases the numbers issuing from 

 a very few holes are so great that even if the species are not those known to be 

 concerned in conveying disease, these efforts are justified if it can be shown that 

 the insects contribute appreciably to man's discomfort. Reasonable comfort is 

 one of the first conditions of health maintenance in the tropics, and the bites of 

 mosquitos, even of those described — perhaps only in our present ignorance — as 

 " harmless," are amongst the many little but important matters which prevent 

 both efficiency and enjoyment of life. 



The island of which the town of Lagos occupies the north-western portion is nearly 

 flat, with a tendency to a whale-back contour in the occupied portion. The highest 

 point is not much over 200 feet above lagoon level, and over the greater area water 

 would be found at any depth from 3 to 10 feet, even at a distance from the lagoon 

 margin. The soil is in the main sandy with areas of mud covered with coarse grass 

 on many low-lying stretches towards the water-side. Being entirely surrounded by 

 the comparatively still waters of lagoons and creeks its margins are at no point 

 exposed to the surf of the open coast, and the rise and fall of the tide has only about 

 a 2-foot range. The most familiar crab of the island is a land-crab, inhabiting in great 

 abundance the low grounds near the margins of the lagoon at any point of the island, 

 and travelling across country to any distance, restricted in the present instance by 

 topographical limitations at most points to several hundred yards. This crab is 

 Cardisoma armatum, Herklots, and is believed to be possibly identical witli the 

 West Indian C. guanhiimi. It belongs to the Family Gecarcinidae, land-crabs 

 common to the warm regions of both hemispheres. The carapace is entire. It is 

 very abundantly captured for food, being generally trapped on the pit-fall principle 

 by means of a kerosene tin sunk flush with the surface in a Hkely spot, but being 

 an inhabitant of filthy localities on Lagos Island its use for the table is not to be 

 recommended. 



