THE LARVA'S ENEMIES 31 



these larvae are found there are sometimes small 

 fish ; but the larvae thrive notwithstanding. 



Then there is an insect that inhabits some fresh- 

 water pools and ponds. It is known as the water- 

 boatman or " backs wimmer " (Notonecta). It lives on 

 mosquito larvae, and will keep a pond quite clear of 

 them. It swims on its back under the water with 

 its face upwards, and catches the breathing mos- 

 quito larva or pupa in its jaws. It is fitted with a 

 pair of paddles which enable it to dart about rapidly 

 in the water ; and it can hop from one puddle to 

 another. It is a water-beetle, but is rendered power- 

 less against mosquito larvae when the pond becomes 

 full of green weed, which hampers its movements. 



Mosquito larvae will not withstand drying. Pour 

 some water containing larvae on to some dry sand. 

 Note how the larvae soon stop wriggling as the water 

 dries up, and how soon they die. They will not live 

 in quickly running water either, for there they 

 cannot breathe. If the wind causes a constant ripple 

 on the surface of the water, they cannot breathe, 

 and soon die off. 



Mosquito larvre have several artificial enemies 

 also. As stated before, they require air to breathe ; 

 they must rise to the surface of the water every few 

 minutes to obtain the necessary supply of oxygen. 

 Now, if the surface of the w r ater is covered with a 

 layer of oil, the supply of oxygen is cut off. The larva 

 then attempts to thrust the tube in its tail out of 

 the water, but it encounters only oil. As a result it 

 dies of suffocation. This artificial enemy, which was 



