286 J. W. SCOTT MACFIE. 



It is probable therefore that the shorter period of siuTival of larvae .submerged iti 

 fluids containing organic mntt(M is in fact due to the smaller quantity of dissolved 

 oxygen present in them. 



The Presence of Wafer-weeds, As might have been expected. water-Aveeds that float 

 on the surface and have only their roots submerged appeared in my ex])erinients to 

 have the same effect as dead organic matter. There are, however, other weeds which 

 float more or less submerged or grow partly in and partly out of the water, and as these 

 are (ireen plants they presumably give off oxygen during the day-time, and the minute 

 chlorophyll-containing organisms so common in pond water presumably do the same. 

 Tlie question then arises whether this oxygen given off by green water-weeds, etc., can 

 prolong the life of mosqnito larvae that are jirevented from reaching the air 

 at tlie surface. 



In a single experiment with S.Jasckda larvae in their fourth phase there did appear 

 to be some prolongation of life. The experiment was carried out in the usual manner, 

 six lar^'ae being confined in six tubes of distilled water. To three of the tubes a small 

 piece of a green water-weed was added, the other three being used as controls. In 

 the tubes containing weed the larvae were found dead after 11, 12, and 13| hourh 

 respectively, and in the controls after 10. 12, and 12| hours. Xow if the weed did 

 not give off oxygen that was available for the use of the larvae it should 

 have acted merelv as organic matter, which has already been shown to accelerate 

 the death of submerged larvae. Since however the larvae lived a trifle longer in 

 the tubes containing the weed, it seems probable that the oxygen evolved by 

 green plants, etc., can be of service to the larvae. 



The leaves of some water- weeds are often found partly submerged, either as the 

 result of growth or of some accidental circumstance. The outer leaves of Pisiia 

 straliotes, for example, droop on to the water as the centre of the plant unfolds and 

 eventually come to lie parallel with the surface of the pool. For some time, however, 

 they are in contact with the water only at their edges, and under them there is an air 

 space to which oil applied to the surface of the pool would not have access. The 

 surfaces of the leaves of Pistia sfratiofes are also unwetable and carry down with them 

 into the water a film of air which appears to be of service to some species of mosquito 

 larvae at any rate. The larvae of Mimomyia sjjhndens, for instance, seem to make 

 use of this film of air for respiration by preference. 



From what has already been said it is clear that the survival of submerged mosc[uito 

 larvae is influenced by a great many different factors, and any attempt to estimate 

 the length of time larvae can live without access to the external air nuist take into 

 consideration not only the species and the age of the larvae, but also the temperature, 

 the degree of oxygenation of the medium, the amount of organic matter present, 

 and probably other conditions. So complex is the question that it is practically 

 impossible to arrange any series of experiments the results of which will be com- 

 parable in detail. B}' carrying out a large number of experiments on similar lines 

 it is possible to arrive at certain general conclusions, and for the present purpose--- 

 namely, as bearing on the efficiency of '' oiling " in anti-mosquito measures- — these 

 are probably sujffiicient. In small tubes of about 8 cc. capacity the mature larvae pf 

 Stegmiyin fosciold at laboratory temj^eratures seldom survived over hours, and 



