XI THE WATER SPRING-TAIL 367 



ponds, for they are plentiful and widely distributed. 

 During the summer months they spring, like Sand- 

 hoppers, from the surface of the water, and then 

 repose tranquilly upon it. They are black, with a 

 mixture of white individuals, which are fresh-moulted. 

 How is it that; they are never wetted by the water ? 



The microscope shows that the body is covered 

 with short and close-set hairs. Into the minute 

 spaces between these the surface-film cannot pene- 

 trate, and the trifling weight of the body is insufficient 

 to press it as a whole beneath the surface. Were the 

 body once wetted, it would be hard for the Insect to 

 force its way through the surface-film into the air. 

 It is equally hard for it to make its way through the 

 surface-film into the water. 



What has the size of the creature to do with its 

 power of swimming or sinking ? you may ask ; the 

 size surely does not affect the specific gravity of its 

 body. No, but it affects the proportion which the 

 contour of its body bears to its weight. In similar 

 figures the contour increases or diminishes directly 

 as any linear dimension, but the weight as the 

 cube. A minute Insect has accordingly a far 

 greater contour in proportion to its weight than a 

 large Insect of precisely similar form, and therefore 

 encounters greater resistance from the surface-film. 

 The resistance in the case of the aquatic Spring-tail 

 is, as we have seen, so great that it can leap from the 

 surface of the water and alight upon it again without 

 ever breaking through. I see that in various books 

 on Natural History this power is attributed to the 

 extraordinary elasticity of the spring. But no elas- 



