No. 3.] Calcutta Municipal Water. 199 



may cease to exist in a given place, something adverse must be intro- 

 duced into some stage of its life history with which it is unable to 

 cope. 



These Lake Flies appear to me to be, at present, living under 

 very advantageous circumstances. They have plenty of water in 

 which to lay their eggs and plenty of weeds to attach them to, mud 

 for their larvae to develop in, and shade for themselves from the 

 heat of the day. There is also plenty of Algae, Sperogera, Volvon, 

 and the like, for the voracious larvae to live upon. It is true that 

 they have enemies — other larvae in the egg-stage and worms in the 

 pupal-stage, etc. Nevertheless, these flies grow and increase in 

 numbers, and will, I think, in due time spread to other pools in 

 Colombo. 



The question now asked is what can be introduced into their life 

 history so disadvantageous as to diminish their numbers. 



The life history of the Fly for practical purposes may be divided 

 into three parts: — 



/. The Perfect Insect— Capable of flying in the air and of much 

 movement. 



//. The Larvae and Pupae — Swimming in the water or living in 

 the mud at the bottom of the stream. 



III. The Eggs — Stationary and at the surface of the water. 



I. The insect stage. 



Many have been the attempts to kill adult mosquitoes on a large 

 scale. They cannot be said to be very successful; and I do not think 

 that any can be applied with success in this case. 



II. The larval stage. 



The young larvae are most resistant, and the older larvae protect 

 themselves by burying in the mud. Kerosene, etc., is quite useless 

 against such larvae. Some of the younger larvae lived for hours in 

 kerosene. 



I, personally, do not see any easily-applied method of dealing 

 with these larvae, except one method, which I thall mention below. 



III. The egg stage. 



This is sedentary and on the surface of the water, and this is the 

 stage at which I propose to attack the insect. 



