THE LIMITATIONS OF KEROSENE AS A LARVICIDE. 28T 



generally ouly 2 to 4 hours, when submerged in their natural medium; Init in tap- 

 water ov distilled water thev survived twice as long or even longer. \ oung laivae 

 that is larvae in the first phase of their development, lived about the same length ot 

 time or a little longer (6 to 7 hours) in a natural medium ; but survived much longer 

 (about 30 hours) in fluids containing little or no organic matter. Smiilarly mature 

 larvae of Cidexfatigans did not survive so long as young larvae, but m the case of this 

 species the period of survival was shorter under all circumstances, the mature 

 individuals dying in less than 2 hours in a natural medium, and seldom surviving as 

 long as 6 hours even in distilled water. 



The Part Played by Cutaneous Respiration during Submergence. 



As has already been explained, these experiments were carried out in very small 

 volumes of fluids, as they were designed to determine the factors mfluencmg the 

 survival and did not aim at estimating the part played by cutaneous respiration. 

 The results however, clcarlv suggested that the larvae di^d m consequence ot the 

 reduction, bv various conditions, of the dissolved oxygen to an amount that ceased 

 to be of use for cutaneous respiration. In actual practice the larvae that it is sought 

 to destroy by " oiling " are not confined in small vessels, and although the medium 

 in which they are found is generally rich in organic matter, it is reasonable to suppose 

 that it must often be a very long time before the dissolved oxygen is exhausted or 

 reduced to a point at which it is no longer serviceable to them. 



The question of the cutaneous respiration of mosquito larvae has been studied by 

 Lima and Sen, and their observations will be discussed presently ; but as they recorded 

 dissimilar results, and as I believe their methods were in some respects unreliable, 

 it seemed advisable to reinvestigate the matter. Some preliminary experiments were 

 therefore carried out, using (A) small tubes, and (B) large bottles ot a ca^pacity ot 

 1800 cc In each experiment a small tube and a large bottle were placed side by side 

 in the laboratory under the same conditions, were filled from the same jug of water, 

 and were sealedwith paraffin from the same sample. Two S. fasciata larvae from the 

 same batch and in the same phase of development were used m each expermient. 

 the one being placed in the small tube and the other in the large bottle. In one 

 experiment of this sort S. fasciata larvae in the fourth phase were used and tap-water 

 that had previously been thoroughly shaken up with air. The experiment was 

 started one afternoon at 2.15 p.m. The larva in the small tube was still alive _at 

 5 p.m., but was dead before 8.15 a.m. the next morning (18 hours). The larva m the 

 larc^e bottle was still alive after 50 hours, but was dead after 51 hours. A slightly 

 youncrer larva, third phase, unintentionally introduced into the large bottle ^^^rviv^d 

 untirthe twentieth day, bv which time a little green groNNth had appeared at the 

 bottom of the bottle which may have helped to prolong its existence. 



When tap-water is being used for such experiments, it must be remembered that its 

 dearee of aeration is liable to great variation, and it is better therefore to use only 

 water that has previously been thoroughly oxygenated by shaking up with air. in 

 one of mv experiments the small tube and the large bottle were filled from the tap 

 in the laboratory in the early morning, a S. fasciata larva (fourth phase) added to each 

 and the vessels sealed ^^^th paraffin in the usual way. An hour later I ^^'^'^^^i^^^^ 

 to find that both larvae were moribund, and within two hours both had died. 



