184 



into solution in the water and poisons the larvae ; that respiration 

 is stopped by the oil entering and blocking the siphon and tracheal 

 tubes ; that the oil acts directly on the tissues as a contact poison on 

 entering the siphon and trachea ; that it is the oil vapour from the 

 inspired oil, Avhich by its extremely rapid penetration of the tracheal 

 tissues produces the lethal effects. 



Experiments to test these theories were conducted with full-grown, 

 larvae of Theobaldia {Culiseta) incidens, the oils used being a series of 

 standard commercial Californian oils including crude, 15*4° Be. ; low- 

 grade stove distillate 29° ; high grade stove distillate 33'3° ; commercial 

 engine oil distillate 38"5° ; kerosene 39'7° ; gasoline 55.5° ; and 

 a by-product obtained from the stills after distillation, with a. reading 

 of 20'3°. Besides these toxic petroleum oils, a standard non-toxic 

 liquid petrolatum of 27° was used. 



The results showed : — That as regards surface tension, so far from 

 this being annulled, the larvae remained at the sm'face for a considerable 

 time, either quiescent or in a definite attempt to pierce the film ; 

 that suffocation is but a slight factor in the larvicidal action of the 

 oils, since larvae kept from the air by simple mechanical means lived 

 for 30 hours, while those under a layer of kerosene died in 45 minutes ; 

 that the solubility of kerosene in water is not a toxic factor in the 

 killing of larvae by its application to the medium in which they live ; 

 that the blocking of the respiratory tubes, even to their very finest 

 subdivisions, which actually does take place by the entrance of the 

 oil into their siphons, is not in itself the cause of the efficiency of the 

 larvicide, for in that case, all oils effecting this would have equal 

 lethal action, whereas kerosene caused death in 45 minutes, while 

 non-toxic petrolatum required 4| hours ; that the direct action of 

 the oil on the tissues as a contact poison is not the cause of death,, 

 because, though it enters and penetrates the tracheal system with 

 great rapidity, the larvae were always dead long before it penetrated 

 the tissue. [Cf. this Review, v, p. 54.] 



Finally, experiments to test the toxic action of oil vapours took 

 the form of exposing the various oils to a temperature of 28° C. + 2° 

 (82° F.) for 103 hours in a constant circulation of air, and using the 

 evaporated residues to make films in the usual manner. It was found 

 that the lethal effect of the oils had decreased in proportion to the 

 percentage of the weight lost through evaporation ; thus kerosene, 

 which had previously given rise to the greatest death rate, now had 

 become the least efficient in its larvicidal properties. Further direct 

 experiments, in which vials containing larvae in water were plugged 

 with absorbent cotton on which were poured 3 c.c. of the various 

 oils, showed that the vapours of gasoline and kerosene confined in the 

 spaces above the water killed the larvae in 153 and 185 minutes 

 respectively, while the vapours of crude petroleum had no ill effects 

 at the end of 72 hours. 



Hence it may be concluded that the practical toxicity of the petro- 

 leum oils as mosquito larvicides increases with an increase in volatility, 

 and that the volatile constituents of the oils contain the principles 

 that produce the primary lethal effects. It is possible that the lethal 

 action of oils having a higher boiling point than 250° C, the volatile 

 constituents of which are practically negligible in quantity, may be 

 due to the effects of stoppage, actual contact and even suffocation. 



