THE LIMITATIONS OF KEROSENE AS A LARVICIDE. 



Table I. 



279 



Experiment showing the action of crude kerosene on the larvae of Stegoniyiafasciata. 

 Five larvae in the first phase were used in each experiment. 



Time after the 



beginning of 



the experiment. 



Crude kerosene (about lOU oc. per sq. metre) applied to 



A natural S. fasciata 

 medium. 



Tap-water containing 

 a fair amount of 

 organic matter. 



Distilled water. 



3 hours 



4 „ 



5 ,, 



7 „ 



8 „ 

 12 „ 

 36 „ 



1 dead, 4 alive 

 4 alive 



1 dead, 3 alive 

 3 alive 



] 2 dead, 3 alive 

 3 alive 



2 dead, 1 alive 

 1 alive 



Experiment discontinued. 



2 dead, 3 alive. 

 1 dead, 2 alive. 

 1 dead, 1 alive, 

 1 alive. 



•» 

 1 dead. 



The oil may be supposed to act in one of several ways ; as for example 

 " by annulling the surface-tension of the water/' by depriving the larvae of access 

 to the air and so drowning them, or by some poisonous effect on the larvae themselves. 



Bij anmiUing the surface-tension. Ross (1911) has suggested that as the result of 

 " oiling " the " larvae and pupae are no longer able to keep the surface by surface 

 tension and quickly drown.'" In the course of a long series of experiments ^rit\l many 

 different species of larvae I have never observed this effect ; on the contrary, indeed, 

 AnopheUne larvae {A. costalis) and larvae with long siphons {Culex invidiosus) Avere 

 apt to be caught and carried to the surface if the layer of oil were thick. AVhen the 

 oil film was thin, larvae such as those of Stegomijia fasciata and CuJexfatigans appeared 

 to have no difficulty in adhering to the surface, and the former were repeatedly 

 observed in what looked like a vigorous attempt to drive their siphons through the 

 film. 



By deprivi)ig the larvae of access to the air. This is perhaps the most common view 

 of the action of " oiling." Celli (1904), for example, states that both oil and petroleum 

 " have a mechanical action only, that is, by intercepting the air from the larvae, which 

 require much oxygen," and emphasises the fact that if the whole surface of the water 

 is not covered by the film of oil the lai"vae do not die. Others have supposed that the 

 oil enters the siphon tubes and blocks them, thus preventing the larvae from breathing 

 air freely. As this theory is necessarily involved in considering the action of keroseiie 

 as a poison, and as it is intimately connected with the question of cutaneous respiration 

 that will be dealt with later, it will not be discussed here. 



