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



251 



(1) Mosquito Larvae in Crab-holes. 



From the samples sent to the laboratory, 14 species have been hatched. The 

 following table gives the names of these and the number of occasions on which each 

 was found. 



Table I. 



Species of Mosquito. 





No. of times found. 



Percentage of total finds. 



Ochlerotatus irritans . . 







95 



34-6 



Culex decens* . . 



. . . 







43 



15-7 



Uranotaenia annulata 



. . 







41 



150 



Ochlerotatus nigricephalus 



. . 







22 



8-0 



Stegomyia fasciata 









20 



7-3 



Anopheles costalis 



. . 







20 



7-3 



Culex insignis 









13 



4-7 



Culiciomyia nehulosa 



. . 







9 



3-3 



Culex thalassius 









4 



1-4 



„ rima 



. . 







3 



M 



„ salisburiensis . . 









1 



•4 



Uranotaenia biUneata var. 



fraseri . 







1 



•4 



Stegomyia luteocephala 



. . 







1 



•4 



Micraedes inconspicuosus 









1 



•4 





■ 



Total 



274 





* Here and throughout this paper 0. invidiosus is merged in C. decens. 



The number of samples is sufficiently large to give fairly representative results, and 

 it may be accepted that 0. irritans is the species most often found. A further claim 

 to priority in this respect is that it is not infrequently the only species found, and that 

 it often hatches out in very large numbers from a single hole. Its occurrence in 

 this habitat may be confidently expected in any month of the year, though perhaps 

 the months of April, May and June have shown a preponderance of finds for this 

 species. 



Its congener 0. nigricephalus has the same seasonal range, but is far behind it in 

 numbers. The former uses many other breeding places, but the only other common 

 source for the latter has been surface pools, and once its larvae were found in a canoe. 

 Both are vicious biters and are commonly caught in houses. 



Culex decens is well entitled to the second place. Its larvae seem to be less hardy 

 than the others and are more difficult to hatch. The journey to the laboratory 

 from the sources of collection is about 6 miles, and the agitation of the bottles used in 

 conveying the larvae is perhaps more than some can stand. C. decens may be 

 found at any time of year, but samples in the months of January to April have shown 

 a greater proportion of this species. Its other habitats show almost as great a 

 diversity as those of S. fasciata, and the adults have been found in dwelling houses 

 in every month of the year. The third place on the list is taken by U. annulata, a 

 species which has also been found in each month of the year, but which seems to 

 have a special prevalence about January and February, when it often exceeds 

 0. irritans. Unlike the latter its adults are comparatively rare in houses except in 

 the immediate vicinity of some prolific nidus. Its other sources are small swamp 

 pools and wells. 



