CRAB-HOLES, TREES, AND OTHER MOSQUITO SOURCES IN LAGOS. 261 



The question of salinity in water samples harbouring mosquito larvae complicates 

 the estimation of true prevalence of different species, some being more sensitive 

 than others to variations of this nature. 0. sitiens appears to be a true salt-water 

 .species, whereas C thalassius is very adaptable, and though breeding abundantly 

 in brackish marshes and lagoons such as those at Bathurst, Accra and Lagos, it is 

 quite commonly found in perfectly fresh water. 0. irritans seems to breed in 

 brackish pools in the Accra lagoon with a salinity of 1 '4 per cent, chlorine ( = 2 '2 

 per cent. NaCl). The following observation, however, suggests the need for dis- 

 crimination as to the actual conditions under which larvae will not only remain 

 alive, but proceed to full development. Very numerous larvae were obtained 

 from two crab-holes close to the edge of the Lagos lagoon. The water was salt 

 to the taste, with a salinity equivalent to 2 '31 per cent. NaCl. Part of the sample 

 was diluted with fresh water, while the rest was kept as obtained. From the 

 former, 0. irritans hatched out abundantly for several days ; from the latter 

 numerous imagines of the same species hatched on the afternoon of the day on which 

 the sample was collected, and on the following two days only, after which no further 

 ■development of the numerous remaining larvae occurred. It is probable that the 

 •crab-holes, originally containing comparatively fresh water, had been filled by 

 a tide, and that the mosquitos which hatched on the first two days were those 

 which had already pupated or which were ready to pupate at the time the sample 

 was taken. In such a situation the water in a crab-hole may show many fluctua- 

 tions in degree of salinity and tonicity, with their accompanying alternations of 

 effect on the development of insect larvae, retarding the immature, but probably 

 hastening the pupation of those far advanced.* 



Mosquitos breeding in Agbo Pots. 



The herbal infusions known as agbo are found in most native houses, especially 

 where there are young children, and the receptacles are commonly concealed to 

 escape the weekly inspection. They are very numerous, and sometimes the pots 

 are of large size. On disturbing an agbo pot or calabash of long standing, the 

 number of insects which escape is almost incredible, and in many cases the fluid 

 is a seething mass of Culicid and Psychodid larvae and pupae, along with the exuviae 

 of their innumerable predecessors. From 450 samples of larvae from agbo the 

 following species were obtained : — 



* Keference may be made to salt-water species occurring elsewhere : — Acarfomyia 

 (Aedes) zammittii, seems to live only in pools left after high water (Dardanelles). Stego- 

 nocops sp. and Aedes fluviat His {Culex Jluviatilis, Lutz) occur in rock-pools with concen- 

 trated sea-water (Panama). Culex sollicitans. Walk., is almost confined to salt pools 

 near sea-beaches (Howard, Dyar and Knab, " The ]\Iosquitoes of N. and Central America 

 and the W. Indies," i, p. 150). C. sollicitans AY a\k., occurs in the mud of salt marshes 

 (India, Karachi). Culex (Aedes) cantator, Coq., C. taeniorJii/nchus, Wd., C. salimirius, Coq., 

 with C. sollicitans, are found in the coastal salt marshes of America (Connecticut, etc.). 

 Species of Taeniorhynchus and Anopheles occur in brackish marshes and mangrove 

 swamps, and Anopheles subpictus (rossi) in brackish wells and salt-water tanks (India). 

 A. [Pseudomyzomyia) ludlowi, a malaria carrier in the Andamans, breeds in salt swamps 

 and mud flats. Anopheles multicolor. Camb. {Pyretophorus sp.) was found in Egypt in 

 water with 2-56 to 3-25 per cent. NaCl (Willcocks, Ann. Trop. Med. Paras, iii, 5, pp. 

 586-587.) 



