II FLIES WITH AQUATIC LARVAE 99 



startled, the larva leaves the surface and sinks slowly 

 to the bottom by gravity alone, which shows that 

 the body is denser than the water. It does not 

 willingly remain below for any length of time, but 

 rises by a jerking movement, striking rapid blows 

 with its tail, and advancing tail foremost. When it 

 reaches the top, it hangs as before, head downwards, 

 and resumes its feeding operations. 



How is it possible for a larva heavier than water to 

 remain floating at the surface without effort, as the 

 larva of the Gnat certainly appears to do ? The 

 possibility of such a thing turns upon the existence at 

 the surface of water or any other liquid of a contract- 

 tile surface-film. (See Introduction.) It is the con- 

 tractile force, or tension, of the film which rounds the 

 rain-drop and the soap-bubble. The film can be 

 stretched over unwetted threads, such as the threads 

 of a piece of dry muslin, or the fine wires of a piece 

 of varnished gauze, and thus supported it will not 

 only refuse to pass through fine meshes, but may 

 be made to hold up a rather considerable weight 



of water. 



If we take a solid body, capable of being wetted 



by water, and place it in water, the surface-film will 

 adhere to the solid. If the solid is less dense than 

 the water, it will float with part of its surface out 







of the water. Under such circumstances the surface- 

 film will be drawn upwards around the solid, and will 

 therefore pull the solid downwards. But if the solid 

 is denser than the water, its upper surface will be lower 

 than the water-level, and we can arrange matters so 

 that the surface-film around it will tend to pull the 



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