12 NATURAL HISTORY OF AQUATIC INSECTS 



venture a little way below the level of the very highest 

 spring-tides ; others are found a little below the 

 ordinary high-water mark, and these are immersed 

 for a few minutes twice a day. But others withstand 

 prolonged immersion, and cannot be drowned in salt 

 water. There is vegetable and animal food in the 

 water, and some creatures will find it and devour it. 

 Insects, nimble, enterprising, and ingenious, have ven- 

 tured in and have succeeded in wresting a share 

 from the slow Mollusks to whom it might seem more 

 properly to belong. 



The life of some aquatic animals is greatly com- 

 plicated by the peculiar properties of the surface-film 

 of water. Like other external conditions, the surface- 

 film may be either a hindrance or an advantage, 

 according to the way in which it is treated. Those 

 who have had experience of the extraordinary versa- 

 tility of living things, and of their power to adapt 

 themselves to the most various conditions, will be 

 prepared to find that plants and animals have been 

 able in various ways to turn to account the properties 

 of the surface-film. 



The Surface-film of Water.^ 



I fear that it is not quite safe to presume that the 

 properties of the surface-film are familiar to ev^ery 

 reader. Let me, at the risk of superfluous explanation, 

 run over some of the elementary facts. 



Take a tumbler and pour water into it until it is 



1 A fuller but quite elementary account of this subject will be j 

 found in Prof. Boys' charming little book on Soap Bubbles, 



