THE LIMITATIONS OF KEROSENE AS A LAIJVICIDE. 293 



The siphon of the larvae of C.faligans is of moderate length, four times as long as 

 its basal diameter, but is nmch shorter than that of C. inridios>,s. For this reason, 

 perhaps, this larva does not seem to be in danger of being caught by the trim of oil 

 and dragged to the surface as is the larva of C. imidiosas. On examinmg the bodies 

 of C.faligans larvae that had died as a result of oiling, however, a globule of kerosene 

 was often found adhering to the end of the siphon. There can 1 think be little doubt 

 but that the larvae rising to the surface to breathe are unable to prevent the oil from 

 entering the tracheal tubes in the siphon and being drawn into their bodies with fatal 



results. -Ill 



Cidex invidiosHS. Five larvae of Cvlex inridiosxs were included with the huvae ot 

 Anopheles eostnJis in the experiment recorded above. After half an hour they were 

 all found floating on the surface in a moribund condition, and after an hour they were 

 all dead. These larvae also are very liable to be caught by the oil and drawn to the 

 surface in a horizontal position, and once this has happened, they a])pear to be unable 

 to liberate themselves. 



The larvae of C. invidwsus have long siphons, eight times as long as the basal 

 diameter, and quite apart fi'om any toxic action of the kerosene, they are unable to 

 sui-vive long without access to the external air. In one experiment by Hen's method 

 five larvae of this species were submerged in ta])-water. T1k'\- were all found dead 

 after half an hour. 



C(dex thahssim. The larvae of C. lluilassius have been found at Accra and 

 Christiansborg in the brackish water of lagoons, in fresli-water pools. and in old domestic 

 utensils. They are not easily destroyed by oiling, and on one occasion in forwarding 

 a sample containing this species the Medical Officer of Health, Accra, Dr. J. h. 

 Alexander, stated that they were from a pool that had been regularly treated witii 

 kerosene. In a laboratory experiment in which the larvae were placed in small tubes 

 containing 8 cc. of their natural medium, three fully developed individuals were still 

 alive after 30 minutes, but were dead after one hour under a layer of kerosene 

 1 cm. deep, whereas three control larvae cut of? from access to air by a layer of paraffin 

 were found dead after i, 5, and G hours respectively. The action of thin films of oil 

 spread over larger volumes of water was similar. 



When confined in similar tubes containing distilled water and covered with a layer 

 of paraffin the larvae lived much longer, for example 'HI 27, and 27 hours respectively 

 in one experiment, and in the system simulating the natural conditions in a pool of 

 water they survived many days and appeared to be quite at ea.-^e. It is not difficult 

 therefore to understand how they may escape the larvicidal action of kerosene. 



It is of some interest, as showing that the size of the anal papillae is no guide to 

 ability to survive without access to air, that in the case of the larvae of C. thahssiiis. 

 which are capable of subsisting so long on cutaneous respii-ation alone, these organs 

 are very small. • The papillae, however, are well supplied with tracheae. Larvae 

 of this species are therefore susceptible to the action of kerosene oil when they come 

 in contact with it, but they are capable of surviving a very long time without coming 

 to the surface to breathe, and this power no doubt accounts for the fact that m actual 

 practice they are not easily destroyed by "" oiling. ' 



Mansonioides africanus. The larvae of Manwnioides ajiicatm^ usually obtam the 

 oxygen they require by thrusting their siphon tubes, which are specially formed for 



