M. E. MacGregor 
351 
acquired. For this reason, although Saprolegnia flourishes in acid water, it 
is rare that one finds Finlaya geniculata, although living in water that is 
distinctly acid, parasitized in a natural state, but if the water be rendered 
less acid by the addition of tap water or even a few drops of iV/10 NaOH the 
change in the P H adversely affects the larvae and they are very often at once 
attacked by Saprolegnict. 
While I would not go so far as to say at present that the abnormal 
development of mosquito larvae in the laboratory is entirely due to changes 
in the hydrogen-ion exponent, it nevertheless has a profound effect upon 
the metabolism of the larvae, and their resistance to diseases which affect 
them. Moreover if their successful development is so adversely affected by 
changes in the reactions of the water in which they normally live, it may 
be possible by employing measures that will make the water of ponds 
acid, and the water of tree-holes alkaline, to find that we have yet another 
means of combating mosquito development. If so, obviously such means 
have a limited practical application, governed as they would be by the size 
of the ponds or other waters to be dealt with. 
In the meantime further investigations are proceeding. 
My thanks are due to Major H. C. Brown, C.I.E., of the Wellcome Bureau 
of Scientific Research, for much help that he has given me in the theory and 
calculations of the hydrogen exponent. 
