CRAB-HOLES, TREES, AND OTHER MOSQUITO SOURCES IN LAGOS. 249 



The method first employed was to pour down the hole a disinfectant such as 

 cyllin, of strength about 1 in 8 or 10, with either cold or boiling water. The latter 

 was found to be more effective in kilUng the crab and had the further advantage 

 that cheap tar or crude creosote could be used. The boiling of the water on the 

 spot, and the finding of dry fuel in rainy weather, however, involved so much loss 

 of time and waste of efiort that a simple solution of cyllin or Kerol came to be 

 preferred for use on a fairly extensive scale. (It may be said, however, that for 

 private owners or occupiers troubled with a few crab-holes large enough to breed 

 mosquitos, the occasional use of a bucket or two of boiling water mixed with 

 Stockholm tar is a tolerably effective procedure. The crab either perishes in the 

 hole or is dealt with in attempting to escape. ) 



From 7th to 31st May 1914, 7,250 crab-holes were treated, and 1,805 from 1st to 

 15th June ; total 9,055. After the middle of June on the other hand an accurate 

 count was not attempted, as the rainfall brought the water near the surface, and in 

 parts of the GoK-course area many holes lost their individuality in open pools and 

 marshy spots. In fairly well-drained compounds they ceased to be found in more 

 than small numbers where they had abounded a few weeks previously, and in many 

 of these a periodic visit to deal with new holes was all that was required thereafter. 

 As the number of compounds was considerable, and as crab-holes formed only one 

 source of the mosquito trouble, it was found desirable in practice to make this work 

 part of a regular routine, and as no means has so far been found to prevent a fresh 

 access of crabs a certain amount of labour devoted to these continues to be necessary. 

 The above figures therefore indicate the tentative work of the first few weeks. 



After the first efforts it was found that in a proportion of cases the crab survived 

 and the hole reappeared in a day or two. Thus in a series of compounds 1,045 

 crab-holes were dealt with by a first treatment with a disinfectant followed by 

 obliteration of the hole. Of these 260 or 24*8 per cent, were found on a second visit 

 to have reappeared. After a second treatment of these still a few persisted, and 

 some of them had to be dealt with by other methods. In one compound situated 

 within 100 yards of the lagoon edge, where various conditions were particularly 

 favourable to the crab, over 1,000 holes were dealt with in the course of several visits. 



An objection to the use of tar or disinfectants in private compounds was found to 

 be discoloration of grass or injury to plants, etc., and eventually the practice adopted 

 for general use was simply to dig out the crab. Even by this method the creature 

 occasionally defied discovery, either being absent, or, as proved by observation, 

 having retreated to another gallery beyond the watery pocket at the bottom of the 

 hole. 



The depth to which it is necessary to dig depends on the time of year and on local 

 circumstances determining the distance of the water from the surface. The burrow 

 is naturally oblique in direction, and a vertical depth of 3 feet has sometimes to be 

 dealt with, the deepest measured being one or more in which larvae were recovered 

 at nearly 4 feet from the surface. One such hole at the depth of 4 feet was followed 

 for a further distance of 2 feet in a horizontal or slightly upward direction. It is 

 obvious that mere obliteration of the hole without destruction of the crab is waste 

 of time, except in virtue of removal for the time being of one mosquito breeding 

 place, soon to be renewed. 



