266 J. M. DALZIEL. 



Anopheles costalis takes a surprisingly high place amongst those obtained from 

 household sources, especially as pools within the compound fence are excluded. In 

 Lagos it can be reckoned upon to supply from 2 to 3 per cent, of the total finds 

 in house to house inspection and generally takes third place numerically. Graham 

 {loc. cit.), finding the average reaching 6 3 per cent, in February, a dry month, 

 suggests that the reduction of puddles out of doors tends to a relative increase 

 of Anopheles larvae in domestic receptacles. The present observations, however, 

 are against this, as the finds of A. costalis here recorded occurred chiefly in the wet 

 months. Thus out of 74 occasions, 52 occurred in the months June to September, 

 and 64 in the months May to October, inclusive. In the year 1916, when no samples 

 were examined in the months May to October, this species is entirely absent from 

 the list. In the Accra list quoted above, A. costalis similarly takes third place 

 amongst those obtained from domestic sources. Of domestic samples, weUs con- 

 tribute nearly half the number for this species in Lagos, viz., 35 out of 74. The 

 water-pot filled at the well may thus receive the larvae, but there is little doubt 

 that direct use is made of pots, tins, etc., within the house. On two occasions the 

 larvae were found in agbo pots, and on six, in catch-pits ; drain, roof -gutter, banana- 

 tree stump, and even a coconut shell furnished occasional samples. In the present 

 series none occurred in tanks and barrels which receive rain-water from roofs. 



The larvae of A. mauritianus were found once only in the roof-gutter of a 

 European's residence. 



The electric cat-fish (Malapterurus electricus) is not infrequently kept in 

 domestic water-pots by the people of Lagos, its presence being supposed to preserve 

 the purity of the water. Although it tends to act as a deterrent to the sanitary 

 inspector, and therefore to neglect of cleansing the utensils, it certainly feeds on 

 mosquito larvae, as was proved by experiment at the Research Institute at Yaba. 



Collections of Adult Mosquitos. 



The native sanitary inspector, whose task is the thorough and regular inspection 

 of a daily block of compounds, is not a suitable person to rely on for collection of 

 mosquitos in native houses, unless this duty is allotted to a definite individual 

 relieved of other duties for the time. Although considerable numbers have been 

 obtained and identified at one time or another, they have usually been collected 

 either by the medical officer of health himself or by the European sanitary inspector, 

 and few of these collections have been representative of the native town. Relative 

 frequency of different species is greatly affected by locality and time of year. Thus 

 in some low-lying streets in Lagos town a collection taken in the rains would 

 certainly contain an overwhelming proportion of A. costalis. On one occasion 

 a collection made by a sanitary student resident in Ebute Metta (on the mainland) 

 consisted almost entirely of Mansonioides africanus, which is a rarity in Lagos 

 town proper. 



Attempts were occasionally made to interest the members of the class of sanitary 

 students in training to coUect adult mosquitos in the premises where they lodged, 

 and cyanide tubes were issued for the purpose. The results were below expectation 

 and the youths soon tired of the good work. The following, however, gives the 

 result of one such efiort representing native houses in different parts of the native town. 



