THE LIMITATIONS OF KEROSENE AS A LARVICIDE. 295 



on reaching the surface may often be seen to execute a series of short, rapid ^^Tif,'gliug 

 movements as if endeavouring to thrust the siplion forcibly upwards. This manoeuvre 

 can most easily be watched when tlie water is covered by a layer of soft paraffiji. It is 

 then seen that the relative!}- sliort stout siphon first comes in contact with the paraffin 

 with its valves closed. The larva, presumably aware that it has not reached the 

 surface, then executes the strong wriggling movements mentioned above and the 

 siphon appears to indent the paraffin and at the same time the valves are seen to open. 

 So vigorous does the effort appear to be that it seems )iot improbablethat it would 

 effectually break through a thin film of oil, and as the valves are not opened until an 

 appreciable time after the thrusting movement has started, there would probably be 

 little risk of oil entering the tracheal tubes. It may sometimes happen, however, 

 that when the film of oil is thick some does enter and this may account for the death 

 of some larvae. AVhen the oil is present in a definite layer, the larvae are of course 

 unable to penetrate it and eventually give up the attempt and sink down through the 

 water. They may then remain quiescent at the bottom for a time, but sooner or later 

 they try again and continue to do this until they die either from lack of oxyge]i or from 

 having at one attempt inhaled some kerosene. This suggestion is supported by the 

 iacts that under a deep layer of kerosene S.fasciafa larvae survive in a natural medium 

 rich in organic matter just about the same length of time as they do under a layer of 

 paraffin, and that larvae often appear to be greatly irritated by the oil, are frequently 

 seen apparently trying to remove it with their mouth-parts from the ends of the 

 siphons, and that oil is often seen to be present at the ends of the siphons of larvae that 

 have died under a layer of kerosene. 



Accra, 



August, 1916. 



References. 



Celli, A. (1904). Malaria. Translated by Eyre ; J. J. Longmans, Green and Co. 

 Lima, S. da Costa (1914). Memorias do Inst. Oswaldo Cruz, Tomo VI, Fasc. I. 

 Macfie, J. W. S. (1915). Observations on the bionomics of Stegomyia fasciata. 



Bull. Ent. Res., Vol. VI, p. 225. 

 Ross, R. (1900). Instructions for the Prevention of Malarial Fever, 'p. 19. 



Memoir I, Liverpool School of Tropical Medicine. 

 Ross, R. (1911). The Prevention of Malaria, p. 270. 

 Sen, S. K. (19U). Observations on Respiration of Culicidae. Lid. Jour. Med. 



Res., Vol. II, p. 698. 



