THE DOMESTIC MOSQUITOS OF ACCRA. 1(37 



The monthly proportions in which larvae of the three types — Anopheline, 

 Stegomyia, and other Cuhcine — have been found in the samples sent to the laboratory 

 from all sources during the years 1912 to 1915 are shown in Table III. It cannot be 

 said that any clear indication is given of seasonal variation, unless it is a diminution 

 in the numbers of both Stegomyia and other Culicine mosquitos during July and 

 August, that is just after the height of the rains. The very small number of Anophe- 

 line larvae identified during these four years indicates that their occurrence in the 

 native compoimds was incidental, as, indeed, might have been expected. 



Table IV, which shows the distribution of the samples containing Stegomyia larvae 

 during the year 1915, might also have given some evidence on this point. It records 

 indeed a very marked falling off in the number of the samples from July onwards, 

 but there does not appear to have been any condition associated with the rainfall, 

 the temperature, or the humidity of the atmosphere (see Chart I) that would explain 

 why a large number of larvae should have been found from December to June, but 

 very few from July to November. On the whole it is probable that this diminution 

 in the number of specimens submitted for examination was due to causes unassociated 

 with the habits of the mosquitos, and with this exception the Table does not seem to 

 show anything that might be interpreted as seasonal variation. 



There is no great variation in temperature or humidity at Accra during the year, 

 and domestic mosquitos are probably independent of the rainfall, since they breed 

 in collections of water provided for them by the hand of man. There seems in fact 

 to be no real reason why there should be any seasonal variation in the distribution of 

 such mosquitos, and so far as the evidence here collected goes, there does not appear 

 to be any such variation. 



Other factors must of course be taken into consideration, such as a possible seasonal 

 variation in the food supply of the larvae or in the numbers of their natural enemies. 

 Such factors probably have but little influence on such dominant species of domestic 

 mosquitos as Stegomyia fasciata and Culex fatigans. Stegomyia fasciata has been 

 stated to breed mainly in clean water, and the presence of these larvae has even been 

 regarded as a proof of purity. This popular error has been exposed by Boyce, and it 

 may be mentioned that very many of the samples found at Accra were thriving in 

 exceedingly impure fluids containing food refuse and various kinds of animal and 

 vegetable debris. Neither S. fasciata nor C. fatigans is dependent on any particular 

 species of algae for its diet. In the laboratory larvae of both these species have been 

 bred in broth media which were sterile at the start, but of course became in a very 

 short time rich cultures of bacteria. Provided the medium did not become actually 

 foul-smelhng, when the larvae perished, the richer it was, the larger (within narrow 

 limits) the mosquitos that were bred from it. Few, if any, of the samples collected 

 in the native compounds contained natural enemies visible to the naked eye ; but 

 Vorticellas, cihates, flagellates, and other microscopical organisms were frequently 

 seen when examining the larvae. Whether any of these organisms, or of the various 

 bacteria found in water, are pathogenic or not seems to be uncertain. Vorticellas 

 certainly accumulate in considerable numbers on larvae whose development has been 

 delayed, and ciliates are usually found infesting the corpses of larvae that have died, 

 but in any case it is unhkely that they would determine a seasonal variation in the 

 mosquitos, since they are found in suitable samples of water all the year round. 



