CRAB-HOLES, TREES, AND OTHER MOSQUITO SOURCES IN LAGOS. 255 



low-lying and water-logged state of considerable areas of the island. It is doubtful if 

 at any time of the year the female Anoj)heles finds it necessary to enter the shadow 

 of a crab-burrow for purposes of egg-laying, at least to a distance beyond the range 

 of ordinary daylight. War against the crab will therefore not be expected to have 

 more than a subsidiary effect on the reduction of Anopheles, which must be dealt 

 with by larger measures of drainage, oiling, etc., in view of the very numerous 

 facilities afforded to that species on Lagos Island. 



Apart from S. fasciata and A. costalis, both carriers of Filaria bancrofti, the crab- 

 holes do not appear in the present instance to be of particular importance in con- 

 nection with filarial disease. 



In addition to supplying an extra and almost domestic source of S. fasciata (in 

 itself a factor of sufiicient importance) and to the possibly noxious — though still un- 

 discovered — quality of some of the other species to which crab-holes afford unlimited 

 breeding-places, there remains anadequate reason in Lagos for the eradication of crab- 

 holes in the mere fact of the numbers of mosquitos which they supply, of types 

 which have been demonstrated as entering houses and preying upon human beings, 

 even if they should be of secondary importance in relation to the spread of endemic 

 diseases. It is quite a common occurrence for the sanitary gang to bring to light a 

 sample of water from a single crab-hole containing many hundreds of larvae in all 

 stages, and the ocular demonstration of the same to the occupier of adjoining premises 

 has at times been adopted in the interests of propaganda, or to convince a complainer 

 that the seemingly interfering methods of the workers are in reality well directed. 

 From such a source when undetected amongst grass or under fohage, perhaps within 

 a few feet of a dwelling-house, there may issue scores of mosquitos daily and there 

 seems nothing to prevent this continuing for weeks or months as long as the burrow 

 is occupied by the crab. There is unfortunately nothing to suggest that the land- 

 crab includes mosquito larvae in its diet. 



A desirable consummation would be to keep the crab at a distance and prevent its 

 access to the neighbourhood of dwelHngs ; but with a creature which exists in such 

 numbers on the neighbouring sea-margin and which travels purposefully and with 

 ease overland, up drains, etc., this seems at present impracticable, and some of the 

 largest burrows may be found alongside a solid sea-wall or cement drain. 



Species of Mosquitos found in Tree-holes. 



Commencing in the rainy season of 1915 a regular search for breeding-places in 

 trees was carried out. A ladder was used and sometimes a syringe with flexible 

 rubber tubing. In a few months most of the trees in the public streets were dealt 

 with, as well as many in private compounds. Hollows in tree-forks and holes in 

 decayed wood or in cut branches were treated either by cement or other material, 

 or by cutting out so as to prevent accumulation of water. In later seasons, so far 

 as trees in public places are concerned, only an occasional routine inspection was 

 required, but doubtless in private premises many escape attention. On only two 

 occasions was a second find made in the same tree. 



In 1915, May to October, mosquito larvae were found on 43 occasions in 15 different 

 species of tree or plant. For 1916 there is no record, but in later years others were 

 (713) e2 



