216 MOSQUITO LABYJE AND THEIR NATURAL ENEMIES. 



(6.) I once placed six fish in a kerosene tin containing numerous mosquito 

 larvae ; the next day all the larvse had disappeared. I did not change 

 the water ; at the end of three weeks there were twenty-three indi- 

 viduals. Shortly after this some of them began to die. They had 

 thus lived and nmltiplied in two or three gallons of stagnant 

 unchanged water. 



(7.) It is almost impossible to state their exact power of destroying iarva3 

 and pup?e in figures, as both larvse and fish vary so greatly in size ; 

 but this power is enormous, for I find two or three fish will soon 

 clear a 10,000 gallon tank and keep it free from larvse.^"* 



The next natural enemy dealt with is a small undetermined Crustacean of 

 the sub-order Decapoda. This is stated to have a special value because it 

 remains in mountain pools in which Girardinus cannot be established, owing 

 to its habit of following the streams down to lower levels. The utility of 

 these Crustacea is illustrated by an experiment, in which six of them were 

 placed in a jar with several hundred mosquito larvae ; the next day not a 

 single larva remained. 



Observations are also cited which suggest that in suitable small pools water- 

 beetles and dragon-fly larvae may serve to prevent the breeding of mosquitos ; 

 and reference is then made to certain other possible inhibitive factors. 



*' One swamp that I have examined is overgrown with thick canes running 

 to a height of twelve feet ; in this I have never found larvae, the absence of 

 light and warmth from the sun evidently preventing their development. I 

 therefore suggest that these reeds might be grown to advantage on certain 

 swampy waste lands. 



" The following illustrates another method which might be applicable in 

 certain situations. Cellia alhipes is very common in the months of December 

 and January. In a certain pool in which they were numerous, I raked the 

 surface free from algae and cleaned the edges of the pool from sedges ; it 

 was a small pool, and this occupied me about fiAC minutes. In a few days I 

 f<earched the pool again but could find no larvae, though they were still 

 numerous in neighbouring pools that I used as a control. I have repeated 

 this experiment with the same results. I have not decided the true reason 

 of this, whether what I removed was their food or their protection from sun, 

 rain or their natural enemies." 



Finally, it is urged that " there is no reason why Neuroptera, Coleoptera 

 and fish that are the natural enemies of mosquitos should not be introduced 

 into other countries ; a general exchange and distribution might be followed 

 by great results. The ^ Millions ' fish could certainly be carried to such a 

 country as India, and there bred in tanks and immunised, protected and 

 distributed to numerous and varied surroundings. The cost of breeding and 

 distributing ' Millions' would be very small .... 



