2U4 J. W. SCOTT MACFIE. 



this pui'posc, into the roots of the water-weed, Pislia slraliotes, but in the laboratory 

 they wore sometimes seen to attach themselves in a similar way to other water-weeds 

 also when Pislia plants were not available. They did not, however, surv'ive long in 

 the absence of Pislia plants, but when these were present they appeared to be quite at 

 home in the laboratory and completed their development. They are not incajiable 

 of breathing air directly, but as a matter of fact they are seldom if ever seen at the 

 .surface unless they have been forcibly disturbed. In the absence of Pislia roots 

 the larvae do not live long if prevented from reaching the external air. In experi- 

 ments carried out by my standard method at laboratory temperatures, larvae in the 

 first phase of development did not survive 20 hours in either their natural medium or 

 in distilled water, and a single fully developed larva in distilled water sui-\'ived 2 hours, 

 but not' 2-1 hours. In jars in the laboratory larvae recently hatched attached 

 themselves to the rootlets of the Pislia plants, and older larvae to the tap-roots, and 

 as the result of actual experiments it was proved that they could moult and even 

 pupate without having direct access to the external air. 



It has been shown above that the larvicidal action is not dependent on any effects 

 produced by solution of the oil. It is not surprising therefore that a layei- of kerosene, 

 no matter how thick, applied to the surface of water in which larvae of M. afrimnus 

 were living attached to Pislia roots was found to be absolutely innocuous. In one 

 such experiment a larva was kept under a film of oil corresponding to 100 cc. per 

 square metre. After three days it was found to have pupated, on the sixth day the 

 pupa appeared to be almost ready to hatch, and on the seventh day it was found dead 

 at the surface. The oil in fact had no effect on the larva, and only acted on the pupa 

 when it left the root of the water-weed preparatory to emergence. 



Slegoini/ia fasciala. Some observations on the action of crude kerosene on the 

 larvae of Slpgomyia fascial a have already been recorded above. It has been shown 

 (p. 9) that in a mass experiment in a natural fluid rich in organic matter there was no 

 appreciable difference between the length of time the larvae survived under a layer of 

 paraffin and under a layer of kerosene ; and an experiment has been described (p. 4) 

 in which some very young larvae were still alive after two days under an unbroken 

 film of oil. 



Two other experiments may be briefly mentioned. In the first, five >S. fasriala 

 larvae were placed in 400 cc. of tap-water and crude kerosene added in a proportion 

 equal to 1 cc. per square metre. After 3| hours one larva was found dead, two more 

 died within the next twenty-four hours, but the remaining two were still alive and 

 apparently unaffected when the experiment was ended on the fourth day. In the 

 second experiment five larvae were placed in tap-w^ater in a jar 7 cm. square, and 

 kerosene added in a proportion equal to 200 cc. per square metre. Three days later 

 one larva w^as still alive, although there still appeared to be a continuous film of oil 

 over the surface of the water and the interior of the jar had a very strong smell of 

 kerosene. On the fourth day this larva pupated. 



When watching mosquito larvae imprisoned beneath, a film of oil one cannot but be 

 struck by the difference in the behaviour of S. fasciala and, for example, C.faliyans. 

 The larvae of C. faligans rising to breathe, as a rule, just touch the layer of oil, and 

 then immediately sink down through the water again ; or if they do adhere to the 

 surface, remain more or less passive. Not so the larvae of S. fasciala. These larvae 



