OBSERVATIONS OX THE BIONOMICS OF STECJOMYIA FASCIATA. 225 



period in these cases, but some larvae of S. fasciata are at present being kept to see 

 liow long it will be before they either die or pupate. At present their larval stage 

 has lasted just over three months- — that is, the larvae have been under observation 

 in the laboratory for that time, but as they were already in the larval 

 stage when they were received, it is impossible to say exactly how many days old they 

 may be. Latterly, a good many of the larv^ae have died, but the last individual 

 which pupated hatched into a healthy mosquito on the following day, exactly 

 100 days after the larva was received at the laboratory. 



So far, the cause of this arrest of development has not been determined. It cannot, 

 I think, be the bottles in which the larvae are kept, since other larvae in identical 

 vessels develop normally. For the same reason, it cannot be either the temperature 

 or the light. Neither does it appear to be due to an exhaustion of the food supply. 

 It is possible, however, that it may be due to a fouling of the medium by the larvae 

 themselves. Many of the larvae that die are found to be infected with ciliates, 

 and to be covered with Vorticella, and it is possible that this may have some bearing 

 on the phenomenon. 



Experiments on the intolerance to common salt of the larvae of S. fasciata. 



In 1914 some experiments were described on the action of common salt on the 

 larvae of Stegomyia fasciata,^ and it was pointed out that solutions of a strength 

 of 2 per cent, were invariably fatal to them. Salt has, of course, been used as a 

 larvicide for untold ages, indeed ever since some primitive man made the classical 

 discovery of its preservative action which culminated in the introduction of salt 

 beef and pork ! The experiments referred to seemed to indicate nevertheless that 

 salt might have a value in the campaign against Stegomyia fasciata, even if its 

 application were restricted to the domestic utensils found littered about the native 

 compounds and a flushing out of open drains with sea water. 



The statements made in this paper referred of course to Stegomyia fasciata only ; 

 for many other species of mosquitos are known to breed in salt or brackish water. 

 At Accra, for instance, OcJilerotatus initans has been found breeding in water 

 •containing 1,400 parts of chlorine per 100,000, C. fatigans in water containing 

 1,600 parts, and C. decens in water containing 2,000 parts. The latter sample 

 <ontained chlorine corresponding to 3 "28 per cent. NaCl, a proportion considerably 

 greater than that found in sea water. 



C. fatigans was found breeding in water containing chlorine in proportions ranging 

 from 2 '6 to 1,600 parts per 100,000. S. fasciata, however, does not seem to possess 

 this power of adaptability. Thirty samples taken at random of water in which 

 iarvae of this species were found breeding were analysed (see Table II.). The chlorine 

 <'ontent ranged from 1'8 to 20 "5 parts per 100,000, and averaged 4*3 parts; but 

 as will be seen by reference to the Table, in 50 per cent, of the samples the chlorine 

 was between 2 and 2*9 parts. 



* Bull. Ent. Kes.. iv, p. 339 (1914). 



